Plant ecological stoichiometry, as a branch of ecological stoichiometry, focuses on the study of elemental content, ratios and relationships within and across plant organs, and the underlying biotic and abiotic drivers. In the 19th century, chemists detected the elemental contents in plant organs via laboratory experiments, sprouting the exploration of plant stoichiometric characteristics. Nowadays, ecologists have explored plant ecological stoichiometric characteristics and their responses to global changes and relationships with plant functional traits, using both field investigation and manipulative experiments. These sustained efforts have largely enriched the knowledge and understanding of plant ecological stoichiometry. In this paper, we briefly introduced the history and reviewed the research progresses of plant stoichiometry since the 19th century. Firstly, we proposed the developmental history of plant ecological stoichiometry as three main periods: sprouting, hypothesis foundation, and theoretical construction periods, and introduced some representative works for each period. Secondly, we overviewed plant ecological stoichiometric characteristics across organs, life forms and environmental gradients. The geometric mean values of leaf nitrogen (N) and phosphorus (P) contents and N:P mass ratios in global terrestrial plants are 18.74 mg∙g^-1, 1.21 mg∙g^-1 and 15.55 (i.e. similar to the Redfield ratio of 16:1), respectively. Leaf N and P contents at either species or community level generally show a decreasing trend with increasing temperature and precipitation, and have large variations among life forms, with higher values in herbaceous than woody plants, and deciduous broad-leaved than evergreen broad-leaved and coniferous woody plants. Compared with leaves, the stoichiometric characteristics of fine roots and other organs in plants remain poorly documented. Thirdly, we reviewed the effects of nutrient addition on plant ecological stoichiometric characteristics. In general, N addition increases soil N availability, then the N content and N:P in plants, thus leading to an increase in plant productivity to some extents. P addition might alleviate the N and P imbalance induced by excessive N inputs, and then increase plant P content. However, long-term nutrient fertilization could perturb the intrinsic stoichiometric characteristics in plants, resulting in the deteriorated nutrient imbalance in tissues and then the subsequent decline in plant productivity. Fourthly, we introduced the main hypotheses of plant ecological stoichiometry. These hypotheses include function-associated hypotheses, environment-associated hypotheses and evolution-associated hypotheses, which delineate the relationships of stoichiometric characteristics with plant growth functions, environmental factors and plant evolutionary history, respectively. Finally, we made an outlook on future research in the area of plant ecological stoichiometry, and highlighted ten potential and important research themes.

Aims Biodiversity loss threatens ecosystem functions. Investigating the effect of biodiversity on the ecological stoichiometry of plant nutrients, therefore, can help reveal the mechanisms of the effect of biodiversity on ecosystem functions.

Methods Using a tree species diversity experiment in subtropical China, Cunninghamia lanceolatafrom plots with different tree species richness (1, 4, 8, 16, 32) were selected as focal tree species. The effects of neighborhood species richness (NSR), functional trait dissimilarities between neighborhood tree species and the focal tree, neighborhood competition index (NCI) on foliar nitrogen (N), phosphorus (P) content and N:P of C. lanceolata were investigated.

Important findings (1) The results showed that the dissimilarity in specific root length (SRL_diss) between neighborhood trees and focal trees significantly increased the foliar P content of C. lanceolata, while the dissimilarity in root tissue density (RTD_diss) significantly decreased the foliar N content of C. lanceolata. (2) Neighborhood competition significantly decreased the foliar N content and N:P of C. lanceolata. (3) The interaction effects of NCI and SRL_diss, as well as the interaction between NSR and SRL_diss significantly reduced the foliar P content of C. lanceolata. The result indicates that the positive effect of SRL_diss on the foliar P content of C. lanceolata decreased with increasing NSR, and the positive effect of SRL_diss on the foliar P content of C. lanceolata decreased with increasing NCI. (4) The interaction between NSR and phylogenetic dissimilarity (NP_diss) significantly increased foliar N:P of C. lanceolata, demonstrating that the negative effect of NP_diss on the foliar N:P content of C. lanceolata decreased with increasing NSR. Our results indicated that the foliar P content of C. lanceolata was significantly enhanced by mixing with tree species with different trait dissimilarities, while foliar N content of C. lanceolata was decreased by neighborhood competition. Tree species richness can help mitigate the adverse effects of interspecific competition on C. lanceolata through niche complementation when mixing with species that have greater trait dissimilarity.

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.

Ams Nitrogen (N), phosphorus (P), and potassium (K) are key elements for plant growth and development. Exploring the ecological stoichiometry characteristics of N, P and K in different phenological stages is of great significance for understanding the physio-ecological processes such as nutrient limitation, resource absorption and utilization, and biomass allocation of plants.
Methods Here, we collected root, stem, leaf and spike samples of Chenopodium quinoa in different phenological stages, and measured the concentrations of N, P and K. We compared the differences of N, P, K contents and their ratios among roots, stems, leaves and spikes and among phenological stages, and analyzed their relationships with the biomass allocations.
Important findings (1) The mean N contents was 9.28, 12.22, 33.68, 31.28 mg·g^-1 in the roots, stems, leaves and spikes, respectively. The breakdowns was 2.64, 3.71, 4.98, 5.68 mg·g^-1 for P contents, and 25.63, 43.80, 74.08, 56.73 mg·g^-1 for K contents, respectively. These resulted in mean N:P of 4.66, 4.20, 7.37, 5.70, N:K of 0.39, 0.31, 0.46, 0.62, and K:P of 13.77, 14.31, 16.82, 9.79 in the roots, stems, leaves and spikes, respectively. (2) The root, stem, and spike N, P and K and the leaf N and P contents decreased significantly with the phenological subsequences, reflecting the obvious dilution effect of biomass. On the contrary, the leaf K contents increased significantly with phenological subsequences, indicating an extremely strong drought resistance mechanism of C. quinoa under drought stress. The allocation ratios of N, P, K and biomass in the roots and stems kept stable, those in the leaves decreased, while those in the spikes increased with the phenological subsequences, indicating that the key resource allocation regulation of leaves and spikes occurred during the flowering stage. As the biomass increased in the filling stage, the nutrient elements gradually transferred to the spikes. (3) The variation source analysis revealed a greater contribution of organs to the variance of N, K contents and N:P, while a less one to the variance of P contents, than the phenological stages. (4) The allocation ratios of N, P, K and biomass were coupled among various organs. Specifically, the allocation ratios of root and leaf biomass showed a positive correlation with those of the root and leaf N, P and K, while a negative correlation with those of the spike N, P and K. The biomass allocation ratio of spike was positively correlated with spike N, P and K allocation ratios, while negatively correlated with root and leaf N, P and K allocation ratios. These results provided theoretical reference for further understanding of crop phenological character and guiding practical production in alpine regions.

