Nearly all tree species develop symbiotic relationships with either arbuscular mycorrhizal (AM) or ectomycorrhizal (EM) fungi to acquire nutrients from soils, and hence influence soil carbon (C) and nitrogen (N) cycles in terrestrial ecosystems. It is crucial to understand the differences in soil C and N cycles between AM and EM forests and the underlying mechanisms. In this review, we first compared the differences in the soil C and N cycles between AM and EM forests, and synthesized the underlying mechanisms from perspectives of the inputs, stabilization, and outputs of soil C and N in forest ecosystems. We also compared the responses of soil C and N cycles between AM and EM forests to global changes. In this field, one major research priority is comparing the structure and function (including the soil C and N cycles) between AM and EM forest ecosystems to provide theoretical basis and solid data for improving forest productivity and ecosystem services. The second research focus is deepening the understanding of the effects of interactions between aboveground litter and belowground mycorrhiza and free-living microbes on soil C and N cycles to reveal the potential underlying mechanisms in forests with different mycorrhizal symbioses. Third, the research methodology and new techniques need refining and applying to explicitly focus on scaling up the fine-scale measurements to better expound and predict the C and N cycles in forest ecosystems. Finally, more studies on the stability of soil organic matter among different mycorrhizal forests are needed to precisely assess responses of the structure and function of forest ecosystems to global changes.

Aims Soil respiration from terrestrial ecosystems is an important component of terrestrial carbon budgets. Compared to forests, natural or semi-natural shrublands are mostly distributed in nutrient-poor sites, and usually considered to be relatively vulnerable to environmental changes. Increased nitrogen (N) input to ecosystems may remarkably influence soil respiration in shrublands. So far the effects of N deposition on shrubland soil respiration are poorly understood. The aim of this study is to investigate the soil respiration of Vitex negundo var. heterophylla and Spiraea salicifolia shrublands and their response to N deposition.
Methods We carried out a N enrichment experiment in V. negundo var. heterophylla and S. salicifolia shrublands in Mt. Dongling, Beijing, with four N addition levels (N₀, control, 0; N₁, low N, 20 kg N·hm^-2·a^-1; N₂, medium N, 50 kg N·hm^-2·a^-1 and N₃, high N, 100 kg N·hm^-2·a^-1). Respiration was measured from 2012-2013 within all treatments.
Important findings Under natural conditions, annual total and heterotrophic respiration were 5.91 and 4.23, 5.76 and 3.53 t C·hm^-2·a^-1 for the V. negundo var. heterophylla and S. salicifolia shrublands, respectively and both were not affected by short-term N addition. In both shrubland types, soil respiration rate exhibited significant exponential relationships with soil temperature. Temperature sensitivity (Q₁₀) of total soil respiration in V. negundo var. heterophylla and S. salicifolia shrublands ranged from 1.44 to 1.58 and 1.43 to 1.98, and Q₁₀ of heterotrophic soil respiration ranged from 1.38 to 2.11 and 1.49 to 1.88, respectively. Short-term N addition decreased only autotrophic respiration rate during the growing season, but had no significant effects on total and heterotrophic soil respiration in V. negundo var. heterophylla shrubland. In contrast, N addition enhanced the heterotrophic soil respiration rate and did not influence autotrophic and total soil respiration in S. salicifolia shrubland.

