Aims Soil respiration is one of the most critical components of carbon cycle in terrestrial ecosystems. The study on temporal dynamics of soil respiration and its linkage with environmental factors in desert steppes under changing precipitation can provide data supports for a deep understanding of the regulatory mechanisms of key carbon cycling processes in fragile ecosystems.
Methods A field experiment involving five precipitation treatments (50% reduction, 30% reduction, natural, 30% increase, 50% increase) was set up in 2014 in a desert steppe in Ningxia. The temporal dynamics of soil respiration rate were explored during the growing season (from June to October) in 2019, and the relationships between soil respiration rate and soil properties and plant characteristics were analyzed.
Important findings Soil respiration rate showed a seasonal variation of an increasing and a decreasing trend across the growing season, with the maximum values (2.79-5.35 μmol·m^-2·s^-1) occurring in late July or early August. Compared with the natural condition, 30% reduction in precipitation did not result in a significant effect on soil respiration rate, reflecting the adaptability of soil respiration to moderate drought. Overall, 50% reduction in precipitation reduced soil respiration rate, whereas increased precipitation (especially the 30% increase) enhanced soil respiration rate, and this positive effect was especially obvious in the early growing season (June to July). Soil respiration rate had a significantly exponential relationship with soil temperature and a significantly linear relationship with soil water content. Soil physicochemical property had a highly independent explanatory power for soil respiration rate (R² = 0.36), and its effect was highly correlated with soil biological property and plant diversity (R² = 0.31). Precipitation could affect soil respiration rate either directly or indirectly through the influences on soil biological property and plant biomass. The results indicated that a moderate increase in precipitation could accelerate soil respiration by alleviating soil water limitation, stimulating soil enzyme activity, promoting microbial activity and plant growth in the desert steppe, and that an extreme increase in precipitation would lead to a decrease in soil permeability and a hindrance to microbial metabolic activity, thus inhibiting soil respiration.

Aims To reveal the relationship between rhizosphere microbial composition, functional characteristics and health status of Malania oleifera.

Methods We collected rhizosphere soil samples of healthy and non-healthy M. oleifera at five different habitats in broadleaf forest, artificial planting forest and karst forest, sequenced the microbial communities using illumina high-throughput sequencing techniques and predicted the microbial community functions using FAPROTAX.

Important findings The results showed that: 1) Amplicon sequence variants (ASV) representative sequence classification analysis showed slight differences in microbial composition among five habitats. The top five bacterial phyla were Acidobacteriota, Proteobacteria, Actinobacteriota, Chloroflexi and Myxococcota. There were significant differences in rhizosphere microbial composition between healthy and non-healthy plants, and the dominant microbial taxa changed significantly. 2) The non-metric multidimensional scaling (NMDS) analysis showed significant differences in microbial components of M. oleiferawith different health status. Redundancy analysis results showed that the healthy plant samples were distributed along the first axis, and the two axes explained 25.83% of the variation in the microbial community as a whole. The contents of soil available phosphorus, total potassium and pH were the main factors affecting the rhizosphere microbial communities of healthy plants. Redundancy analysis of non-healthy plants showed that 51.84% of the variation in microbial community was explained by the two ordination axes. Soil total potassium content and available phosphorus content represented the important factors affecting the rhizosphere microbial communities of the non-healthy plants. 3) The correlation heatmap showed that soil pH, available phosphorus content and total potassium content were significantly correlated with the abundance of Chloroflexi, Planctomycetota, Methylomirabilota and Desulfobacterota in healthy plants. However, the abundance of Desulfobacterota, Acidobacteriota, Desulfobacterota, Latescibacterota and Gemmatimonadota were significantly affected by soil pH, available nitrogen content, available phosphorus content, total phosphorus content and total potassium content in non-healthy plants. 4) FAPROTAX functional prediction results showed that the abundance of phototrophy, photoautotrophy, aromatic compound degradation, cyanobacteria and oxygenic decreased significantly in healthy rhizosphere microorganisms, whereas fermentation, ureolysis and human pathogens increased significantly. The results demonstrate that the rhizosphere microbial community undergoes significant changes in different health conditions.

Aims Nitrogen (N) and phosphorus (P) nutrient availability is a key factor governing forest productivity and carbon sequestration. However, scientific knowledge on the nutrient limitation in forest ecosystems under variable environments is still lacked. The mountain ecosystems, characterized by the dramatical changes in multiple environmental factors along increasing altitude such as climate, vegetation and soil properties, provide a natural experiment platform for understanding forest nutrient limitation and its drivers.
Methods In this study, we examined the nutrient limitation of a typical subalpine coniferous forest (Abies fargesii var. faxoniana forest) along an altitudinal gradient (from 2 850 m to 3 200 m) in the southeastern Qingzang Plateau, by simultaneous detection of above-ground leaf N, P status and underground microorganisms extracellular stoichiometry, and analyzing the changes of forest nutrient limitation and the main driving factors along the altitude.
Important findings The results showed that: 1) as altitude increases, the concentration of leaf N and P decreased, while leaf N:P increased from 12.33 to 15.00, indicating a shift from N limitation to N-P co-limitation and an enhancement of P limitation with increasing altitude. (2) Vector model analysis showed that the vector angles of microbial extracellular enzyme stoichiometry were all exceed 45° at different altitudes, and as altitude increases, the vector angle showed an increasing trend, indicating that microorganisms were limited by P and the P limitation increases with altitude. (3) Temperature is the dominant factor driving nutrient limitation of Abies fargesii var. faxoniana forest. Collectively, both leaf and soil microbial nutrient evidence indicated that an enhancement of P limitation in subalpine coniferous forests with increasing altitudes in western Sichuan. This finding could provide an important theoretical basis for guiding forest nutrient adaptive management in subalpine coniferous ecosystems under the scenarios of global climate change.

Plant species change soil abiotic and biotic properties which in turn influence the performance of plants, leading to so-called “plant-soil feedbacks” (PSF). It is the prerequisite of plant-soil feedbacks that plant species can cause specific changes in soil microbial communities which are characterized by specialized soil pathogens and mutualists. Specialized microbes can have substantial effects on host plants, but likely do not influence the performance of non-host plants. PSF have been used to interpret ecological processes of different scales since the concept was proposed in the 1990s, such as succession, interspecific competition, biological invasion and effects of global changes on terrestrial ecosystems. In recent years, community ecologists and theoretical ecologists have started to integrate the research of PSF and community ecology, resulting in fundamental progress. In this review paper, we introduce soil microbe-mediated PSF and its implications for plant species coexistence, community structure and ecosystem functions. Classical PSF theory assumes that soil microbes can generate stabilizing process which promotes plant coexistence. However, recent studies show that soil microbes can also cause fitness difference between plant species which can influence species coexistence through equalizing process. Community ecologists predict that rare species have less negative or more positive PSF than abundant species, thereby leading to negative correlations between plant landscape abundance and PSF. However, empirical evidence demonstrates inconsistent patterns such as negative, positive and neutral correlations, and coevolution of plants and soil pathogens is key to reconcile these patterns. Soil microbes are also considered as a fundamental factor regulating succession. Dilution of soil microbial effects is a mechanism of positive plant diversity-productivity relationships. Specialist pathogens and mutualists accumulate in the soil of monocultures, but their negative and positive effects are diluted in multi-species mixtures, thereby increasing and decreasing biodiversity effects on productivity, respectively. We suggest three directions for future studies: empirical testing for specialization of plants and soil microbes, multi-dimensional species coexistence and eco-evolutionary dynamics in plant-soil feedbacks.

Aims Under the background of changing carbon cycle process in forest ecosystems caused by global environmental change, the microbial carbon use efficiency (CUE) in forest rhizosphere soil is critical to determine the strength of microbial anabolism and catabolism in forest ecosystems. However, the variation and influencing factors of microbial CUE in rhizosphere soils at different altitudes remain undetermined.

Methods Rhizosphere soil at six different altitudes spanning four forest belts in Taibai Mountain was sampled to determine the physical and chemical properties, extracellular enzyme activity, and characteristics of microbial community and vegetation. Based on the stoichiometric ratio, the soil microbial CUE was estimated. Furthermore, the variation in microbial CUE of rhizosphere soil along the altitude gradient was analyzed to quantify the influencing factors of microbial CUE.