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 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 The study of the spatial distribution pattern of soil nutrient stoichiometry along the altitude gradient is helpful to clarify the status of nutrient limitation in the mountainous ecosystems, to reveal the potential influencing factors to nutrient limitation, and to provide a scientific basis for regional ecological protection and vegetation restoration.
Methods The sampling plots were established along an altitude gradient from 3 100 m to 3 700 m in Pailugou watershed of the Qilian Mountains. Soil samples from 0-10 and 10-20 cm layers were collected separately to analyze the distribution of soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP) contents, and stoichiometric characteristics among different altitude and soil layers. Correlations between these nutrient characteristics and climatic factors, aboveground biomass, and soil pH were analyzed.
Important findings Results showed that: (1) Soil nutrients were concentrated on the soil surface and decreased with soil depth. The SOC and TN contents increased initially and then decreased as altitude increased. Soil TP content increased with increasing altitude. (2) Soil N:P increased from 3 100 m to 3 400 m, reached the maximum value at 3 400 m, and then decreased with increasing altitude and soil N:P was less than 14, suggesting an increase in N limitation along the altitude gradient. The C:P in soil was lower at high altitude than that at medium or low altitude, while C:N decreased with increasing altitude. (3) SOC content was positively correlated with TN content and negatively correlated with TP content. TN and TP content had no significant correlation. (4) Mean air temperature and total precipitation in the growing season showed positive correlation with SOC content and C:N:P, negative correlation with TP content, and no correlation with TN content. SOC and TN content had positive correlations with the aboveground biomass of shrublands. Soil pH was negatively correlated with TP content, but did not affect SOC and TN content significantly. Our results indicate that the productivity of alpine shrublands in the watershed was mainly limited by N and an appropriate amount of N application could alleviate the limitation.

Aims In order to explore the effects of different water and salt environments on the plant functional traits and their ecological stoichiometric characteristics at the lakeshore zone of Bosten Lake. The dominant plants and soil environmental factors in this area were selected to clarify the strategy of plant adaptation to the environments in this region.

Methods Eighteen sample plots were set up to investigate plant diversity. A total of 24 plant species including 8 shrubs and 16 herbaceous species were examined. The relationship between functional traits of plant leaves and soil environmental factors was tested using redundancy analysis method.

Important findings Our results showed that leaf functional traits varied considerably with the increasing water and salt content. Among plant traits, the chlorophyll content (SPAD), leaf thickness (LT) and specific leaf area (SLA) were the greatest in low water and salt environments, while the leaf water content (LWC), leaf dry matter content (LDMC) and leaf dry mass (LDM) were greater in medium and high water and salt environments. The content of carbon (C), nitrogen (N) and phosphorus (P) of plant leaves and their stoichiometric ratios were highly variable, with the C:N being 9.35-26.51 and the range of C:P being 50.13-228.95. The range of N:P was 2.31-11.99, with the largest variation in C:P. The Leaf C content was significantly and positively correlated with LT, LDMC and LDM, and leaf N content was significantly and positively correlated with SPAD and LT, while leaf P content was significantly and positively correlated with LWC. Whereas, C:N and C:P were both significantly and positively correlated with LDMC, while N:P was not correlated with any of the leaf functional traits. SLA was not correlated with any of the leaf ecological stoichiometric characteristics. The correlation between environmental factors and the functional traits of the dominant plant leaves revealed that the environmental factors affecting the functional traits of plants differed between species.

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 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 Myriophyllum aquaticum has high tolerance to high concentration of ammonium (NH₄⁺) and thus becomes the preferred species for swine wastewater treatment. It is of great significance to explore the effects of exogenous NH₄⁺ on the photosynthetic pigment composition and the stoichiometric characteristics of nitrogen (N) and phosphorus (P) of the M. aquaticum for improving the purifying efficiency of artificial wetlands system of M. aquaticum.

Methods Six NH₄⁺ concentration levels (0, 0.1, 1, 5, 15, 30 mmol·L^-1) were set up in this study. After 21 days of indoor cultivation, the contents of chlorophyll, N and P of M. aquaticum were measured to analyze their characteristics of changes.

Important findings The results showed that the relative stem height and biomass increased initially and then decreased, which were well fitted by a curve equation. Moreover, the peak of them appeared at 16.22 and 12.58 mmol·L^-1 exogenous NH₄⁺, respectively, via fitting non-linear Gaussian equation. With the increase of exogenous NH₄⁺ concentrations, the chlorophyll content in the leaves decreased significantly, but increased in the stems. In addition, there was a wide variation of chlorophyll a than chlorophyll b. The chlorophyll a/b did not change significantly among different NH₄⁺ treatments, except the value in the stems of 5 mmol·L^-1 NH₄⁺ treatment, which was significantly decreased. With the increase of exogenous NH₄⁺ concentrations, the N contents in the leaves and stems were significantly increased by 85%-235% and 127%-373%, respectively, and the P content in the leaves was increased by 49%-51% in comparison to the control (CK). When the concentrations of exogenous NH₄⁺ was no more than 15 mmol·L^-1, the N content and N:P ratio of the leaves and stems increased rapidly, so were relative stem height and relative biomass. Correlation analysis showed that the contents of N and P, and N:P ratio were negatively correlated with total chlorophyll content in the leaves, but positively correlated with total chlorophyll content in the stems. In conclusion, M. aquaticum grew well with larger biomass and higher absorption of N and P, when the concentrations of exogenous NH₄⁺ were in the range of 12-16 mmol·L^-1. Therefore, the constructed wetlands system planted with M. aquaticum can effectively remove N and P from polluted wastewater and achieve the purpose of efficient water purification.

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 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 Soil nitrogen (N) plays a vital role in regulating the structure and function of ecosystems and is affected by N deposition. Most previous studies focus on the responses of the N content in bulk soil to N deposition, but the responses of the N content in different soil organic matter (SOM) fractions remain unclear. We aimed to investigate how long-term N addition influenced soil N of different SOM fractions in a semi-arid grassland.

Methods A manipulated N addition experiment with 4 levels of N addition (0, 8, 32 and 64 g·m^-2·a^-1) has been conducted for 13 years in Duolun country, Nei Mongol. SOM was separated to particulate organic matter (POM) and mineral associated organic matter (MAOM) by density fractionation. The plant and soil properties were also measured.