Aims The optimal patterns of plant community for water use and nutrient utilization, the responses of soil carbon and nitrogen turnover processes to forest succession, and the mechanisms of soil organic carbon accumulation, are three critical issues in forest ecosystem study. It is difficult to accurately detect these ecological processes with conventional methodologies in the short term, yet the application of ¹³C and ¹⁵N natural abundance technique may yield important information about these processes.Methods This study was conducted in Dinghushan Biosphere Reserve. We investigated the natural isotopic abundance of both ¹³C and ¹⁵N of plant-soil continuum along a successional gradient from Pinus massoniana forest (PF) to coniferous and broad-leaved mixed forest (MF), and monsoon evergreen broad-leaved forest (BF). We also analyzed the correlations of foliar stable carbon isotope ratio (δ¹³C) and stable nitrogen isotope ratio (δ¹⁵N) with foliar elemental contents and the variations of soil δ¹³C and δ¹⁵N along soil profiles at different successional stages.Important findings A significant positive correlation between foliar δ¹³C and foliar C:N was observed. In both litter and soil, the δ¹³C values tended to decrease along the forest succession, with the order as PF > MF > BF. Foliar δ¹⁵N was positively correlated with foliar N content. The δ¹⁵N values of litter and upper soil (0-10 cm) increased with successional status. Both soil δ¹³C and δ¹⁵N values increased with increasing soil depth at all three forests. Our results imply that 1) trade-off between water use efficiency and nitrogen use efficiency did not necessarily exist in subtropical forests of China; 2) the application of isotopic technique could assist understanding of the mechanisms of soil carbon accumulation in subtropical forests, especially in old-grow forests; 3) the ¹⁵N natural abundance of plant-soil continuum could be a potential indicator of soil nitrogen availability and ecosystem nitrogen saturation status.

Aims Exotic plant invasion has been a global eco-environmental issue, which declines biodiversity and influences ecosystem structure and function. Over the past decades, more and more studies showed that influences of exotic plant invasion on soil nitrogen (N) cycles, and soil N availabilities can facilitate the success and aggravation of invading plants.
Methods Based on differences in soil N contents between invaded and uninvaded areas in natural ecosystems at the same study sites, this study explored magnitudes and ecophysiological mechanisms of soil N variations under exotic plant invasion.
Important findings Based on the data integrated from 107 papers, we found that contents of soil total N, ammonium-N, nitrate-N, inorganic N, microbial biomass N under exotic plant invasion were increased by (50 ± 14)%, (60 ± 24)%, (470 ± 115)%, (69 ± 25)%, (54 ± 20)% respectively relative to those under no invasion. The increment in the soil nitrate-N pool was highest, suggesting higher nitrification rate, which potentially promoted plant nitrate-N utilization and the coexistence of nitrate-preferring species. The increment of soil nitrate-N pool under invasion was higher in the temperate zone than the subtropical zones significantly. Invasion of N₂-fixing plants caused obviously larger increments of soil total N and nitrate-N contents compared to invasion of non-N₂-fixing plants. The invasion of woody and evergreen invasive plants caused larger increments of soil total N than herbaceous and deciduous plants, respectively. The increases in soil ammonium-N under invasion did not differ substantially among different life forms and showed no clear relationship with the percentage of N₂-fixing plants. Differently, soil nitrate-N showed much larger increments under invasion and showed positive linear relationship with the percentage of N₂-fixing invasive plants. The N₂-fixing function, litter quality and quantity of invasive plants are important factors regulating soil N mineralization and nitrification under invasion. This study provides novel insights into the mechanisms underlying the success and aggravation of plant invasion and into the relationships between soil N dynamics and plant functional traits in ecosystems under exotic plant invasion.

Climate warming has significantly alerted the terrestrial carbon dynamics, resulting in enhanced vegetation productivity, especially in the northern hemisphere. However, most of the prior modeling studies have neglected the effects of nutrient availability, such as the phosphorus limitation, on carbon processes, which potentially leads to an overestimation of the capacity of terrestrial ecosystems to sequester additional carbon. Here, we reviewed recent progress in phosphorus limitation and its interactions with carbon dynamics in the context of climate change, with a focus on the process-based modeling approach. We comparatively analyzed quantitative representations of phosphorus-associated biological processes in some models (i.e., Carnegie-Ames-Stanford Approach (CASA), Community Land Model (CLM), and Jena Scheme for Biosphere-Atmosphere Coupling in Hamburg (JSBACH)), such as photosynthesis and distribution of assimilates, phosphorus uptake by plants, the transformation of phosphorus pools in soil, phosphorus inputs and outputs, etc. We also discussed the key characteristics of these models and summarized the mathematical representations of the terrestrial phosphorus cycle. In addition, we identified and discussed the limitations, uncertainties and future needs in process-based modeling in terms of nutrient and carbon dynamics. Our study highlighted the importance of including phosphorus limitation in regional carbon estimation and provided deep insights related to biogeochemical modeling at broad scales.