Important findings The results showed that the microbial CUE of rhizosphere soil exhibited an overall upward trend with the increase in altitude. The microbial CUE increased by 4.36% from 0.505 at the lowest altitude to 0.527 at the highest altitude, but decreased at 1 603 and 2 405 m. Based on the Mantel analysis, we identified four categories of factors (i.e., altitude, soil matrix, vegetation and microbe) that related to microbial CUE in rhizosphere soil. The variations of microbial CUE in rhizosphere soil are affected by multiple environmental factors, with the dominant factor being soil matrix (such as dissolved organic carbon (DOC) content, ammonium nitrogen (NH⁺₄-N) content), followed by vegetation. Furthermore, the altitude factor and the microbial factor explained 2.6% and 3.1% of the CUE change, respectively. Although the microbial factors exerted no significant impact on microbial CUE, soil matrix, vegetation and microbe jointly explained 47.0% of the microbial CUE change. The variance partitioning analysis (VPA) quantitatively revealed the contribution of environmental factors to the change of microbial CUE, where soil matrix and vegetation explained 17.0% and 5.7% of the variation, respectively. While the interaction between soil matrix and vegetation accounted for 31.9% of the changes in microbial CUE. The above results indicated that the high-altitude rhizosphere soil in Taibai Mountain has a high carbon sequestration potential, and the carbon sequestration of forest rhizosphere soil may decrease with the intensification of global warming. The vertical temperature difference and the vertical differentiation of the vegetation belt induced by altitude gradient will alter the growth and metabolism environment of microorganisms in the rhizosphere soil. The comprehensive effect of multiple environmental factors dominated by soil matrix impacts the CUE of soil microorganisms, and ultimately changes the assimilation and catabolism processes of soil carbon. The results of this study can provide a scientific basis for the carbon assimilation capacity and carbon sequestration potential of forest soil microorganisms in Qinling Mountains, as well as the forest soil carbon cycle under the background of global change.

Aims Understanding the effects of shrub encroachment on plant and soil microorganisms in the forest-grassland ecotone will help improve the understanding and management of shrub encroachment in the forest-grassland ecotone.

Methods In this study, different levels (light, moderate and heavy) of shrub encroachment were selected in the forest-grassland ecotone of Dongling Mountain Nature Reserve in Beijing, to explore the effects of shrub encroachment on plant diversity, soil microbial diversity, plant individual trait and soil nutrients by plot method and high-throughput sequencing technology. The correlation between plant diversity, soil microbial diversity, plant individual trait and soil nutrients were also developed in order to further explore the effects of shrub encroachment on plants and soil microorganisms and associated mechanisms.

Important findings Our results showed that: 1) Shrub encroachment significantly reduced the diversity of plants with the different responses of arbor, shrub and herb, among which the diversity index of herb plants decreased in the largest level. 2) Shrub encroachment significantly increased the diversity of soil fungal microorganisms. 3) Shrub encroachment significantly increased the height and crown width of shrubs, while soil total nitrogen and organic carbon contents increased significantly with the increasing shrub encroachment level. 4) The partial least squares path model (PLS-PM) revealed that shrub encroachment had a direct impact on plant and soil microorganisms, whereas plant individual trait and soil nutrients did not have a direct impact on them. The redundancy analysis (RDA) further showed that shrub height made great contribution to the interpretation of changes in plant diversity, and soil total nitrogen content was the main factor in affecting soil microbial diversity. There was a stronger correlation of soil fungal microbial diversity with plant diversity than its relationship with soil bacterial microbial.

Aims The mixed decomposition of organic residues is crucial to the material cycle in terrestrial ecosystems. This study focuses on the decomposition process of mixed straw in farmland ecosystems.
Methods In this study, a 6-month litter bag experiment on the decomposition of maize (Zea mays) straw, potato (Solanum tuberosum) straw and maize-potato mixed straw was set up in maize monoculture, potato monoculture and maize-potato intercropping plots respectively. Biolog-Eco microplate method was applied to evaluate the microbial carbon metabolic activity as influenced by straw type and decomposition environment.
Important findings The results showed that the mixture of potato straw and maize straw resulted in a synergistic effect on the decomposition process. The decomposition rate and microbial metabolic activity of mixed straw were higher than those of single straw type, which facilitated the utilization of carbohydrate and carboxylic acid substrates by microorganisms. Such synergistic effect is weakened over time. Random Forest and structural equation modeling showed that soil dissolved organic carbon, nitrate nitrogen, ammonium nitrogen contents and straw carbon to nitrogen ratio were key factors driving straw decomposition. In general, straw decomposition is promoted by straw mixing.

Aims This study aimed to study the effects of two young mixed plantations of Parashorea chinensis (an endangered native tree species) on the functional characteristics and carbon source utilization of the soil microbial community, so as to select the suitable afforestation mode of P. chinensis and maintain sustainable management of forests by changing pure Eucalyptus plantation into mixed plantations with heterogeneous structure.

Methods The functional diversity of soil microbes and their utilization of six carbon sources in mixed and pure plantations of P. chinensis were compared and analyzed using the Biolog-ECO technique, and a correlation analysis was further carried out incorporating soil physicochemical properties.

Important findings (1) The Shannon-Wiener, Simpson and McIntosh diversity index of the microbe community in mixed plantations of P. chinensis and E. grandis × E. urophylla were the highest, and their soil microbial functional diversity was significantly higher than that of pure plantations. (2) The carbon source utilization and microbial quantity of soil microbes in the mixed plantations were higher than those in the pure plantation, and decreased with the deepening of the soil layer. The utilization of phenolic acid by soil microbes in mixed and pure plantations was the highest, followed by amines, with polymers at the lowest level, and the difference was that mixed and pure plantations were more dependent on amino acids and carboxylic acids, respectively. (3) The soil moisture conditions and the content of organic matter, total nitrogen, total potassium and available nutrients in the mixed forest of P. chinensis were higher. In addition, the vertical distribution of other nutrients except total phosphorus and total potassium showed obvious surface aggregation. (4) Environmental factors analysis showed that soil pH, organic matter and potassium were the main driving factors causing significant differences in soil microbial functional diversity and carbon source utilization between mixed and pure plantations, as well as different soil layers. In summary, the mixed afforestation model of P. chinensis has a significant impact on soil microbial community and their habitats. Especially, P. chinensis mixing with E. grandis × E. urophylla may effectively improve the metabolic activity and functional diversity of soil microbes, and promote the decomposition of organic matter. Compared with the pure forest, the mixed forest improved soil quality and fertility to some extent, and created a better soil environment and light conditions for the growth of young P. chinensis saplings.

Aims This study aimed to explore the response of ammonia oxidizing bacteria (AOB) to litter decomposition and nitrogen application in the topsoil of desert steppe, and the main environmental factors influencing the AOB were also analyzed. Thus, this study will contribute to better understanding of the mechanism of nitrogen cycle function gene abundance in response to nitrogen deposition in arid and semi-arid desert steppe regions.
Methods We conducted the nitrogen fertilization experiment in the desert steppe in Yanchi, Ningxia. The four plant litters, including Sophora alopecuroides, Artemisia scoparia, Stipa breviflora and Agropyron mongolicum were selected as plant litter input treatments. The nitrogen fertilization treatments, including control (0 g·m^-2·a^-1) and nitrogen application (9.2 g·m^-2·a^-1), were applied to explore the response of AOB to nitrogen deposition and different litter inputs in top soil (0-5 cm) of desert steppe by fluorescent quantitative PCR and high-throughput sequencing.
Important findings Our results showed that there were 3 phyla, 4 classes, 6 orders, 7 families, 8 genera and 17 species of AOB in the topsoil of desert steppe under nitrogen application and litter decomposition. The AOB communities were mainly derived from Nitrosomonas and Nitrosospira, which were from beta-Proteobacteria, and Nitrosospira was the dominant species. Compared with the control, the gene copy number of AOB was significantly decreased under nitrogen application, indicating that nitrogen application inhibited nitrification in the topsoil of the desert steppe. Whereas, the response of AOB-ammonia monooxy genase subsuit A (amoA) gene abundance varied under different plant litter decomposition. Litter input could alleviate the inhibition of nitrogen enrichment on soil AOB-amoA gene abundance to a certain extent, but did not change the trend of soil AOB-amoA gene abundance. A redundancy analysis confirmed that soil organic carbon, total phosphorus, ammonium nitrogen and nitrate nitrogen contents were the key environmental factors affecting niche separation of AOB-amoA gene abundance in desert steppe soil. The results showed that nitrogen addition could significantly reduce the gene abundance of AOB-amoA, thereby affecting the nitrification and the direction of soil nitrogen transformation in the topsoil of desert steppe.

Selenium is an essential micronutrient element for humans. The range between “beneficial” and “harmful” levels of selenium is very narrow, so biofortification through plants is a safe and effective way to supplement selenium. This article reviewed the processes of selenium uptake, transport, and metabolism in plants. Plants mainly absorb selenate, selenite, and organic selenium from soil. The root system has different absorption mechanisms for different forms of selenium, and the absorption process is participated by different transporters. The absorbed selenium is mainly transported in plants in the form of selenate ion, transported to the aboveground through the xylem and the phloem, and metabolized under the action of various enzymes. Finally, part of the selenium absorbed by the roots is stored in the plant as organic selenium, and the other part is released into the atmosphere in the form of selenide. This article also focuses on the effects of different types of rhizosphere microorganisms on plant selenium biofortification. Arbuscular mycorrhizal fungi, ectomycorrhizal fungi, and rhizosphere promoting bacteria can promote the absorption of selenium in plants to a certain extent, but their internal mechanisms are still unclear. Based on the current research status, the future research focuses are put forward: 1) the process of selenium absorption by plants and its gene regulation; 2) the underlying mechanism and application potential of microorganisms on selenium biofortification in plants.