Important findings The results showed that N addition had no significant effect on the carbon (C) content in bulk soil, POM, or MAOM. With increasing levels of N addition, the N content in bulk soil and in POM increased significantly. Furthermore, we found that the increased N content of POM was mainly associated with greater aboveground biomass following N addition. The N content of MAOM is mainly correlated with soil texture, but was not affected by N addition. These results suggest that continuous N addition can increase the soil N in bulk soil, but the increased N is mostly distributed in labile POM pools, which can be vulnerable to land use and climate change.

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.

Aims Non-structural carbohydrates (NSC) are available carbon in plants and can be utilized as an energy source during plant metabolism, so NSC are important components for plant growth and metabolic activities, particularly under environmental stress. Moreover, NSC in senescent leaves as litter-fall provide available carbon for soil microorganisms involving in soil organic matter formation and biogeochemical cycles in forests. Therefore, study on the variation in NSC between fresh and senescent leaves is of great significance for understanding the carbon metabolism during plant growth and carbon biogeochemical cycles during early decomposition of plant litter. The objective of this study was to determine the difference in NSC content between fresh and senescent leaves of 11 subtropical tree species and the variation between leaves with different plant functional types.

Methods A common garden was established in the stands with similar soil development, aspect, slope, and management history at the Sanming Research Station of Forest Ecosystem and Global Change in February 2012. A total of 13 representative subtropical tree species (2-year-old) were planted in the common garden, which were designed according to random blocks with 4 replicates for each tree species (a total of 52 plots with approximately 0.1 hm² for each plot). In this study, the fresh and senescent leaves of 11 tree species, including evergreen broadleaved species Schima superba, Lindera communis, Elaeocarpus decipiens, Michelia macclurei, Castanopsis carlesii and Cinnamomum camphora, deciduous broadleaved tree species Liriodendron chinense, Liquidambar formosana and Sapindus mukorossi, and coniferous tree species Cunninghamia lanceolata and Pinus massoniana were collected in August 2019. The contents of NSCs, including soluble sugars and starch, in fresh and senescent leaves of the 11 tree species were determined.

Important findings The NSC content was significantly higher in fresh leaves than that in senescent leaves for all of the studied tree species. The NSC contents in fresh leaves were 68.7-126.3 mg∙g^-1, while those in senescent leaves were 31.4-79.5 mg∙g^-1. Notably, the variation in soluble sugar between fresh leaves and senescent leaves was much greater than that of starch. Specifically, the average content of soluble sugar in fresh leaves was 3.3 times greater than that of senescent leaves, and the average starch content in fresh leaves was 1.2 times greater than that of senescent leaves. Moreover, the NSC contents in both fresh and senescent leaves varied significantly among trees with different plant functional types. For example, the NSC contents in both fresh and senescent leaves of evergreen and deciduous broad-leaved trees showed similar levels, while the NSC contents in evergreen coniferous trees were significantly lower than those in broad-leaved trees. In fresh leaves, the average NSC contents in evergreen and deciduous broad-leaved trees were 99.7 and 96.8 mg∙g^-1, respectively, while the average NSC content in evergreen coniferous trees was 75.4 mg∙g^-1; In senescent leaves, the average NSC contents in evergreen and deciduous broad-leaved trees were 47.2 and 50.7 mg∙g^-1, respectively, while the average NSC content in evergreen coniferous trees was 33.3 mg∙g^-1. These results suggest that NSC, an important carbon metabolic component for trees, could be transferred from senescent leaves to fresh leaves before senescence; this is a significant strategy for carbon storage during plant growth. However, the NSC content was significantly lower in subtropical coniferous trees (such as Cunninghamia lanceolata and Pinus massoniana) than in broad-leaved trees, regardless of fresh and senescent leaves, suggesting that the initial substrate quality is lower in these coniferous litters with less labile components following forest plantation in subtropical China. This difference in NSC content in foliar litter has significant influence for litter decomposition and soil organic matter formation mediated by microbial metabolism and turnover. These results are of great significance for improving the theory of carbon metabolism during plant growth and for understanding the dynamic changes of carbon components during the early decomposition of leaves litter in subtropical forests.

Aims Plants of the same life-form may utilize different forms of nitrogen to avoid or reduce the competition for resources, thus achieving co-existence. Studying on whether niche separation exists in nitrogen uptake by plants of the same life-form in desert ecosystems is helpful to understand the survival strategy of desert plants and the effect of nitrogen on the survival of desert plants.

Methods Two annual plants, Ceratocarpus arenarius and Suaeda glauca, are widely distributed in Gurbantünggüt Desert. ¹⁵N isotope tracer method was used to study the nitrogen uptake strategies of two desert annuals in different months and from different soil layers.

Important findings The results showed that the nitrogen absorption rates of the two plants in shallow soil were higher in July than those in June. Comparing the absorption rates of different nitrogen forms, plants preferred inorganic nitrogen to organic nitrogen. Ceratocarpus arenarius preferred nitrate nitrogen, and the highest nitrogen absorption rate was 3.81 μg·h^-1 per gram dry root, while S. glauca preferred ammonium nitrogen, and the highest nitrogen absorption rate was 4.74 μg·h^-1 per gram dry root. The contribution rates of nitrate nitrogen out of total nitrogen uptake ranged from 35.7% to 43.9% for C. arenarius; while the contribution rate of ammonium nitrogen out of total nitrogen uptake ranged from 40.0% to 48.3% for S. glauca. The two annual plants can not only utilize inorganic nitrogen, but also directly absorb organic nitrogen in soil. The findings showed that the nitrogen uptake capacity of annual plants in Gurbantünggüt desert was different and diversified, and all of them could absorb the soluble organic nitrogen sources in the soil.

Aims The content of carbon (C), nitrogen (N) and phosphorus (P) in different plant organs and their stoichiometric characteristics can reflect the nutrient allocation and balance within the plant. In this paper, the response of C, N and P in different organs of tea plants to three management modes was investigated by field experiment. The purpose was to explore the variation characteristics of C, N and P contents and their stoichiometric ratios in roots, stems and leaves of tea plant and its allometric growth relationship under different management modes.

Methods We set up three different management modes in Tieguanyin tea plantations in Anxi, Fujian: routine management mode (M1), intercropping mode (M2) and modern technology mode (M3). In this paper, we investigated C, N, and P contents in the roots, stems, and leaves and their stoichiometric characteristics, nutrient variations and the allometric relationships of tea plants under different management modes.