Nitrogen (N) deposition has profound impacts on the phosphorus (P) cycling in forest ecosystems. Especially, the aggravated P limitation on tree growth under N addition has caused much attention to researchers. This article reviews the effects of N addition on plant P content in forest ecosystems. The result showed that N addition increased soil available P and facilitated the absorption of P by plants by promoting soil phosphatase activity, thereby increasing plant P content. Furthermore, changes in tree P content following N addition were also affected by species, life forms as well as experimental duration. Due to the inconsistency, the underlying mechanisms of changes in P content under N addition were further summarized as follows: 1) changes in soil available P content induced by exogenous N input affected the source of plant P; 2) N input affected the P uptake capacity of plants by affecting plant root exudates, mycorrhizal symbiosis and root morphological structure; 3) plant P utilization efficiency was also influenced with changes of P re-distribution and P re-absorption. Overall, for the changes in plant P under increasing exogenous N inputs, alterations of soil available P under N addition was the primary factor, while changes in plant P uptake capacity and P utilization efficiency ulteriorly regulated plant P content.

Aims Soil respiration is an important indicator for evaluation of ecosystem health in the grazing grasslands of arid regions, and thus can be used to assess dynamics of ecosystem functioning during the restoration of degraded grasslands from enduring intensive grazing.
Methods This study was carried out in a Nei Mongol desert grassland with four grazing intensity treatments, i.e., control, light, moderate, and heavy grazing intensity designated as CK, LG, MG, and HG, respectively. Our objectives of this study were to explore the responses of soil respiration in these treatments with additional nitrogen (N) and water (W) addition. The plant community was dominated by a grass species, Stipa breviflora.
Important findings Our results showed that: (1) previous grazing intensity had significant impacts on soil respiration in 2016 and 2017, but not in 2015. Grazing increased soil respiration. Moreover, both nitrogen and water addition significantly enhanced soil respiration in MG plots, while only combined addition of nitrogen and water significantly increased soil respiration in HG plots. (2) Neither grazing intensity nor addition of nitrogen and water changed the seasonal dynamics of growing season soil respiration in this desert grassland. Soil respiration showed a single-peak curve model, and the peak occurred in July with both rain and heat. (3) The effects of nitrogen and water addition varied in different growing seasons. Nitrogen addition had no significant effects in the first two years (2015 and 2016), while showed significant effects in the third year (2017). Water addition had significant effects in years with normal precipitation (2015 and 2017), while had insignificant effect in the year with high precipitation (2016). Combined addition of nitrogen and water showed stronger effects than only addition of water in CK, LG, and HG plots, indicating that the synergistic effects of nitrogen and water addition on soil respiration. (4) The sensitivity of soil respiration to soil temperature at 10 cm depth (i.e., the Q₁₀ value) ranged between 1.13 and 2.41, with an average value of 1.71. Without addition of nitrogen and water, Q₁₀ values in grazing plots were all lower than in CK plots, with the lowest value occurring in HG plots. With the addition of water and combined addition of water and nitrogen, the Q₁₀ value increased significantly by 100%. Taken together, our results indicated that soil moisture was the leading environmental factor affecting soil respiration in this desert grassland, while nitrogen played an effective role only after the minimum requirement of water availability was met. Results from this study will provide important helpful information for the restoration and rational utilization of the degraded desert steppe.

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.

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.

In the past several decades, the development of nitrogen (N) stable isotope techniques has improved the understanding of N cycling in terrestrial ecosystems. This review briefly introduced the history of N stable isotope techniques in studying N cycling in terrestrial ecosystems and summarized typical studies focusing on different aspects of ecosystem N cycling in recent years, including using 1) ¹⁵N natural abundance to identify plant N sources, indicate N status of ecosystems, and quantify N transformation rates; 2) ¹⁵N enriched tracers to study N fates, redistribution and gaseous loss from ecosystems. In the end, this review points out challenges and future applications of N stable isotope techniques on studying N cycling in terrestrial ecosystems.

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.