Aims Understanding the effect of tourism disturbances on soil microbial diversity and community structure is necessary for the restoration and management of environmental resources in tourist areas. Therefore, we conducted a field survey in Beijing Songshan National Nature Reserve, to reveal the effect of different tourism disturbance intensity on soil microorganisms in a Pinus tabuliformis forest.

Methods Three intensity groups, high disturbance (HD), low disturbance (LD) and no disturbance (ND), were conducted in the P. tabuliformis forest. We investigated microhabitat conditions and measured soil physicochemical properties. Next generation sequencing technique was used to determine the diversity and community structure of soil microorganisms. Then, we evaluated the impact of tourism disturbance intensity on soil microorganisms.

Important findings 1) HD significantly reduced soil fungal alpha diversity, and LD significantly reduced soil fungal phylogenetic diversity. Soil fungal diversity showed a decreasing trend, and soil bacterial diversity showed an increasing trend with increasing disturbance intensity. 2) For soil fungal community structure, the dominant phylum of three intensity groups were Basidiomycota and Ascomycota. HD disturbance significantly affected the relative abundance of Ascomycota, but had no effect on Basidiomycota, LD had no effect on both of them. LEfSe analysis showed that indicators of ND were Pseudogymnoascus and three species (Oidiodendron griseum, Acrodontium hydnicola, Metacordyceps chlamydosporia); indicator of HD was Clavariaceae; there was no indicator in LD. 3) For bacterial community structure, the dominant phylum of three intensity groups were Proteobacteria, Actinobacteria and Acidobacteria, but HD and LD had no effect on them. LEfSe showed that indicators of ND accounted for 82.05% of total indicators, and the most indicative ones were Gaiellales and Solirubrobacterales; indicators of HD accounted for 17.95% of total indicators, they mainly manifested as pathogenic indicator bacteria and bacterial groups related to human activities, the most indicative ones were Flavobacteriia and one genus of Verrucomicrobia; there was no indicator in LD. 4) Partial Least Squares Path Modeling (PLS-PM) found disturbance intensity significantly impacted microhabitat and alpha diversity of soil fungi. Redundancy analysis showed that soil and microhabitat condition explained 71.35% and 74.47% of variations in community structure of fungi and bacteria under different intensity group, respectively. Tree diameter at breast height, herb cover and litter cover were the main factors that altered fungal and bacterial community structure. In conclusion, tourism disturbance significantly reduced alpha diversity and impacted community structure of soil microbiota in the P. tabuliformis forest, and the degree of influence associated with disturbance intensity and the kind of microorganisms. Moreover, the impact was also controlled by microhabitat and soil physical and chemical properties. Therefore, future attentions should be paid to the restoration of microhabitats and soil conditions in tourism areas.

Litter often decomposes more rapidly in its native habitat (“home”) than in non-native habitats (“away”), a phenomenon called the “home-field advantage”. To explore the driving mechanism of home-field advantage of litter decomposition is important to predict the process of plant nutrient return and ecosystem carbon budget. This study reviewed the research progress on the home-field advantage of litter decomposition in recent years by discussing the quantification of home-field advantage, the controlling factors, and related driving mechanisms. There are four common metrics to describe home-field advantage in litter decomposition, and the use of linear model analysis to calculate home-field advantage is more appropriate. Litter quality (chemical composition, etc.) and soil microbial community structure are the main factors influencing the home-field advantage of litter decomposition, and soil fauna, climatic conditions, decomposition time, plant life form and growth form can also influence the intensity of the home-field advantage. Greater differences in litter quality usually generate stronger home-field advantage. Microbial taxa in the soil drive the home-field advantage of litter decomposition, but the role of soil microbes is often mediated by animal and climatic disturbances. In addition, the existence of phyllosphere microbes makes the home-field advantage of litter decomposition stronger. The litter chemical convergence hypothesis, decomposer control hypothesis and substrate quality-matrix quality interaction hypothesis are major hypotheses explaining the home-field advantage in litter decomposition, but they are not impeccable. We believe that the association between litter and soil microbial community is the driving force behind home-field advantage. The current researches on the factors and relative contribution of home-field advantage are not deep enough and usually focusing on a single ecosystem. Future investigations should explore deeper on the factors and their relative contributions of home-field advantage, and focus on more ecosystem types to improve the understandings of the mechanism of home-field advantage.

Aims Heavy metal chromium (Cr) contamination has toxic effects on crops and will disrupt soil microbial community homeostasis in agricultural soils. Nevertheless, the mechanisms underlying the responses of different crops and their rhizosphere soil microbial communities to Cr stress were different. By using time series data, this study analyzed the effects of Cr stress on ‘Jingu 21’ growth, functional pathways of differentially expressed genes (DEGs) in cereals and soil microbial community structure and function. Our objective was to elucidate the response mechanism of millet (Setaria italica) and soil microbial community, and provide a theoretical basis for the growth of cereals under Cr stress and the ecological restoration of the contaminated soil.

Methods We collected millet seedlings and soil samples from the pot experiments with planted millet before (CK) and 6-hour and 6-day after Cr stress (Cr_6h, Cr_6d), and determined the physiological traits of seedlings and soil physicochemical properties. The gene expression and the functional pathways enriched in the seedlings were investigated by transcriptome analysis; the dynamics of microbial community composition, structure, diversity as well as function in time series and their correlations with soil physicochemical properties were studied by high-throughput sequencing analysis.

Important findings 1) Transcriptome analysis showed that Cr stress induced up-regulation of gene expression (54% for up-regulated DEGs); GO enrichment analysis showed that DEGs significantly down-regulated the expression of photosynthesis-related genes in CK & Cr_6h, Cr_6h & Cr_6d samples, and also significantly up-regulated the expression of defense and damage regulation-related genes, and down-regulated the expression of cell wall and cell membrane and cell division-related genes in Cr_6h & Cr_6d samples. 2) High-throughput sequencing revealed a significant change in the composition of soil bacterial and fungal communities at the phylum and genus level during the time series of Cr stress. The α diversity of bacterial communities showed a phase change from stress to stability (Shannon-Wiener diversity index for CK, Cr_6h, Cr_6d were 6.09, 5.93, 6.05, respectively. Simpson diversity index for CK, Cr_6h, Cr_6d were 0.006 8, 0.007 8, 0.006 8, respectively; Chao diversity index for CK, Cr_6h, Cr_6d were 2 818.49, 2 630.73, 2 769.38, respectively), while the α diversity of fungal community decreased significantly (Shannon-Wiener diversity index for CK, Cr_6h, Cr_6d were 4.17, 3.81, 3.23, respectively). The distribution of β diversity of bacterial community in the Cr stress time series was significantly different from that of fungal community. 3) Correlation analysis between soil physicochemical properties and microbial communities showed that soil physicochemical factors were significantly correlated with a variety of fungal flora, but weakly correlated with bacterial flora. Cr stress significantly inhibited the photosynthesis of millet seedlings by reducing chlorophyll content, photosystem activity and affecting structural components such as thylakoid, and inhibited the proliferation and differentiation of leaf cells by down-regulating the expression of cell wall and microtubule-related components. At the same time, Cr stress also activated the plant defense system to overcome toxicity. Meanwhile, soil bacterial and fungal communities adapted to Cr stress through changing community composition and diversity, and their response levels and strategies differed in the stress time series.

Aims Globally, coastal salt marshes have been considered as major blue carbon sinks and contributors for climate change mitigation. Understanding the effects of soil moisture and salinity on soil CO₂and CH₄ emissions will advance better understand of long-term storage of soil carbon in coastal salt marshes.

Methods We conducted a simulation experiment with a gradient of water treatments (25%, 50%, 75% and 100% soil saturated water content) and salt treatments (9 g·kg^-1and 18 g·kg^-1). And we investigated soil carbon mineralization rates, soil properties, microbial biomass and community structure of typical salt marsh soils in the Yellow River Delta.