Important findings The results showed that the N and P contents in roots, stems and leaves of tea plants under M2 and M3 management mode were significantly higher than those under M1 management mode, but no significant differences were observed for the C contents; the order of C:N, C:P and N:P ratios in roots, stems and leaves of tea plants was M1 > M2 > M3. The contents of C, N, and P varied significantly among different organs of tea plants. According to the analysis of variation sources, the management mode factors showed significant impacts on the content variation of all the three elements. The allometric relationships of N and P in roots, stems and leaves (N-P^1.7456, p< 0.01; N-P^1.0987, p< 0.01; N-P^1.1993, p< 0.01) suggested that the nutrient requirements of different organs were similar. Soil pH and bulk density were important factors affecting C:N, C:P and N:P, while soil water content and salinity had great impacts on C content in roots and leaves of tea plants. In general, intercropping, as well as modern drip irrigation and fertilizer management technology, can improve the nutrient absorption efficiency of tea plants, and have positive effects on solving the problem of soil nutrient imbalance.

Aims Nutrient resorption in different organs of plants is important for plant nutrient use strategies and elemental biogeochemical cycles. Previous researches on nutrient resorption have focused on leaves, but neglected culms. Additionally, leaves of different species have been collected at the same time during the peak growth stage in previous studies, ignoring the different peak times of nutrient content of different species, which leads to the underestimation of nutrient resorption efficiency.

Methods In order to explore the seasonal variation of nutrients and nutrient resorption efficiency in the culms and leaves of herbaceous plants, 22 common plants in Hulun Buir grassland were chosen as the research objects to determine the temporal dynamics of nutrient content during the growing season, and the resorption efficiency of nitrogen (N) and phosphorus (P) in plant culms and leaves.

Important findings The content of N, P in plants had obvious temporal dynamics during the growing season, showing an increasing trend first and then a decreasing trend. For most of the 22 species, the maximum content occurred in the middle and late August, but the peak time differed among different species. The resorption efficiency of N in leaves was higher than in culms, but that of P did not differ between leaves and culms. Nutrient resorption efficiency of plants was closely related to the nutrient content at the senescence stage, but not to that at the growth stage. In previous studies, different plants were sampled at the same time during the growth stage, which led to underestimation of N and P resorption efficiency of culms and leaves. This study re-examined the sampling strategy in nutrient resorption studies, and showed that the sampling time of mature tissues in the growth stage could be determined according to the peak nutrient content time of different species.

Aims Plants absorb mineral nutrients such as nitrogen (N) mainly through their roots. The nutrient uptake of plants with different root morphologies differs. Many studies have shown that arbuscular mycorrhizal fungi (AMF) can help their symbiotic associates absorb mineral N. However, there is little research on whether the effect of AMF on nutrient uptake of plant roots is affected by root morphology.

Methods In this study, we selected three rice mutants and one wild type (root hairless (rhl1), lateral rootless (iaa11), adventitious rootless (arl1) and wild type (Kas)) to investigate the role of root morphology in plant nutrient uptake. Subsequently, we used the ¹⁵N isotope labeling method to explore the effects of arbuscular mycorrhizal fungi and N addition (low N: 20 mg·kg^-1 NH₄⁺-N; high N: 100 mg·kg^-1 NH₄⁺-N) on N uptake of rice mutants with different root morphologies.

Important findings The results show that the leaf ¹⁵N concentrations of rhl1,Kas, iaa11 and arl1 were increased by 60%, 72%, 128% and 118%, respectively, under the high N compared to the low N treatment. This result indicates that the addition of N significantly promoted rice N uptake with the most evident effect occurring in iaa11 and arl1. The average effect sizes of AMF on rhl1, Kas, iaa11 and arl1 were 17%, 31%, 42% and 51% under the low N level, indicating that root morphology can alter the effect of AMF on plant N uptake. Compared to the low N treatment, high N significantly downregulated the AMF effect on N uptake by rice plants with different root morphologies, indicating that N addition may mediate the complementary effect of AMF and root morphology on plant nutrient uptake. In conclusion, our data provide direct experimental evidence of funcitonal complementarity of mycrrohzal fungi and their associated roots with different root morphogy.

Changes in soil nutrient availability and primary succession of vegetation often co-occur during the processes of natural soil development. A low availability of nitrogen (N) and phosphorus (P) resources is common in the very early and late stage of soil development, respectively. Plants have evolved different nutrient-acquisition strategies (NASs) in response to low nutrient availability. Although the changes and responses of plant NASs to soil nutrients may affect primary succession and species diversity, the temporal trends and underlying mechanisms of plant NASs with soil development remain unknown. We reviewed 104 studies mainly carried out on soil chronosequences to clarify changes in plant NASs with soil age and its ecological significance. We classify plant NASs into Fine root, Microbial, Specialized root, Carnivorous and Parasitic strategies. We argue that the diversity of plant NASs changes with increasing soil age following a dumbbell-pattern, while reaching the maximum in the late stage of soil development. The role of Microbial and Fine root strategies in plants acquiring nutrients gradually decreases with increasing soil age, while the minimum and maximum role of Specialized root strategies in plants acquiring P is in the intermediate and late stages of soil development, respectively. The effects of NASs on interspecific relationships of plants vary with soil age. Specifically, pioneer plants with biological N fixation and specialized root strategies usually increase available soil N and regolith-derived nutrients to facilitate the colonization of subsequent plants in the early stage of soil development. During the early-intermediate stage, NASs mainly affect plant competitiveness in acquiring relatively abundantly available nutrients from soil. The facilitation and competition affected by NASs contribute to plant species turnover in the first two stages. In the late stage, diverse NASs enable plants to acquire distinct forms of nutrients from different soil spaces and complementary NASs enable plants to take up soil nutrients mobilized by their neighbors. Together with the interactions between NASs and soil pathogens, these processes contribute the coexistence and diversity of plant species in this stage when most soil nutrients have a very low availability. We propose that it is necessary to quantify the relationships between changes in soil nutrient availability (including concentrations and fractions) and plant NASs with soil age. More studies are also needed to quantify contributions of NASs to primary succession, diversity of plant species and soil development.

Aims Exploring the seasonal dynamics in leaf carbon (C), nitrogen (N) and phosphorus (P) concentrations and their ecological stoichiometric characteristics will enhance our understanding about physiological and ecological processes such as plant growth and development and nutrient uptake and utilization as well as dynamic equilibrium relationship among plant stoichiometry.
Methods Here, we collected leaf samples of 18 dominant plant species semimonthly through growing season (i.e. from June 2nd to Sept. 2nd) from a long-term fenced site in a typical steppe in Xilinhot of Nei Mongol, China. Leaf C, N and P concentrations were measured. Seasonal changes in leaf C, N and P concentrations and their ratios were explored and their differences between different species groups were analyzed using one-way ANOVA. The relationships between leaf C, N and P concentrations and their ratios were analyzed using correlation analysis. Lastly, the allometric relationships between the concentrations of different elements were analyzed using Standardized Major Axis.
Important findings Seasonal trends in leaf C, N and P concentrations and their ratios were not consistent with each other and also differed between different functional groups. Specifically, the variation of leaf N and P concentrations for all functional groups showed obvious dilution effect. Monocotyledons and perennial grasses had lower leaf N and P concentrations but much higher leaf C:N and C:P mass ratio than dicotyledons and perennial forbs, respectively. Leaf N concentration was positively correlated with leaf P concentration while leaf C:N and C:P mass ratios were negatively correlated with leaf N and P concentrations respectively, indicating the internal coupling mechanism between nutrient elements in plants. Allometric analyses showed that leaf N concentration and C:N mass ratio, leaf P concentration and C:P mass ratio as well as leaf N and P concentrations all maintained the same growth rate respectively among species through most time of growing season.