Exchanges of energy and matter between terrestrial biosphere and atmosphere and hydrosphere create critical feedbacks to Earth’s climates. To quantify how terrestrial ecosystems respond and feedback to global changes, terrestrial biosphere model (TBM) has been developed and applied in global change ecology during the past decades. In TBMs, myriad of biogeophysical, biogeochemical, hydrological cycles and dynamics processes on different spatial and temporal scales are represented. The TBMs have been applied on assessing and attributing past changes in terrestrial biosphere, and on predicting future changes and their feedbacks to climates. Here, we provide an overview of processes included in TBMs and TBMs applications on carbon and hydrological cycles, as well as their application on exploring human impacts on terrestrial ecosystems. Finally, we outline perspectives for future development and application of TBMs.

As an important component of terrestrial ecosystems, natural grasslands cover 30% of the global land. Thus, grasslands play a significant role in global carbon cycle, climate change, water retention, soil and water conservation, livestock production and so on. Grazing, as one common use of grasslands, brings fundamental impacts on plant individuals, populations, communities, biodiversity, soil quality and microbes, and then affects structural and functional processes of grassland ecosystems through different kinds of grazing livestock, grazing intensity, period, and system. We explored the effects of grazing on grassland ecosystem by using the methods of bibliometric analysis and literature review. To summarize the effects of grazing on grassland structure and functional processes, our study 1) reviewed the research stages on the impacts of grazing on grassland ecosystems since the 1950s; 2) extracted the hot topics, important research areas and keywords of previous research; 3) revealed the cutting-edge and limitations of domestic research on the effects of grazing on plants growth, community characteristics, carbon, nitrogen and nutrient cycling, productivity and soil quality; 4) proposed the future research directions and priority areas from the aspects of precise grazing management, validation of related hypothesis, and global change research. This study can provide scientific basis for grassland grazing ecology research, adaptive management and sustainable development in 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.

Terrestrial ecosystems are characterized by a series of spatiotemporally continuous, multiple scaled, and mutually connected processes. Since most of these ecological processes are regulated by temperature, climate warming will profoundly impact terrestrial ecosystems at global scale. Recently, how key processes in terrestrial ecosystems respond and/or adapt to climate warming has become a fundamental question in global change ecology. Here, we reviewed the recent research progress related to such question. This review focuses on key ecosystem processes, such as plant ecophysiological processes, phenology, community dynamics, productivity and carbon allocation, decomposition of litter and soil organic carbon, nutrient cycling, and carbon-nitrogen coupling. Based on a literature review, we propose perspectives for future research to tackle fundamental questions, such as the predictability of plant traits on ecosystem processes, coupling between biogeochemical cycles, mechanisms driving ecosystem responses to extreme climate and asymmetric warming, and ecological forecasting with models. We finally suggest more research efforts on warming adaptation rather than response on China’s specific ecosystems, and on the integration of experiments, observations, and models for coordinating studies across scales.

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 The purpose of this study was to understand the effects of the second generation wild boar grazing on species diversity and soil physicochemical properties of coniferous-broad-leaved mixed forest in the Jiajin Mountain, and to provide reference for the maintenance of ecological stability and scientific grazing of the mixed coniferous and broad-leaved forest in this area.

Methods On the basis of comprehensive investigation, according to the number, area, behavior characteristics and activity range of wild boar, four grazing disturbance intensity (from strong to weak was followed by I, II, III, IV) were divided, and no disturbance state was set as control (CK) to explore species diversity and soil physicochemical properties under different grazing disturbance intensity.

Important findings (1) 172 species of vascular plants, belonging to 55 families and 117 genera, were recorded, with the families, genera and species of trees, shrubs and herbs reaching the highest under slight disturbance intensity (IV). (2) The responses of tree, shrub and herb diversity index to the disturbance gradient were basically the same, but the richness index (S), Shannon diversity index (H') and Simpson dominance index (D) reached the maximum under the IV level disturbance intensity, which were higher than those in CK. The diversity level tended to decline with the increase of disturbance intensity (IV-I). In addition, for Pielou evenness index (E), there was no significant difference among different disturbance intensity levels. (3) Compared with CK, soil water content, maximum water content and total nitrogen content decreased under wild boar grazing. The greater the grazing pressure, the greater the proportion of decline. The content of soil total porosity, total phosphorus, available phosphorus and organic matter content increased in the IV level of interference, and decreased significantly under the I-III level interference; the soil density increased with the increase of disturbance intensity. (4) Redundancy analysis showed that soil organic matter content, available phosphorus content, soil density, soil water content, soil porosity, total phosphorus content, total nitrogen content and diversity index were significantly correlated. The light disturbance intensity is beneficial to the richness of community species diversity, the improvement of soil fertility and soil structure, which is a positive factor for the maintenance of ecological stability of the mixed coniferous and broad-leaved forest in the Jiajin Mountain. The study provides a reference for the forest ecological environment protection and sustainable development in this area under the background of increasingly expanding grazing and animal husbandry and increasingly frequent disturbance of human activities.