Important findings We found that: (1) There was no interaction between soil moisture and salinity content on soil CO₂, CH₄ emissions and CH₄:CO₂, and soil CO₂ emissions showed a unimodal curve along the soil moisture gradients and a significant decrease with increasing soil salinity content. The increased soil moisture significantly promoted soil CH₄ emissions, but the increased soil salinity content significantly inhibited soil CH₄emissions. (2) There was a weak significant interaction between moisture and salinity content on dissolved organic carbon (DOC). Under low water treatment, DOC content decreased with increasing soil salinity content, but increased under high water treatment. There was a significant positive relationship between soil CO₂ emissions and DOC content. (3) Soil microbial biomass exhibited a trend of first increasing and then decreasing with the increasing soil moisture, while soil salinity content significantly decreased microbial biomass. There was a significant positive correlation of microbial biomass with CO₂ and CH₄ emissions. (4) Both soil moisture and salinity treatments modified soil microbial community structure. Soil moisture and salinity treatments significantly increased and decreased the number of bacteria and α diversity index, respectively. Both soil CO₂ and CH₄ emissions were positively correlated with the number of bacteria and α diversity index. The climate is gradually drying and warming in this region due to climate change. Therefore, we speculated that changes in microbial biomass and community structure, soil moisture and salinity content may have potentially profound effects on the carbon-sink function at coastal salt marsh.

Aims Grassland is an important component of the terrestrial ecosystems in China, and plays a vital role in ecosystem productivity and functioning. During the past decades, 90% of natural grasslands have been degraded as a result of climate change and anthropogenic activities. Grassland degradation altered soil nutrient balance, exerting substantial impacts on ecosystem structure and functions. Our objective was to explore the responses of soil and microbial carbon (C), nitrogen (N) and phosphorus (P) stoichiometry to grassland degradation across the Qingzang Plateau alpine grasslands.

Methods We collected soil samples (0-10 cm) along the degradation sequence (i.e., non-degradation, moderate degradation and heavy degradation) from five sites across the “Three-River Source” region. By determination of soil and microbial C, N and P, we examined the changes in their contents and stoichiometric ratios with grassland degradation. We further synthesized data from the whole Qingzang Plateau alpine grasslands to validate the measured results using a meta-analytical approach.

Important findings Grassland degradation significantly reduced soil organic C, total N and total P contents and their stoichiometric ratios. Although microbial C and N content declined with degradation, change in microbial P content was limited along the degradation gradient. The microbial C:N:P ratios showed minimal responses to degradation. No obvious relationships were observed among soil and microbial C:N:P ratios. The above results indicate that soil microbes have the ability to maintain a given elemental composition despite variation in soil elemental composition following grassland degradation. From a long-term perspective, the nutrient-balance based soil quality promotion technology is able to effectively enhance grassland restoration and improve ecosystem service.

Aims The aim of this study was to explore the effects of native tree species on soil microbial biomass carbon (MBC) and nitrogen (MBN) content in subtropical Sichuan.

Methods In the present study, Cinnamomum japonicum, C. longepaniculatum, C. austrosiense, Alnus cremastogyne, C. camphora, Toona ciliata and T. sinensis in a common garden were selected as the research objects; whereas the abandoned land as the control. The effects of tree species on soil MBC content and MBN at different depths (0-10, 10-20, and 20-30 cm) were analyzed using the method of common garden.

Important findings (1) Tree species significantly affected the content of soil MBC and MBN, as well as their ratio. Compared with the abandoned land, tree species exhibited positive or neutral effects and the effects of tree species were particularly obvious in C. japonicum.For example, the contents of MBC and MBN in 0-10 cm soil layer were 108.2% and 139.6% higher than those in the abandoned land, respectively. (2) The content of soil MBC and MBN in the both tree and abandoned land generally decreased with an increase with soil depth; however, the characteristics of MBC:MBN varied with tree species. (3) The content of soil MBC and MBN varied with tree species and soil layer. The variations caused by tree species were stronger than that caused by soil layer. Compared with other species, C. japonicum was more conducive to the growth and reproduction of soil microorganisms.

Aims Phosphorus is one of the major limiting nutrients for plant growth in subtropical areas, whereas increasing nitrogen deposition may be a limiting factor in determining the availability of soil phosphorus. Here, focusing on soil microorganisms and plant fine roots, we explored the transformation of soil phosphorus to unravel the maintenance of soil phosphorus supply and plant productivity with low availability under nitrogen deposition.

Methods At the Fuzhou Changʼan Mountain in Fujian Province, China, control (0 kg·hm^-2·a^-1), low nitrogen (40 kg·hm^-2·a^-1), and high nitrogen (80 kg·hm^-2·a^-1) treatments were set up to simulate nitrogen addition. Soil and root samples of Cunninghamia lanceolata seedlings were then collected to comprehensively analyze soil phosphorus and nutrient contents as well as microbiological-plant root characteristics.

Important findings The results showed that the contents of soil labile organic phosphorus, moderately labile inorganic phosphorus and occluded phosphorus were significantly increased, whereas those of primary mineral phosphorus and moderately labile organic phosphorus decreased under the low nitrogen treatment as compared to the control treatment. However, there were no significant changes under the high nitrogen treatment. Redundancy analysis indicated that soil acid phosphatase activity, relative abundance of mycorrhizal fungi, soil microbial biomass phosphorus content, and root biomass were important soil microbiological-plant root characteristics factors that could explain the changes in soil phosphorus components. Variance partitioning analysis revealed that the soil microbiological-plant root characteristics synergy explained 57% of the alternations in soil phosphorus components, whereas correlation analysis showed a significant positive correlation between the relative abundance of mycorrhizal fungi and root biomass. Overall, these results suggest that mycorrhizal colonization is promoted under a low level of nitrogen input and the synergistic action of mycorrhizal fungi and C. lanceolata fine roots promotes the conversion of moderately labile organic and primary mineral phosphorus to labile phosphorus, thus maintaining the growth of C. lanceolata seedlings.

Aims Long-term nitrogen (N) deposition induces soil nutrient imbalance and profoundly affects nutrient cycling processes, ecological functions and the sustainable development of forest ecosystems. Although previous studies have found that N deposition increased phosphorus (P) limitation of forest trees in southwest mountainous areas, China, whether soil microorganisms showed synergistic response with plants remains unclear.

Methods In this study, we measured soil available nutrients, soil microbial biomass carbon (C), N, P and extracellular enzyme activities in a typical subalpine coniferous plantation (Pinus armandii) with chronic N addition treatments in southwest China. Furthermore, three models of ecoenzymatic stoichiometry, i.e., enzymatic ratio model, vector analysis model and threshold element ratio model were used to evaluate changes of microbial nutrient limitation under N addition.

Important findings The results showed that: 1) N addition significantly increased the P-acquiring enzyme activities by 52.5% and 53.2% in rhizosphere soil and bulk soil respectively, leading to a decrease of enzymatic N:P ratio by 7.8% and 4.8% compared to the control in rhizosphere soil and bulk soil respectively. 2) Vector model analysis found that vector angles of two soil compartments under N addition exceeded 45°, and the vector angles of rhizosphere soil and bulk soil were 52.2° and 49.0°, respectively. 3) The C:P threshold ratios (TER_C:P) of microbes in two soil compartments were significantly reduced by N addition. Consequently, the ratio of TER_C:P to available C:P (Av_C:P) was much less than 1, and the response of rhizosphere microbes was more significant. Collectively, all three models of ecoenzymatic stoichiometry indicated that N deposition aggravated P-limitation of microbial metabolism, and the extent of P limitation was more intense in the rhizosphere soil, which was closely related to nutrient contents and stoichiometric ratios of soil and microbes. The findings of this study provide an important scientific basis for adaptive management of forest ecosystems under global climate change.

Aims The formation of the geographical range boundary of species has always been an important topic in evolutionary biology. Although plant-microbe interactions have been extensively studied, we have a poor understanding of how plant's geographic range limits affect soil microorganisms. Cephalostachyum pingbianense is a rare bamboo species documented that produces bamboo shoots all year round in the wild, and is endemic to southeast Yunnan Province, China. The species is of great significance to study narrow endemic species in Bambusoideae. Here, we aim to reveal the relationship between the range limits of C. pingbianenseand soil fungal community.

Methods We assayed soil physical and chemical properties at the center, edge and beyond the range of C. pingbianense, and changes of fungal community were analyzed by means of Internal Transcribed Spacer (ITS) sequence based Illumina MiSeq high-throughput sequencing techniques.

Important findings (1) Soil pH and available phosphorus content at the range edges was significantly lower than other sites. (2) At the range center, species diversity of soil fungi was the highest, and relative abundance of Mortierella was significantly higher than other sites. At the range edges, species diversity of soil fungi was the lowest, and relative abundance of Basidiomycota was greater than 65.0%. (3) Soil pH played a crucial role in driving the variation of fungal community, which was negatively correlated with the relative abundance of ectomycorrhizal fungi, and positively correlated with the relative abundance of saprophytic fungi. Soil acidification and phosphorus deficiency may be important soil properties controlling the distribution range of C. pingbianense. Mortierella may be important mutualists of C. pingbianense, which can desorb phosphorus from soil minerals and reduce acidification of soil.