Aims The nutrient characteristics of nitrogen (N) and phosphorus (P) in plant leaves are affected by soil available N, P contents and soil N:P. However, little is known about the effects of changes in soil N, P contents and N:P on N, P stoichiometry in leaves and nutrient resorption efficiency.
Methods In this study, pot experiments were conducted to explore the response of the stoichiometry and nutrient resorption characteristics of Achnatherum splendens leaves to three levels of nutrient addition (low, 1.5 g·m^-2·a^-1; moderate, 4.5 g·m^-2·a^-1; and high, 13.5 g·m^-2·a^-1) and N:P (5, 15, 25).
Important findings The results showed that higher level of nutrient addition significantly increased the P contents in green leaves and N, P contents in senescent leaves, but significantly deceased the N resorption efficiency (NRE) and P resorption efficiency (PRE). The increases in soil N:P significantly decreased the P contents in senescent leaves and NRE, but increased N:P in green and senescent leaves and PRE. At the same nutrient addition level, soil N:P was significantly positively correlated with PRE, but showed no significant correlation with NRE. At the same level of N:P, the level of nutrition addition was negatively correlated with NRE, but exhibited no significant correlation with PRE. The changes in plant demand for N and P caused by changing environment can be indicated effectively by plant leaf NRE:PRE, and the characteristics of nutrient stoichiometry and resorption of A. splendens leaves are affected together by nutrient addition levels and soil N:P.

Aims The objectives of this study were to estimate the adaptation strategies of Broussonetia papyrifera to the poor soil nutrients in karst rocky desertification area in Southwest China, and to explore the response of stoichiometric characteristics of fine roots and rhizosphere soil to the degree of rocky desertification.
Methods The contents of carbon (C), nitrogen (N), phosphorus (P), calcium (Ca), magnesium (Mg) and C:N:P ratios of fine roots and rhizosphere soil of B. papyrifera were studied.
Important findings Results indicated that, except Ca content, the nutrient content of the fine roots and rhizosphere soil of B. papyrifera in karst rocky desertification environment remained at the low level. N:P ratio of fine roots was 12.59, which indicated that the growth of B. papyrifera was co-limited by N and P. With the increase of the degree of rocky desertification, the content of C and N as well as C:N and C:P ratios of fine roots decreased first and then increased; while the content of K and P increased first and then decreased; and neither the content of Ca, Mg nor N:P ratio changed significantly. The content of N, P, K, Ca in rhizosphere soil was different when the degree of rocky desertification was different, while none of C, Mg and C:N:P ratios in rhizosphere soil changed significantly. Additionally, the content of C, P, Ca, Mg as well as C:N and C:P ratios in the fine roots were positively correlated with their values in rhizosphere soil, while N content in fine roots was negatively correlated with its value in the rhizosphere soil. Moreover, the content of K in fine roots was relatively stable, and was hardly affected by the nutrients of rhizosphere soil.

Aims The dynamics and driving factors of soil enzyme activities and stoichiometry in the micro-scale elevation gradient is of great significance in the study of nutrient cycling processes.
Methods In the present study, the Quercus aliena var. acuteserrata forest belts at the elevation of 1 308, 1 403, 1 503, 1 603, 1 694 and 1 803 m in Taibai Mountain were sampled to determine the contents of carbon (C), nitrogen (N), and phosphorus (P) in leaves, litters, roots and soils, and the activities of alkaline phosphatase (AKP), β-1,4-glucosidase (βG), cellobiohydrolase (CBH), β-1,4-xylosidase (βX) and β-1,4-N-acetylglucosaminidase (NAG).
Important findings Our results showed that altitude had a great impact on the activities of five soil enzymes. CBH and βG increased first and then decreased with the altitude, while βX showed the opposite trend. The NAG and AKP activity showed a downward trend from 1 408 to 1 694 m and increased with elevation since 1 803 m. The total enzyme activity index exhibited a decreasing trend with altitudes increases. The correlation analysis results indicated that soil enzyme activities and their stoichiometry were controlled by plant, soil C, N, P resources, and soil water and heat conditions. Among these factors, the content of soil organic carbon had high correlation with these parameters and was the main factor affecting the change of soil enzyme activities in the Quercus aliena var. acuteserrata forest. In short, the soil enzyme activities and stoichiometry were different along the micro-scale elevation gradient, affected by the C, N, and P resources of plant and soil.

Aims Drought is a limiting factor for plant growth in southern karst areas. Climate change may affect the amount and distribution pattern of precipitation in these areas. It is important to understand the stoichiometric characteristics of soil and plants and how they respond to increasing precipitation in karst areas.
Methods In Jianshui karst areas in southern Yunnan, a water addition experiment was conducted since April 2017 and the concentrations of carbon (C), hydrogen (H), nitrogen (N), phosphorus (P), sulfur (S), potassium (K), calcium (Ca), magnesium (Mg), aluminum (Al), sodium (Na), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu) in the soils and leaves of two dominant shrubs (Bauhinia brachycarpa and Carissa spinarum) were measured in the dry season (April) and rainy season (August) in 2018.
Important findings Water addition affected the content of C, N and Na in the soil. Compared with the dry season, the concentrations of Na and S in the soil significantly decreased in the rainy season. The remaining soil elements did not show any significant differences between treatments and seasons. With the increase of soil moisture content, the concentrations of K decreased while Ca in both plant species increased. These results also indicated that soil moisture changes could significantly affect plant ecological traits. With soil moisture changes, the stabilities of leaf elements were related to their contents. The closer the leaf element contents are to the corresponding maximum or minimum values, the smaller are the coefficients of variation. And the variation coefficients of P, S and Mg with the concentrations close to 1 mg·g^-1were the highest. Under the changes of soil moisture conditions, the stability of C, N, P and other major elements in C. spinarum was significantly higher than that B. brachycarpa. Changes in soil water content, which was caused by both rainfall changes and water addition, had different effects on different the contents of different elements in both soil and plants. These results may shed light on the restoration of soil and plants in karst regions.