Aims Soil extracellular enzymes and enzyme stoichiometry are indicators of soil nutrient availability and microbial substrate limitation. Subalpine treeline ecotones are special areas which are sensitive to global change. However, the patterns in soil enzyme activities and stoichiometry, and their key drivers remain unclear in the subalpine treeline ecotones.

Methods In this study, soils from a subalpine treeline ecotone in Gongga Mountain in Southeast of Qingzang Plateau were collected. The activities of five hydrolases (β-1,4-glucosidase (BG), cellobiohydrolase (CBH), xylosidase (XYL), β-N-acetyl glucosaminidase (NAG), leucine aminopeptidase (LAP)) and two oxidases (polyphenol oxidase (POX), catalase (CAT)) were detected. The stoichiometric ratios of soil extracellular enzyme activities (carbon and nitrogen enzyme activity ratio and carbon quality index) were calculated.

Important findings Our results showed that LAP, POX and CAT activities of the shrub soils were significantly lower than those of the treeline and forest soils, XYL activity was the lowest at the treeline, and the activities of other extracellular enzymes did not differ significantly among locations in the treeline ecotone. The lnBG/lnLAP of the shrub soil was significantly higher than those of the forest and treeline soils, lnBG/ln(NAG + LAP) did not vary significantly at the treeline ecotone, and the carbon quality index was highest at the treeline. Soil extracellular enzyme activity stoichiometric ratios were not significantly related to microbial nutrient status. Non-metric multidimensional scaling analysis showed that total carbon, total nitrogen, nitrate nitrogen content and lignin to nitrogen ratio of plant leaves were the main factors influencing soil extracellular enzyme activities in the treeline ecotone. The main drivers of the stoichiometric ratios of extracellular enzyme activities were soil dissolved nitrogen, carbon to nitrogen ratio, and lignin to nitrogen ratio of plant leaves. In summary, some soil enzyme activities and their stoichiometric ratios varied significantly along the treeline ecotone, which was mainly influenced by the changes in vegetation type, possibly via its influences on plant-associated microbial communities. Treeline migration induced by future climate change may change extracellular enzyme activities and thus affect soil nutrient cycling.

Aims Responses of soil carbon (C), nitrogen (N), and phosphorus (P) contents and their stoichiometric ratios to climatic variables (mean annual precipitation (MAP) and mean annual air temperature (MAT)) along soil depths are important for understanding the effects of climate change on terrestrial ecosystem functions.
Methods To explore the responses of soil C, N, and P contents and their stoichiometric ratios along soil profile to MAP and MAT at a regional scale, we investigated these variables for four soil layers (0-20, 20-40, 40-60, and 60-80 cm) at 44 sites in grasslands on the Mongolia Plateau.
Important findings (1) Soil C and N contents decreased while soil P did not change with increasing soil depth. Soil C:P and N:P decreased while soil C:N was relatively stable with increasing soil depth. (2) Soil C, N, and P contents, as well as C:P and N:P, were positively correlated with MAP, but negatively correlated with MAT. Soil C:N was negatively correlated with MAP but did not correlate with MAT. The correlations between climate variables and soil C, N, and P contents and their stoichiometric ratios were weakened with increasing soil depth. (3) The effect of MAP or MAT on soil C, N, and P contents and their stoichiometric ratios were different among four soil depths. The total interpretation of the variations in soil C, N, and P contents and their stoichiometric ratios explained by MAP or MAT decreased with increasing soil depth. These results indicate that climatic variables had a top-down regulation on soil C, N, P contents and their stoichiometric ratios, and the effect of MAP was more important than that of MAT on soil C, N, P contents and their stoichiometric ratios in grasslands on the Mongolia Plateau.