Aims Nitrogen (N) availability is an important limiting factor for grassland ecosystem productivity, and soil microorganisms are the main driving factor on soil N transformation. With the increase of atmospheric N deposition, the response of soil microbial characteristics to different nitrogen input levels is still unclear especially in saline-alkaline grassland.

Methods The experiment was conducted in Youyu Loess Plateau Grassland Ecosystem Research Station, Shanxi Province. Eight different nitrogen addition levels were set, which were 0, 1, 2, 4, 8, 16, 24 and 32 g·m^-2·a^-1, respectively. The Ammonia-oxidizing microorganisms (i.e. ammonia-oxidizing bacteria (AOB) and ammonia- oxidizing archaea (AOA)) abundance, soil bacterial and fungal abundance, as well as soil microbial biomass carbon (MBC) and nitrogen (MBN) content were measured in the growing season (May to September) in 2020 to explore the effects of different levels of N addition on soil microbial characteristics.

Important findings Our results showed that: (1) Sampling month had a significant effect on soil AOB, bacteria, fungal abundance and MBC, MBN content due to the variation in soil temperature and soil water content in the growing season. (2) N addition had a significant effect on soil AOB abundance, while had no effects on soil MBC, MBN content, and bacterial and fungal composition. (3) Higher N addition (24 and 32 g·m^-2·a^-1) significantly increased the abundance of ammonia-oxidizing bacteria (AOB) on the early growth stage (May to August), while having no effect on late growth period (September). (4) Soil microorganisms were mainly regulated by soil cations concentrations and soil pH values, which explained the variation of soil microorganisms by 21.8% and 17.2%, respectively. We found that soil microorganisms were not sensitive to N addition in saline-alkaline grassland, while AOB showed a significant increase under higher N addition, indicating that higher N addition might promote soil N transformation.

Interactions between plants and coexisting microorganisms have significant impacts on plant growth, development, and health. Human domestication has resulted in significant differences between modern crops and their wild ancestors in physiological and genetic characteristics and growth environment, which will inevitably affect the interaction between crops and their microbiomes. Understanding the impact of domestication on the diversity and community structure of microbiome and the mechanisms involved is an important theoretical basis for application of microbiome during crop improvement and breeding. In this review, we summarize the research progress of the effects of domestication on the community composition and diversity of root and shoot microbiome (bacteria and fungi) in crops. We also analyze the involved action pathways in shaping crop microbiomes by domestication, considering the domestication effect on crop morphology, root configuration, exudates and other physiological characteristics, and the change in growth environment. The research directions that need to be focused on in this field were proposed.

Aims Microbial biomass and their stoichiometric characteristics not only are important parameters of soil nutrient cycling, but also can contribute to prediction of climate changes, improvement of model accuracy, and understanding of terrestrial nutrient cycling. Our objective was to investigate microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and microbial biomass phosphorus (MBP) concentrations and their stoichiometric characteristics in alpine wetlands in the Three Rivers Sources Region.

Methods Using data from 50 sites, we explored MBC, MBN, MBP, their stoichiometry and their relationships with the controlling factors of alpine wetlands in the Three Rivers Source Region.

Important findings Our results showed that 1) MBC, MBN, MBP concentrations were 105.11, 3.79, 0.78 mmol·kg^-1, respectively, and MBC:MBN, MBC:MBP, MBN:MBP, MBC:MBN:MBP were 50.56, 184.89, 5.42, 275:5:1, respectively. 2) Soil physical and chemical properties could significantly affect MBC, MBN and MBP concentration. Soil moisture had significantly negative effects on both MBC:MBN and MBC:MBP, while soil density had positive effects on both MBC:MBN and MBC:MBP. Soil total nitrogen content had negative relationship with MBC:MBP, while having weak effects on MBC:MBN. Soil physical and chemical properties also had weak effects on MBN:MBP. 3) Generally, soil microbial community composition had significant effects on MBC, MBN and MBP concentration. Soil microbial community composition had similar effects on MBC:MBN and MBC:MBP. Total phospholipid fatty acid (PLFA) content, gram-positive bacteria, gram-negative bacteria, bacteria, actinomycete, arbuscular mycorrhizal fungi concentration, and other PLFA content had negative effects on MBC:MBN and MBC:MBP, while fungi:bacteria had positive effects on both MBC:MBN and MBC:MBP, but fungi had weak relationships with both MBC:MBN and MBC:MBP. Except for arbuscular mycorrhizal fungi, MBN:MBP had weak relationships with soil microbial community composition. Soil physical and chemical properties, and soil microbial community composition had significant effects on soil microbial biomass and their stoichiometric characteristics in Three Rivers Sources Regions in the alpine wetlands, which are greatly helpful for deeply understanding of terrestrial high altitude nutrient cycling.

Aims Variations in temperature and snow accumulations in winter will change the structure and function of the soil-microbial system. As a key biological factor in the terrestrial ecosystem, microorganisms play an important role in regulating soil nutrient cycles. However, they are very sensitive to environmental disturbances, especially to winter climate changes. It is in great need to study the response of soil nutrients and microbial properties of typical semi-arid grasslands to climate change in winter, in order to predict the ecological process and functional changes of grassland ecosystem in the long term.
Methods In the present study, the semi-arid grassland in the Yunwushan National Nature Reserve in Ningxia Province was taken as the research object. The four treatments including warming (W), snow reduction (S), interaction of warming and snow reduction (WS), and control (CK) were set to explore the responses of soil nutrients, enzyme activities and soil bacterial communities in the 0-5 cm soil layer of the typical grassland of the Loess Plateau to variations in winter temperature and snow cover.
Important findings Our results indicated that: (1) Warming, snow reduction and their interaction in winter increased the 0-5 cm soil temperature, lowered the relative humidity of the soil, but significantly increased the number of soil freeze-thaw cycles. (2) Compared with the control, other different treatments generally reduced the microbial biomass and bacterial diversity, which led to reduced activity of soil β-1,4-glucosidase (BG), β-1,4-N-acetylglucosaminidase (NAG) and alkaline phosphatase (AKP). The content of soil organic carbon, total nitrogen, available phosphorus, and nitrate nitrogen in the soil increased, while the content of nitrate nitrogen decreased. (3) The soil bacterial species in the study area were mainly Acidobacteria, Proteobacteria, Actinobacteria and Gemmatimonadetes. The dominant bacteria at the class level included Acidobacteria, γ-Proteobacteria, Thermophiles and σ-Proteobacteria. Redundancy analysis (RDA) results showed that available phosphorus (AP) content had the most significant impact on the bacterial community composition, with an explanation rate of 21.3% for the community variation. In conclusion, winter climate change can significantly affect soil temperature and humidity, especially the freezing and thawing cycles, which might further influence soil nutrients cycles, enzyme activities, and soil bacterial diversity. These results are of great significance for enriching and expanding the understanding of the process and mechanism of climate change on grassland ecosystem, as well as predicting the mid and long-term dynamic changes of typical grassland ecosystems.

Aims Soil microorganisms are an important component of terrestrial ecosystems and play a critical role in regulating multiple ecological processes such as nutrient acquisition, carbon cycle, and soil formation, especially in the tropical forests where soils are highly weathered with poor nutrients. The objective of this study was to examine the response of soil microbial community under long-term simulated acid rain (SAR) and investigate the most important factors influencing microbial community structure.
Methods Based on a long-term (10-year) field SAR experiment, we investigate the response of soil microbial community structure to soil acidification in the south subtropical monsoon evergreen broad-leaved forest of Dinghushan National Nature Reserve. Four levels of SAR treatments were set by adding the following amount of H⁺: 0 (CK), 9.6, 32 and 96 mol·hm^-2·a^-1.
Important findings 1) The SAR treatment significantly reduced the pH value of soil (i.e., increased soil acidification). 2) Soil acidification did not significantly influence microbial carbon (C) content, but changed microbial nitrogen (N) and phosphorus (P) contents, leading to significant increases in microbial C:P and N:P in topsoil (0-10 cm). This result indicated that soil acidification might aggravate microbial P limitation. 3) Soil acidification also altered the microbial community structure and significantly increased the fungal/bacterial ratio in the subsoil (10-20 cm). Further analysis showed that soil pH and available P content were the most important factors affecting the soil microbial communities under the SAR treatment.