Aims The leaf stoichiometry and potential driving factors play a vital role in understanding the distribution patterns of plant community and predicting the plant responses to environmental changes. In this study, we aimed to investigate the spatial distribution patterns and driving factors of leaf carbon (C), nitrogen (N) and phosphorus (P) stoichiometry of coniferous species on the eastern Qinghai-Xizang Plateau, China.
Methods We collected leaf and soil samples from 29 coniferous tree species at 84 sampling sites on the eastern Qinghai-Xizang Plateau. Linear fitting was used to analyze the variation patterns of leaf stoichiometry along geographical and climatic gradients. Partial redundancy analysis was used to characterize the relative contributions of climate and soil factors to leaf stoichiometry variation patterns.
Important findings (1) At the level of family and genus, C and N concentrations as well as C:N of leaves were significantly different across distinct conifer species. The leaf N:P was less than 14, indicating that conifer species in the study region were mainly N-limited. (2) Leaf N and P concentrations showed a consistent distribution pattern along environmental gradients. Specifically, N and P concentrations of leaves were significantly decreased with elevated latitude and altitude, while remarkably increased with the increase of mean annual temperature (MAT) and mean annual precipitation (MAP). In comparison, leaf C concentration had no significant correlation with latitude, altitude, MAT or MAP. (3) The leaf C:N and C:P showed an opposite distribution pattern with leaf N and P concentrations, which significantly increased with elevated latitude and altitude, while markedly declined with the increase of MAT and MAP. Leaf N:P had no significant correlation with altitude, MAT or MAP. (4) The main driving factors of leaf C, N, P concentrations and their stoichiometric characteristics were different. Specifically, soil properties were the main driving factors accounting for the variations of leaf C concentration and N:P. The variations of leaf N and P concentrations as well as ratios of C:N and C:P were primarily explained by climatic factors. Collectively, variations of leaf stoichiometry of coniferous species along environmental gradients in the study region provided a compelling support for the Temperature Biogeochemistry Hypothesis. These findings largely improved the understanding of the distribution patterns and driving mechanism of leaf stoichiometry under changing environments.

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.

Aims The aims of this study were to explore how vegetation restoration affects leaf, litter and soil C, N, P stoichiometry dynamics and nutrients cycling, and to characterize the homeostasis and nutrient use strategy of plants at different vegetation restoration stages in the mid-subtropical area of China.
Methods Four vegetation types representing the successional sequence in the secondary forests were selected using the “space for time substitution” approach in central hilly area of Hunan Province, China, which consists of Loropetalum chinense + Vaccinium bracteatum + Rhododendron simsii scrub-grass-land (LVR), Loropetalum chinense + Cunninghamia lanceolata + Quercus fabri shrubbery (LCQ), Pinus massoniana + Lithocarpus glaber + Loropetalum chinense coniferous-broad leaved mixed forest (PLL), and Lithocarpus glaber + Cleyera japonica + Cyclobalanopsis glauca evergreen broad-leaved forest (LCC). Permanent plots were established in each community. The organic carbon (C), total nitrogen (N) and total phosphorus (P) contents in leaf, undecomposed litter layer and 0-30 cm soil layer were quantified at each stage. The response and nutrient use strategy of plant to environmental changes were estimated by allometric growth, nutrient use efficiency and nutrient reabsorption efficiency.
Important findings 1) Along vegetation restoration, the leaf C:N, C:P ratios decreased significantly and the highest values were in LVR. Leaf C, N, P contents, soil C, N contents and soil C:N, C:P, N:P ratios increased significantly, in which leaf C, N contents and soil C, N contents, N:P in LCC were higher than those in LVR, LCQ and PLL, and leaf P content and soil C:N, C:P in PLL were higher than those in LVR, LCQ and LCC. Leaf N:P (>20) indicated that all restoration stages were P limited. C, N, P contents and their stoichiometry of litter fluctuated greatly. 2) The relationships between litter and leaf or soil nutrients and their stoichiometry were weak, and the significant correlations were found in the relationships between leaf and soil nutrients and their stoichiometry. Leaf C, N and P were positively correlated with soil C, N, C:N (except leaf C, N contents), C:P and N:P, while leaf C:N was negatively correlated with soil C, N, C:P and N:P, leaf C:P was negatively correlated with soil C content, C:N and C:P, and leaf N:P were negatively correlated with soil C:N. 3) During vegetation restoration, leaf N and P had significantly allometric growth relationship (p < 0.01) with the allometric index being 1.45. The use efficiency of N and P nutrients in leaf showed decreasing trends and reabsorption efficiency showed increasing trends, and the lowest N use efficiency was observed in LCC and the lowest P use efficiency was in PLL, but the highest N, P reabsorption efficiency were both in PLL. 4) The leaf N content had weak homeostasis, and leaf P content had strong homeostasis to maintain P balance in plant under P limited in soil. Vegetation restoration had significant effects on leaf, litter and soil C, N, P contents and their stoichiometry. The C, N, P contents and their stoichiometry had significant correlations between leaf and soil. Plants could adapt to the shortage of soil nutrient supply mainly by reducing nutrient use efficiency and improving nutrient reabsorption capacity. The N and P cycles of the leaf-litter-soil system gradually reached the “stoichiometric equilibrium” during vegetation restoration.

In order to understand the research progress of litter decomposition and its underlying mechanisms, this paper presented a bibliometric analysis of litter decomposition in China from 1986 to 2018 based on the four common literature databases, including CNKI, ISI Web of Science, ScienceDirect and Springer Link. Litter decomposition researches are mainly from forest ecosystems (65%), and focus on above-ground litter. This suggests that the studies on below-ground litter decomposition should be strengthened in the future. About 68% studies focused on the litters from dominant species, which couldn’t represent the natural decomposition characteristics due to the mixed effects among litters from multiple species. Besides carbon, nitrogen and phosphorus, we should pay more attention to other key chemical components related with decomposition (e.g. K, Fe, Mn, lignin, tannin, etc.) and the heavy metal elements related with environmental pollution. Meanwhile, ecological stoichiometry is an effective method to interlink the biogeochemical cycle in the plant-litter-soil system. Nitrogen deposition and climate change are hot topics in the field of litter decomposition, especially the interactions of multiple factors (e.g. nitrogen, phosphorus, etc.), temperature sensitivity of litter decomposition and underlying mechanisms in permafrost under climate warming context.