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 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 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 contents of carbon (C), nitrogen (N) and phosphorus (P) in different plant organs and their ecological stoichiometric characteristics are important for understanding of the relationships among soil nutrients in their cycling process. The purpose of this study was to explore the variations of ecological stoichiometry of plant and soil C, N, and P in a jasmine (Jasminum sambac) plantation and their stoichiometric homeostasis under three different treatments.

Methods We set up three treatments: control, straw addition and biochar addition, and measured growth characteristic parameters of jasmine and C, N, P contents in different jasmine organs and in the soil and then analyzed their ecological stoichiometric characteristics.

Important findings Results showed that compared to the control, the straw addition treatment significantly increased the leaf biomass of jasmine by 73.33%, and decreased the soil salinity and soil temperature by 37.04% and 1.41%, respectively. Additionally, the biochar addition treatment significantly increased the plant height, leaf area, leaf and stem biomass of jasmine by 26.11%, 29.42%, 239.59% and 96.04%, while the soil density and soil temperature were significantly lower under the biochar addition treatment than under the control by 18.33% and 1.79%, respectively. Under different treatments, there was no significant difference in leaf or stem C content, or leaf N content. Root and soil C and N contents were significantly higher under biochar addition treatment than under straw addition and control treatments. The P contents of jasmine leaf, stem, root were in the order of biochar addition treatment > control treatment > straw addition treatment, while the soil P content was in the order of biochar addition treatment > straw addition treatment > control treatment. Compared with the control treatment, the biochar addition treatment decreased the C:P of leaf, stem, root and soil, and significantly decreased the N:P of jasmine leaf and stem, while increasing the N:P of root and soil. The overall internal stability of C, N, and P in different organs of jasmine was in the order of C > N > P, and the C:N, C:P and N:P were in the order of N:P > C:P > C:N. In summary, the application of biochar addition increased the absorption and assimilation of N and P by above-ground plants, and further promoted the C sequestration and homeostasis in the plant-soil system.

Aims In this study, we sought to determine changes in soil extracellular enzyme activities and their stoichiometric characteristics during the process of karst rocky desertification and their ecological response to environmental variation.

Methods Soil ecosystems at five stages of rocky desertification were selected for investigation and we applied the theory and methods of ecological stoichiometry to systematically study the effects of rocky desertification on the activities of six extracellular enzymes (β-1,4-glucosidase, cellobiohydrolase, β-1,4-xylosidase, β-1,4-N- acetylglucosaminidase, leucine aminopeptidase, and acid phosphatase). We also analyzed correlations between enzyme activities and environmental factors.

Important findings The results revealed that the extracellular activities of β-1,4-glucosidase, cellobiohydrolase, β-1,4-xylosidase, and leucine aminopeptidase, in the no, potential, and slight stages of rocky desertification were significantly higher than those in the moderate and severe stages. In contrast, the stoichiometric characteristics of soil extracellular enzymes showed no significant differences among the different stages of rocky desertification. The quality of soil at the different stages of desertification could be roughly divided into three categories, namely, the biochemical properties of non-rocky desertification soil were superior to those at the potential and slight stages, which in turn were superior to those at the moderate and severe stages. In addition, soils at the no, potential, and slight stages of rocky desertification were found to be phosphorus deficient (the enzyme vector angle was greater than 45°), whereas soils at the moderate and severe stages were deficient in nitrogen (the enzyme vector angle was less than 45°). Moreover, we established that during the process of rocky desertification, the changes in soil extracellular enzyme activities and their stoichiometric characteristics were mainly influenced by soil total nitrogen, available phosphorus, nitrate nitrogen, ammonium nitrogen, and litter phosphorus contents. The observed changes in soil extracellular enzyme activities were found to be correlated with changes in nutrient contents, and tended to be limited by litter phosphorus content. On the basis of these findings, as measures to alleviate the associated nutrient limitations, we would recommend supplementation with phosphorus in the potential and slight stages of rocky desertification and the supplementary application of nitrogen in the moderate and severe stages during the restoration of rocky desert ecosystems.