Aims Soil enzymes, which are mainly produced by plant roots and soil microbes, involve in the organic matter degradation and element cycling and other key processes in plant-soil systems. Study on the relationships between soil enzyme activity and plant community composition and microbial activity under changing precipitation pattern and increasing nitrogen (N) deposition can provide a new insight for evaluating the influencing mechanism of global change on the biogeochemical cycling in plant-soil systems.
Methods Based on a field experiment involving five precipitation treatments (50% reduction, 30% reduction, natural precipitation, 30% increase, and 50% increase) and two N addition treatments (0 and 5 g·m^-2·a^-1) conducted in a desert steppe of Ningxia since 2017, the changes of soil enzyme activities (sucrase, urease, and phosphatase) were studied and their relationships with plant community composition and microbial ecological stoichiometry were analyzed in 2018 and 2019.
Important findings Compared with decreasing precipitation, increasing precipitation had greater impacts on the three enzyme activities, but its effects were interacted with N addition and sampling year. Increasing precipitation had no significant impacts on the three enzyme activities in 2018, but enhanced them in 2019. By contrast, N addition had less influences on the three enzyme activities, especially in 2019. The biomass of Astragalus melilotoides was negatively correlated with urease and phosphatase activities, while the biomass of Cleistogenes squarrosa had positive correlation with the three enzyme activities. Except the Patrick richness index, plant community diversity indices were generally negatively correlated with the three enzyme activities. Soil enzyme activities were more greatly affected by soil pH, soil total phosphorus (P), and microbial biomass carbon (C):N:P. Therefore, short-term precipitation change and N addition have little effects on the soil enzymes in the studied desert steppe (especially under reducing precipitation); increasing precipitation and N addition could pose direct influences on soil enzyme activities by increasing plant biomass, changing plant diversity, regulating microbial biomass ecological stoichiometry, and enhancing soil P availability. Given the diversity and functional complexity of soil enzymes, it is necessary to deeply analyze the influencing mechanism of global change on enzyme activities by measuring the long-term responses of various enzyme activities.

Aims How soil microbial diversity assembly, maintain and change is a key topic of ecology. A large number of studies show that soil microbial biodiversity is controlled not only by soil environment but also by plant species. However, due to strong covariation between the two factors in the field, it remains a challenge to isolate and clarify the role of plant diversity in regulating soil microbial biodiversity. Hence, here, we aim to clarify how plant diversity affects soil microbial diversity in environment-consistent artificial communities.
Methods In this study, we examined differences in species diversity of soil bacteria and fungi among plots subjected single- and mixed-sowing of three grass species with fertilization treatments after 13 years’ experiment on the eastern Qingzang (Tibetan) Plateau. We also analyzed the relationships between soil microbial diversity and edaphic factors as well as plant community attributes.
Important findings (1) The species richness and diversity of soil bacteria, not including soil fungi, significantly and consistently decreased in mixed-sowing plots relative to single-sowing plots, with higher relative abundances in proteobacteria and actinobacteria but lower in acidobacteria, bacteroidetes and planctomycetes in the mixed- sowing plots. (2) Soil pH and total nitrogen content significantly decreased while soil total phosphorus content increased in mixed-sowing plots relative to single-sowing plots. Fertilization significantly increased soil available phosphorus while decreased soil pH and soil humidity. However, variations in these edaphic factors contributed little in variation of soil microbial diversity. (3) Fertilization significantly increased plant aboveground biomass while decreasing richness of present plant species, which was also negatively associated with soil bacterial diversity. In short, this long-term field experiment clearly showed that mixed-sowing of common grass species did not promote diversity of soil microbes. This study provides new insight into management of grasses mixed-sowing artificial grasslands.

Aims Degraded peatlands recovery is an important environmental issue of current concern. Exploring the response of Zoigê degraded peatlands prokaryotic microbial community structure to water level recovery could provide foundation for the ecological restoration.
Methods For exploring the response of prokaryotic microbial community structure to water level recovery in the short-term, we selected Zoigê degraded peatland and designed two water level recovery (10 and 30 cm) with a control group (-10 cm) in situ test from year 2014 to 2015. We collected 0-15 cm soil samples and determined soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP) content and soil pH, we also analyzed microbial community structure by using 16S rRNA high throughput sequencing technique.
Important findings The results showed that water level recovery could improve the content of SOC, TN, TP and its stoichiometric ratio to a certain extent, however, there was no significant difference with the control group. The dominant microorganisms at the phylum level were Acidobacteria, Proteobacteria and Verrucomicrobia. Short-term water level recovery (10 and 30 cm) had no significant effect on the alpha diversity of prokaryotic microbial, but significantly reduced the relative abundance of Verrucomicrobia and Spartobacteria, while having an increase in methanogenic bacteria. Soil pH and water level were negatively correlated with the abundance of Verrucomicrobia and Spartobacteria. Prokaryotic microbial community structure is sensitive to soil C:P, N:P and SOC content. In a word, short-term water recovery hasn’t changed prokaryotic microbial alpha diversity, but increased the methanogenic bacteria, which will probably stimulate methane production pathways. Soil C:P, N:P and SOC content control the structure variation of prokaryotic microbial community in degraded peatlands during short-term water level restoration process. Our findings enrich the understanding of the structure of prokaryotic microbial community in response to short-term water level.

Aims Human disturbance is one of the main obstacles to the forward succession of karst grassland, exploring the response of grassland to disturbance in terms of soil microorganism can provide the basis for the restoration and rational utilization of karst land. Our objective was to study the effects of different human disturbances on soil microorganisms and the underlying mechanisms in a karst grassland ecosystem of northwestern Guangxi, China.
Methods Three patterns of disturbances (burning, mowing, and mowing plus root removal) and one control treatment (enclosure) were conducted at the long-term monitoring plots in the Huanjiang Observation and Research Station for Karst Ecosystems, Chinese Academy of Sciences. We analyzed the changes of soil microbial diversity and community structure by high-throughput sequencing, and determined their relationships with environmental factors (slope position, soil physicochemical properties).
Important findings 1) For α diversity, at both middle and lower slope positions, the burning treatment significantly reduced the fungal Chao1 index, while the mowing treatment significantly reduced the bacterial Shannon index and Pedigree diversity index. However, the mowing plus root removal treatment significantly reduced the fungal Chao1 index and the bacterial Shannon index, respectively, at middle and lower slope positions. 2) For microbial community structure, burning, mowing and mowing plus root removal treatments significantly reduced the relative abundance of Acidobacteria at both middle and lower slope positions, while the fire treatment significantly reduced the relative abundance of Ascomycota from 74.49% to 34.72% at the lower slope position. 3) Redundancy analysis showed that soil microbial biomass carbon explained 29.8% and 26.8% of the changes of bacterial and fungal α diversity, respectively, and 31.7% of the changes of bacterial community structure. Root biomass explained 13.9% and 10.3% of the changes of bacterial α diversity and fungal community structure, respectively. In conclusion, the three studied human disturbances have significantly negative influence on soil microbial α diversity as well as having a significant change in and changed community structure, and the degree of influence varied among the pattern of disturbances and the type of microorganisms. Moreover, the effects were also regulated by slope position. Long-term human disturbances mainly affected the diversity and structure of soil microbial communities by changing soil microbial biomass carbon and root biomass. The decreases of α diversity and Ascomycota will not be conducive to the maintenance of soil ecosystem stability, and the decrease of Acidobacteria will not facilitate to soil organic matter degradation and iron cycling. Therefore, the long-term human disturbances such as burning and mowing will induce the functional degradation of grassland ecosystem.

Aims The aim of the present study was to investigate the responses of understory vegetation to soil nutrients and bacterial communities.
Methods This study investigated the understory vegetation biomasses and species composition as well as soil physical and chemical properties in 17-year-old Chinese fir plantations with three densities (high-density (KH), medium-density (KM), and low-density (KL)) in Kaihua, Zhejiang. The changes of bacterial community structures were analyzed via 16S rDNA high-throughput sequencing techniques.
Important findings The result showed that the total above-ground biomass of the understory vegetation ranged from 0.10 to 2.10 t·hm^-2 and the dominant plant species varied in three Chinese fir plantations. The soil pH and available phosphorus content were significantly different between high-density and low-density forest stands. Correlation analysis showed that soil pH was positively correlated with the biomass of herbs, shrubs and the total of understory vegetation, while the content of soil organic matter was just positively related with the last two factors, and the available potassium content was only affected by the biomass of shrub. Based on the analysis of the soil microbial community, the Acidobacteria, Proteobacteria, Actinobacteria, and Chloroflexi were the dominant phyla in the three Chinese fir plantations. Redundancy analysis showed that soil pH, available nitrogen, available phosphorus and available potassium contents played a crucial role in regulating the soil bacterial community structures. Gp2, Gp1, Gp3 and Gp6 were the dominant subgroups of Acidobacteria, accounting for 51.32%- 57.38% of the Acidobacteria. With the decline of the Chinese fir density, the biomass of understory vegetation and the proportion of Gp1 increased, while the proportion of Gp2 and Gp6 decreased and the relative abundance of Gp6 was negatively correlated with soil pH. Obviously, the moderate reduction in stand density of pure Chinese fir forests was beneficial in the growth of understory vegetation and in maintaining a reasonable bacterial community structure, which helps to maintain the soil fertility of the Chinese fir forests and to achieve sustainable management in the long run.