Aims Soil microorganisms in forest ecosystems play vital roles in regulating above- and belowground ecosystem processes and functions such as soil nutrient cycling, litter decomposition, net ecosystem productivity, and ecosystem succession. We aim to investigate broad-scale seasonal patterns of soil microbial biomass carbon (C), nitrogen (N) and phosphorus (P) stoichiometry. Methods By synthesizing 164 samples of soil microbial biomass C, N and P content derived from the published literature, we investigated global seasonal patterns of soil microbial C, N, P content and their ratios across three vegetation types of global forests. Important findings Soil microbial biomass C, N and P content in temperate and subtropical forests were lower in summer and higher in winter. Soil microbial biomass C, N and P content in tropical forests were lower than those in temperate and subtropical forests in four seasons. Soil microbial biomass C and N content in tropical forests were relatively the lowest in autumn, and soil microbial biomass P content was relatively constant in all seasons. The soil microbial biomass C:N of temperate forest was significantly higher than that of other two forest types in spring, and that of tropical forest was significantly higher than that of other two forest types in autumn. Soil microbial biomass N:P and C:P in temperate forests remained relatively constant in four seasons, while those in tropical forests were higher than those in other three seasons in summer. The soil microbial biomass C content, N content, N:P and C:P of broad-leaved trees were significantly higher than those of conifers in four seasons, while the soil microbial biomass P content of conifers was significantly higher than that of broad-leaved trees in four seasons. There was no significant difference in soil microbial biomass C:N between broad-leaved and coniferous trees in both spring and winter, but the soil microbial biomass C:N of coniferous trees was significantly higher than that of broad-leaved trees in summer and autumn. For the change of soil microbial biomass, season is not but forest type is the main significant factor, suggesting that the seasonal fluctuation of soil microbial biomass changes with the inherent periodic change of trees. Asynchronous nutrient uptake by plants and soil microorganisms is a trade-off mechanism between nutrient retention and ecological function maintenance.

Aims Ecological stoichiometry focuses on the balance of chemical elements in ecological processes, in which the stoichiometric ratios of carbon (C), nitrogen (N) and phosphorus (P) are important features of ecological functions. The objectives of this study were to determine the stoichiometric characteristics in different organs and components of mixed evergreen and deciduous broadleaved forests, and to examine the discrepancy in stoichiometric ratios among different components of the ecosystem and plant organs. Methods We measured the concentrations of C, N and P in different plant organs, litter and soil in a mixed evergreen and deciduous broadleaved forest in Shennongjia of Hubei Province, China, and computed the stoichiometric ratios using the biomass-weighted mean method. Important findings The C concentration, C:N and C:P of different components were ranked in the order of plant community > litter > soil, and concentrations of N and P and N:P in the order of litter > plant community > soil. There were little differences in C concentration among various organs, with the coefficient of variation (CV) much lower and less variable than that for N and P concentrations. Both N and P concentrations were highest in leaves with the lowest CV value; N:P was highest in the bark, but with the lowest CV value in branches. Additionally, there were considerable differences in N and P concentrations in leaves between evergreen and deciduous species. Compared with other forest types, this forest had lower C:P and N:P ratios in plant community, higher C:P and N:P ratios in litter, and the C, N and P stoichiometric ratios in soils were consistent with, and the C:N ratio in ecosystem was lower than, that in subtropical evergreen broadleaved forests. Our findings demonstrated the patterns of differences among components in stoichiometry using the integral biomass-weighted mean method differ from that using the arithmetic mean method in selective organs. Furthermore, the distribution and homeostasis of C, N and P concentrations and their stoichiometric ratios could be closely related to the physiology of different organs.

AimsAs a dominant understory layer, shrubs is important in the material turnover and nutrient circulation of forested ecosystems. It is essential to explore stoichiometric characteristics of carbon (C), nitrogen (N) and phosphorus (P) of the shrubs and their driving factors, including microenvironments and soil nutrients.
MethodsThe leaves, branches, roots of the shrubs and the soils they rooted were sampled from seven dominant forest types of Qinghai, China, and the tissue contents of C, N and P were examined. One-way ANOVA was used to explored the difference of the shrubs and the soils among the forest types using, respectively. Redundancy analysis (RDA) was used to analyze the effects of soils and environmental factors on the stoichiometric characteristics of C, N and P of shrubs.
Important findings Our results showed that there were no significant differences in the P content and C:P of the leaves, branches and roots among all the seven dominant forest types, while the N content and N:P of shrubs in the Populus davidiana, Sabina chinensis and Picea asperata forests were significantly higher than those in Betula platyphylla, Populus tomentosa, Betula albosinensis and Picea wilsonii forests, while the C:N ratios were the other way around. The shrubs in Sabina chinensis forest were limited by the soil P content, but that in the other six forest types was limited by the soil N content. The contents of soil organic C (SOC) and soil total N (TN) were significantly different among the seven forest types, while the soil total P (TP) was not. Correlation analysis showed that the N content, the C:N and N:P of understory shrub tissues (leaves, branches and roots) were significantly correlated with soil TN content, soil C:N and N:P, while tissue P contents and the C:P ratios were correlated with soil TP contents. Redundancy analysis (RDA) showed that the stoichiometric characteristics of C, N and P the understory shrub layer were synthetically affected by soils and environmental factors, of which the soil C:N, altitude, mean annual temperature and mean annual precipitation were the main influence factors.

Aims The objective was to explore the stoichiometry of rhizosphere soil enzymes under major understory vegetation and their responses to plantation types and seasons.

Methods Rhizosphere soils of understory shrubs (Loropetalum chinense, Adinandra millettii and Eurya muricata) and herbs (Woodwardia japonica and Dryopteris atrata) were sampled in the early growth stage (April) and the vigorous growth stage (July) in Cunninghamia lanceolata, Pinus massoniana and Pinus elliottii plantations at Qianyanzhou Ecological Research Station, Taihe, Jiangxi. Potential activities of β-1,4-glucosidase (BG, carbon (C) acquiring enzyme), β-1,4-N-acetylglucosaminidase (NAG, nitrogen (N) acquiring enzyme) and leucine aminopeptidase (LAP, N-acquiring enzyme), acid phosphatase (AP, phosphorus (P) acquiring enzyme) and their stoichiometric ratios were measured. Soil physical and chemical properties were also analyzed.