Aims Caragana korshinskii is the dominant species in the hilly area of northwest Shanxi, which plays very important roles in maintaining community species diversity, ecosystem stability and soil environment restoration.

Methods In order to investigate the stoichiometric characteristics of carbon (C), nitrogen (N), phosphorus (P) in the ecosystem of C. korshinskii artificial forest with different ages and their effects on leaf photosynthesis, the plant leaves and soil of C. korshinskii artificial forest with different ages (0, 6, 12, 18, 40, and 50 years) were collected. The variations of C, N, P stoichiometry in plant leaves and soil were analyzed.

Important findings With the increase of plantation time, the contents of C and N in the leaves of C. korshinskii increased significantly, while the contents of P increased firstly and then decreased. The contents of C, N and P in the leaves ranged from 434.14-452.26, 15.72-28.11 and 1.32-1.95 g·kg^-1, the contents of C and N in leaves reached the maximum value after 50 years of plantation, while the content of P in leaves reached the maximum value after 18 years of plantation. Leaf C:N increased first and then decreased, and reached the maximum in 18 years, while N:P increased significantly and reached the maximum in 50 years. The photosynthetic pigments (chlorophyll a, chlorophyll b, carotenoid and total chlorophyll) contents of leaves decreased significantly with increasing time of plantation, and leaf C and N contents had a significant effect on the changes of photosynthetic pigments contents. Soil water content increased first and then decreased with increasing time of plantation in the 0 -20 cm soil depth, and decreased significantly after 18 years of plantation. Soil organic carbon (SOC) and total nitrogen (STN) contents in the 0-20 cm soil depth profile increased with increasing time, and soil total phosphorus (STP) contents had no significant change with time. Soil C:N, C:P and N:P increased with the increase of forest age. Soil water content of C. korshinskii was significantly positively correlated with SOC and STN contents, and SOC and STN contents were also significantly positively correlated. The N and P contents of C. korshinskii leaves were significantly positively correlated. Leaf C and N contents were significantly positively correlated with SOC, STN and STP contents, and negatively correlated with soil water content. Leaf stoichiometric ratio was significantly positively correlated with corresponding soil stoichiometric ratio. The results of this study are of guiding significance to systematically understand the nutrient changes of artificial C. korshinskii forest ecosystem and to regulate and manage forest nutrients.

Aims Precipitation regime alteration and increasing nitrogen deposition have substantially altered the structure and function of grassland ecosystems. However, the responses of stoichiometry in soil and vegetation remain elusive, which limits the accuracy in predicting functional changes of alpine meadow.

Methods Based on a manipulation experiment platform of nitrogen addition (10 g·m^-2·a^-1) and precipitation change (precipitation reduction by 50% and increase by 50%) in an alpine meadow on the southern foot of Qilian Mountains, organic carbon (SOC), total nitrogen (SN), total phosphorus (SP) contents in topsoil (0-10 cm), and foliar carbon (LC), nitrogen (LN), phosphorus (LP) and potassium (LK) contents of dominant plant species, including Gentiana straminea, Elymus nutans, Oxytropis ochrocephalaand Kobresia humilis,were continuously surveyed from 2017 to 2020.

Important findings The soil stoichiometry varied significantly among different years, but was not affected by experimental treatments. The aboveground plant biomass showed inter-annual variations and was significantly affected by nitrogen addition. The responses of leaf stoichiometry were species-specific. Foliar stoichiometry of a resource-conservative species, E. nutans, showed limited variations, while that of the sensitive species, K. humilis, fluctuated significantly. To exclude the impacts of temporal variations, we conducted the analysis based on the relative changes (Δ) between treatment plots and the control plots from the same year and the results showed that nitrogen addition significantly increased ΔPB by 15.6%. Precipitation reduction significantly decreased ΔLC of O. ochrocephala by 6.8% while increased ΔLP of K. humilis by 19.8%. Our findings suggest that only nitrogen addition increased aboveground biomass and precipitation reduction altered LC and LP contents in some plant species. The temporal or species-specific effect, rather than experiment treatments effect, dominated the stoichiometric variations of soil and vegetation, highlighting the complex responses of alpine meadow to precipitation regime alteration and nitrogen addition.

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 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.