Aims In recent years, under the background of climate change and human activities, the trend of biodiversity loss is increasing. Such accelerated loss in biodiversity could bring serious consequences to ecosystem functions. At present, the research on ecosystem function ignores the important driving role of carbon and nitrogen cycling in soil and microorganism on the above ground ecosystem functions. Any changes of soil carbon, nitrogen and microorganism may affect the ability of belowground community, which can have substantial effects on the aboveground ecosystem functions. Our aim was to explore the driving factors and key mechanism of abovegroud ecosystem functions (AEF) in alpine grassland.
Methods From July to August 2015, we conducted a transect survey in alpine grasslands to measure plant community and soil properties across Qingzang Plateau. There were in total 115 sample sites. The aboveground ecosystem function was calculated based on the aboveground biomass, leaf carbon, leaf nitrogen and leaf phosphorus. The effects of key elements such as soil organic carbon, total nitrogen and biomass on the aboveground ecosystem function were analyzed. Combined with mean annual precipitation and air temperature, we explored important drivers of AEF and related mechanisms.
Important findings Precipitation has a greater impact on aboveground ecosystem functions, while air temperature has a minor impact. Mean annual precipitation, soil microbial nitrogen content and aridity index had relative higher importance to aboveground ecosystem functions. Specificially, mean annual precipitation, soil microbial nitrogen content and aridity index accounted for the variations of 21.1%, 10.9% and 10.1%, respectively. The findings indicated that soil properties might play more important roles than plant community and productivity to aboveground ecosystem functions. Considering the cascading impacts of climate factors on soil nutrients cycling and microorganisms, soil microbial biomass nitrogen content plays an important role in regulating AEF of alpine grassland, Qingzang Plateau.

Aims As a key factor of nutrient cycling in ecosystems, soil enzyme activity is an important indicator of soil quality and ecosystem function. However, there have been very few studies on the differences of soil enzyme activities among different types of alpine grassland ecosystems. Thus, the aims of this study were to compare the differences of soil enzyme activities among five different types of alpine grassland and to reveal their influencing environmental factors on the Qingzang Plateau.
Methods Totally, 21 samples of five alpine grassland types, including alpine meadow, alpine steppe, alpine meadow steppe, alpine desert steppe and alpine desert on northern Qingzang Plateau, were selected for field in-situ investigation and sampling. The activities of 14 enzymes involved in the cycling of carbon (C), nitrogen (N) and phosphorus (P) were determined, and the relationships between enzymatic activities and environmental factors in alpine grassland were established.
Important findings The activities of C-acquisition (invertase, cellulase, β-1,4-glucosidse, polyphenol oxidase and peroxidase), P-acquisition (alkaline phosphatase) enzymes and two N-acquisition (arylamidase and nitrite reductase) were significantly different among different alpine grassland types. Moreover, correlations were found among C-acquisition, N-acquisition and P-acquisition enzymes. A significant positive correlation was found between invertase and alkaline phosphatase, and between cellulase and N-acetyl-α-D-glucosaminidase. A significant negative correlation was found between polyphenol oxidase and nitrite reductase, N-acetyl-β-D- glucosaminidase. Soil organic matter (SOM) content, gram-negative bacteria content, ratio of nitrogen to phosphorus, gram-positive bacteria content, bacteria content, actinomycetes content, total nitrogen content and fungi content were the key factors influencing soil enzyme activity among the 19 environmental indicators, and SOM content had the greatest impact (explained 11.9%). The results demonstrated that the activities of C-acquisition, P-acquisition and two N-acquisition (arylamidase and nitrite reductase) enzymes were significantly different among different types of alpine grassland, and soil enzyme activities were mainly controlled by SOM content, microbes and N elements in alpine grassland ecosystems.

Aims Soil microbial community composition and structure were regulated by temperature and plant species. Picea asperata and Abies faxoniana were planted in the monoculture and mixture plantations of the subalpine region in southwestern China. However, the effects of these two species and their interactions on soil microbial community under future climate warming remain unclear.
Methods An experiment was conducted to examine the effects of warming and plant species on soil microbial community composition with two levels of temperature (unwarming and warming with infrared heater) and four planting patterns (single A. faxoniana, single P. asperata, mixture of A. faxoniana and P. asperata, and unplanted bare land). Root zone soil of different planting treatments were sampled to estimating the microbial biomass and microbial community composition by the phospholipid fatty acids (PLFAs) content analysis.
Important findings The results indicated that: (1) Both P. asperata and A. faxoniana mono-planting significantly increased the biomass (PLFAs content) of main soil microbial groups and the whole community, regardless of warming, but the PLFAs content was only increased by mixed planting in unwarming plots. On the other hand, warming enhanced fungi (F) in unplanted plots and gram-negative bacteria (GN) in the P. asperata plots, respectively. However, warming significantly decreased soil microbial biomass in A. faxoniana and the mixed planting plots. (2) Principal component analysis (PCA) showed that effects of planting P. asperata and A. faxoniana on soil microbial community composition were greater under unwarming than under warming conditions. All the planting treatments significantly decreased the ratio of gram-positive/gram-negative bacteria (GP/GN) and increased the ratio of fungi/bacteria (F/B) in unwarming plots. However, significant effects on GP/GN and F/B ratios were only observed in A. faxoniana plots under warming condition. (3) PLFAs content was positively correlated with soil organic carbon, and F/B ratio was significantly correlated with soil pH and inorganic N. These results showed that the effects of warming on soil microbial biomass and composition varied among the tree species, and the effects of P. asperata and A. faxoniana were weakened under warming condition than under unwarming condition. Our results provide a vital theoretical basis for further study on the responses of soil microbial communities to vegetation and global climate change in southwestern China.

Due to huge consumption of fossil fuels and chemical fertilizers, substantial amount of anthropogenic reactive nitrogen (N) has been released into the environment. Therefore, N deposition has gradually increased worldwide and become one of the most important issues of global change. China has been a N deposition hotspot, and N deposition is projected to last long duration, which poses serious threats to ecosystem stability and functionality. In this synthesis paper, we summarized the impacts of N deposition on aboveground vegetation, soil microorganisms and biogeochemical cycling of major elements (carbon, N and phosphorus) in terrestrial ecosystems by outlining the progresses in the research field during the past 40 years. Results indicate that the accumulation of reactive N compounds induced by N deposition alters the soil environment, ecological stoichiometric balance and species co-occurrence patterns, thereby changing biodiversity and ecosystem functions. The rates, forms and duration of N deposition and the homeostasis of biosystem together with abiotic environments determine the direction and extent of the ecosystem response to N deposition. Through analysing local and foreign studies in this research area, we explore the weaknesses of relevant research that are being conducted in China. To advance the basic research on and risk management of N deposition, we propose the establishment of a N deposition monitoring and research network across the country with consideration of different ecosystems to promote regional and global risk assessments. Future research should highlight the combined multiple factors with N deposition and conduct direct and in-depth mechanism studies.

Aims The agro-pastoral ecotone is considered as fragile ecosystems which are strongly affected by agriculture and animal husbandry. The saline-alkali grassland is a unique grassland type in the agro-pastoral ecotone. A large amount of fertilizers are used to increase productivity in this area, which also promotes the emission of reactive nitrogen (N) gases and leads to the changes in soil carbon and N cycles. Mowing is a primary management practice in the agro-pastoral grassland in northern China. In order to explore the impact of N addition and mowing on carbon dynamic in this saline-alkali grassland located in the agro-pastoral ecotone, we determined the response of soil respiration to N addition and mowing.
Methods This study area is located in Youyu County, an agro-pastoral grassland ecosystem in northern China. The field experiment was set up in May, 2017. The treatments included: control (without mowing and mowing), addition of urea, addition of slow release urea, addition of urea + mowing, addition of slow release urea + mowing. Each treatment included 6 replicates. Therefore, there were totally 36 plots in this experiment. Soil respiration rate, soil temperature, soil moisture content, microbial biomass, inorganic N content, above-ground and below-ground biomass were measured under different treatments, and the cumulative carbon emissions and CO₂ fluxes were calculated.
Important findings Our results showed that: (1) Short-term (2017-2018) N addition significantly increased soil respiration rates and soil cumulative carbon emissions. Meanwhile, soil respiration rates and cumulative carbon emissions were significantly higher under urea treatment than those under slow release urea addition. (2) Mowing significantly reduced soil respiration rates and cumulative carbon emissions. (3) The interaction of short-term N addition and mowing had no significant effect on soil respiration rate. Therefore, short-term N addition can promote soil carbon release from the saline-alkali grassland in the agro-pastoral ecotone of northern China. Mowing can reduce soil respiration and decrease cumulative of carbon emissions. This may be because that mowing reduced the input of litter and further reduced soil substrate for microbes, which led to a decrease in soil microbial activity. However, long-term effect of N addition and mowing on soil carbon dynamics in saline-alkaline grasslands in the agro-pastoral ecotone still needs to be further explored.