Important findings The results found that (1) rhizosphere soil extracellular enzyme activities associated with C and N acquisition and BG:AP (enzyme C:P) were significantly different among understory species, but P acquisition were not. Both forest stand types and sampling seasons influenced BG:(NAG+LAP) (enzyme C:N). Interactions of understory species, forest stand types and seasons observably affected enzyme C:P. Principal component analysis showed that rhizosphere soil enzyme activities and ecoenzymatic stoichiometry differed significantly among different understory species (Loropetalum chinense was obviously different from Eurya muricata, and both of them were evidently different from other understory species), forest stand types (Cunninghamia lanceolata was different from Pinus massoniana and Pinus elliottii plantations) and sampling seasons. Soil NO₃ ^--N, NH₄ ⁺-N, DOC content and C:N were the main edaphic abiotic factors influencing the rhizosphere soil enzyme activities and ecoenzymatic stoichiometry. (2) Standardized major axis analysis showed that there were significantly linear relationship among lg(BG), lg(NAG+LAP) and lg(AP) of rhizosphere soils of understory species. lgBG:lg(NAG+LAP):lgAP(enzyme C:N:P) was approximately 1:1:1.3. Rhizosphere soil enzyme C:P and (NAG+LAP):AP (enzyme N:P) of understory species were 0.14 and 0.15, respectively. The regression slopes of lg(BG), lg(NAG+LAP) and lg(AP) deviated significantly from 1 because AP activities were much higher than BG activities and NAG+LAP activities. This study found that rhizosphere soil enzyme activities and ecoenzymatic stoichiometry were affected by understory species, forest stand types and sampling seasons in which substrate availability played an important role. Compared with C- and N-acquiring enzymes, microorganisms allocated more resources to the production of P-acquiring enzymes, which implied that the growth and activity of soil microorganisms were much more limited by P in rhizosphere soil of understory vegetation in subtropical plantations.

Aims The objectives of this study were to characterize the carbon (C) : nitrogen (N) : phosphorus (P) stoichiometry of the “host branches-haustorias-parasitic branches-parasitic leaves” continuum and to better understand nutrient relationship between hemiparasite plants and their hosts.

Methods The study site is located in the Xujiaba area of Ailao Mountain, Yunnan Province. Two common hemiparasite plants Loranthus delavayi and Taxillus delavayi were selected, and the C, N and P concentrations of host branches, haustorias, parasitic branches and parasitic leaves were measured.

Important findings The results showed that, the tendency of C, N, P stoichiometry characteristics of host branches-haustorias-parasitic branches-parasitic leaves were species specific, and were not identical between the two hemiparasites. The host branches of the same parasitic plant have similar C, N, and P stoichiometry characteristics, and the host species have no significant effect on the stoichiometry of hemiparasites. There was a close coupling relationship between the C, N, P stoichiometry characteristics in the host branches, and the haustorias was weaker than the host branch, the parasitic branch was weaker than the haustorias, and there was no significant correlation between the N and P concentrations in the parasitic leaf. There was a significant negative correlation between the host branches and the parasitic leaves of C concentration. The C, N, P stoichiometry characteristics of the haustorias were more similar to the parasitic branches, and it had a very significant positive correlation with the host branches. As a key part of the host and parasitic plants, the haustorias had a significant correlation with the host branches, which reflected the importance of the host branch nutrients to the parasitic plants. The element stoichiometry and their relationship of the haustorias were more similar to those of the parasitic branches, which embodied that haustorias as a parasitic plant organ had physiological functions similar to those of the parasitic branches. These results provided important data for in-depth study of nutrient utilization strategies and ecological adaptability of hemiparasitic plants.

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 Sand-fixing shrubs play an irreplaceable role in ecological restoration and eco-environmental protection in arid and semiarid regions of northern China. Determination of the stoichiometric homoeostasis of dominant sand-fixing shrubs along soil nutrient gradients could provide insights into ecological adaptability and pattern of changes of sand-fixing vegetation in Horqin Sandy Land.
Methods We measured N and P concentrations in leaves of two dominant sand-fixing shrubs Caragana microphylla and Artemisia halodendron, and the total and available N and P concentrations in soils beneath the canopy of each shrub. The differences between the two shrubs in N and P concentrations and N:P of leaves and soils as well as in stoichiometric homoeostasis were examined.
Important findings Caragana microphylla had higher leaf N concentration and lower leaf P concentration, thereby higher leaf N:P, than A. halodendron. Soils beneath the shrub canopies, regardless of the species, had higher total and available N and P concentrations relative to soils outside the canopy cover. Moreover, the total and available N and P concentrations in soils beneath the C. microphylla canopy were higher than that beneath the A. halodendron canopy. The stoichiometric homoeostasis indexes (H) were ranked in the order of H_P > H_N:P > H_N in A. halodendron and H_N:P > H_N > H_P in C. microphylla, respectively, suggesting N limitation in A. halodendron and P limitation in C. microphylla. Therefore, Caragana microphylla could be used as nursing plants in degraded N-limiting soil because of high H_N. However, due to excessive uptake of N, Caragana microphylla might suffer from P limitation, and adequate P supply should be considered during the restoration process in sandy land.

Aims In order to discuss the underlying mechanism of desertification effect on the ecological stoichiometry of soil, microbes and extracellular enzymes, we studied the changes of soil, soil microbial and extracellular enzyme C:N:P stoichiometry during the desertification process in the desert grassland in Yanchi County, China.
Methods The “space-for-time” method was used.
Important findings The results demonstrated that: (1) Soil C, N, P contents and soil C:P, N:P significantly decreased, but soil C:N gradually increased with increasing desertification. (2) Soil microbial biomass C (MBC):soil microbial biomass P (MBP), soil microbial biomass N (MBN):MBP and soil β-1,4-glucosidase (BG):β-1,4-N- acetylglucosaminidase (NAG) gradually decreased, soil BG:alkaline phosphatase (AP) and NAG:AP basically showed an increasing trend with increasing desertification. (3) Desertification increased the soil microbial carbon use efficiency (CUE_C:N and CUE_C:P) gradually, while soil microbial nitrogen use efficiency (NUE_N:C) and soil microbial phosphorus use efficiency (PUE_P:C) basically decreased. (4) Soil, soil microbial and soil extracellular enzyme C:N stoichiometry (C:N, MBC:MBN, BG:NAG) were significantly negatively correlated with the soil, soil microbial and extracellular enzyme N:P stoichiometry (N:P, MBN:MBP, NAG:AP), the soil and extracellular enzymes C:N (C:N, BG:NAG) were significantly positively correlated with the soil and extracellular enzymes C:P (C:P, BG:AP). Soil N:P was significantly positively correlated with the soil MBN:MBP, but was significantly negatively correlated with the soil NAG:AP. The analysis demonstrated that soil microbial biomass and extracellular enzyme activity changed with soil nutrient during the desertification process in the desert grassland. The covariation relationship between soil nutrient and C:N:P stoichiometry of microbial-extracellular enzyme provides a theoretical basis for understanding the underlying mechanism of C, N, P cycling in the soil-microbial system in desert grasslands.