Aims Altitude-induced changes in temperature, moisture, vegetation types and other conditions would significantly affect soil carbon (C_soil), nitrogen (N_soil), phosphorus (P_soil) concentrations and their stoichiometry. How soil microorganisms adapt to the variability of soil resource stoichiometry by regulating their biomass and extracellular enzymatic stoichiometry remains uncertain. The objective of this study was to quantify the altitudinal trends of soil-microbe-exoenzyme C:N:P stoichiometry and to explore the correlations among soil-microbe- exoenzyme stoichiometry.Methods In the present study, we investigated the C_soil, N_soil, P_soil concentrations, microbial biomass C (C_mic), N (N_mic), P (P_mic) concentrations, and the activities of C (β-1,4-glucosidase, BG), N (N-acetyl-β-glucosaminidase, NAG), and P (acid phosphatase) acquiring extracellular enzymes for microorganisms in four ecosystems along an altitudinal gradient on Mt. Datudingzi, Northeast China. These four ecosystems are a mixed broadleaf-coniferous forest at 800 m, a coniferous forest at 1 100 m, a Betula ermanii forest at 1 600 m and a grassland at 1 700 m.Important findings The results showed that: (1) altitude had no significant effect on C_soil and C_mic concentrations but had significant effects on soil and microbial biomass N and P concentrations. (2) The activities of BG and NAG decreased significantly with increasing altitude, likely due to the high elevation induced low temperature that inhibits microbial activities. (3) Altitude had significant effects on soil C:N, microbe C:N:P, and exoenzyme C:N:P; exoenzyme C:N:P decreased with the increasing stoichiometric imbalances between microorganisms and soils (ratios of soil C:N:P to microbe C:N:P, respectively). Overall, these results suggested that microorganisms can adapt to the variability of soil C:N:P by regulating their biomass C:N:P and exoenzyme C:N:P, and supported the microbial resource allocation theory.

Forests are large and persistent carbon (C) sink mainly through the C sequestration of tree growth, which can mitigate the rising rate of CO₂ concentration in the atmosphere. According to C availability in trees, two mechanisms involved in controlling tree growth are attributed to limitations to C input and C utilities. Since many environmental factors influence the activities of C-source and C-sink of trees interdependently, it is difficult to quantify how the sensitivity of C-source or C-sink activity to environmental changes affects tree growth. Therefore, it is of significance to understand physiological mechanisms underlying potential limitations to tree growth in order to predict tree growth and forest C sink under global change scenarios. In this review, the debates on the C-source and C-sink limitations to tree growth were firstly introduced. Second, we discussed responses of tree growth to biotic and abiotic stresses, such as defoliation, drought and low temperature from the perspective of C-source/sink limitations. Finally, we proposed three priorities for future studies in this field: (1) to explore the regulating mechanisms on the allocation of non-structural carbohydrates (NSC) in trees, and to determine what conditions and what extent trees actively allocate the photosynthates to NSC storage at the expense of growth; (2) to strengthen studies on the tree C-sink, and determine the photosynthates allocated to all components of tree C-sink, especially the missing C-sinks such as the activities of roots and related microorganisms; and (3) to implement studies on interactions among C metabolism, mineral nutrition and hydraulics physiology, and fully understand the C-water-nutrient coupling and its effects on tree growth.

Microbiome is the combination of all microorganisms and their genetic information in a specific environment or ecosystem, which contains abundant microbial resources. A comprehensive and systematic analysis of the structure and function of microbiome will provide new ideas in solving the core issues in the fields of energy, ecological environment, industrial and agricultural production and human health. However, the study of microbiome largely depends on the development of relevant technologies and methods. Before to the advent of high-throughput sequencing technology, microbial research was mainly based on techniques such as isolation, pure-culture and fingerprint. However, due to the technical restrictions, scientists could only get limited knowledge of microorganisms. Since the beginning of 21st century, the revolutionary advances in the technology of high-throughput sequencing and mass spectrometry have greatly improved our understanding on the structure and ecological functions of environmental microbiome. However, the application of microbiomics technology in microbial research still faces many challenges. In addition, the descriptive studies focusing on the structure and diversity of microbiome have already matured, and the study of microbiomics is facing a critical transition period from quantity to quality and from structure to function. Hence, this paper will firstly introduce the basic concepts of microbiomics and a brief development history. Secondly, this paper introduces the related technologies and methods of microbiomics with their development process, and further expounds the applications and main problems of microbiomics technologies and methods in ecological study. Finally, this paper expounds the frontier direction of the development of microbiomics technology and methods from the technical, theoretical and application levels, and proposes the priority development areas of microbiome research in the future.

Aims Long-time continuing cropping of peanut (Arachis hypogaea) would result in soil deterioration, which would seriously affect the productivity and the quality of peanut. Arbuscular mycorrhizal fungi (AMF) have been used as a fertilizer that may improve root microenvironment, increase nutrient uptake and stress resistance of the plants. This study investigated the effects of Funneliformis mosseae on peanut rhizosphere microenvironment under continuing peanut cropping.
Methods We conducted an experiment to examine soil properties, peanut productivity and quality between the treatments of: (1) peanut seedlings inoculated with F. mosseae in continuous cropping soil, and (2) peanut seedlings without the inoculation.
Important findings We observed that F. mosseae significantly enhanced the activity of sucrase, urease, alkaline phosphatase and nitrate reductase in soil, significantly increased the soil contents of total nitrogen, total phosphorus, total potassium, available phosphorus and available potassium. Meanwhile, the abundances of Aspergillus, Fusarium and Gibberella in the rhizosphere soil of continuous cropping were decreased, while the abundances of Gaiella was significantly increased comparing to the treatment without F. mosseae inoculation. In addition, F. mosseae significantly increased the peanut yield and quality, including protein, oleic acid and linoleic acid content. Our results suggested that F. mosseae improve the peanut rhizosphere environment, alleviate the obstacles of continuous cropping.

Recently developed in recent decades, the carbon isotope tracing technology is one of the most reliable methods, which has been widely used in the study of carbon (C) cycling in terrestrial ecosystems due to its high specificity and sensitivity. Here, the principle, analysis method and application process of C isotope tracing technology in C cycling in terrestrial ecosystem have been reviewed. Four different methods are currently being used in laboratory or field conditions, including natural abundance method, Free-Air Concentration Enrichment (FACE) technology coupling with ¹³C dilution method, pulse and continuous labeling with ¹³C enriched CO₂, and labeling with ¹³C enriched substrates. Results of field experiments and lab incubation experiments employing carbon isotope tracing technology were combined in order to quantify the transformation and distribution of photosynthetic C in plant-soil system. Furthermore, these techniques also help to understand the contribution of plant photosynthetic C to soil organic matter, the stabilization of soil organic matter and its microbial mechanism, to illustrate the dynamic changes of soil organic carbon (SOC), evaluate the contribution of new and old organic C to soil C storage, and estimate the micromechanism of SOC input, conversion and the stabilization in terrestrial ecosystems. Carbon cycle is affected by climate, vegetation, human activities and other factors, and therefore it is imperative to further develop a sensitive, accurate, multiscale and multidirectional isotope tracing system by combining carbon isotopes with mass spectrometry, spectroscopy and molecular biological technology. We have summarized the coupled application of carbon isotope tracing technology and the insitu detection involving molecular and biological approaches, and discussed the existing issues of carbon isotope tracing technology.

Biomarkers are biogenic organic compounds that carry the chemical structures specific to their biological sources and survive long-term preservation in environmental and geological systems. The abundance of biomarkers may indicate the relative contribution of specific biological sources to the natural organic matter while their chemical and isotopic compositions may also inform on the transformation stage of organic matter and the environmental settings. Compared with conventional bulk analysis, biomarkers offer highly specific and sensitive tools to track the sources, transformation and dynamic changes of natural organic matter components and have therefore been widely used in ecological and biogeochemical studies in the past decades. In particular, combined with ecosystem observations and control experiments, biomarkers have shown great potentials in revealing changes in microbial activity and carbon sources, soil organic matter dynamics, stabilization mechanisms and response to global changes. The recently-developed biomarker-specific isotope analysis also exhibits a great promise in revealing ecosystem carbon and nitrogen turnover and food web structures. This review summarizes several major categories of commonly used biomarkers, their analytical methods, applications in ecosystem studies and existing pitfalls, and discusses future directions of research to provide guidance for biomarker users in ecology and environmental sciences.