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.

The biological sciences developed very fast during the 20th century and have become increasingly sophisticated and predictive. Along with this trend, areas of research also have become increasingly specialized and fragmented. However, this fragmentation and specialization risks overlooking the most inherent biological characteristics of living organisms. One can ask if the living organisms on the earth have unified and essential characteristics that can connect the disparate disciplines and levels of biological study from molecular structure of genes to ecosystem dynamics. By exploring this question, a new science, ecological stoichiometry, has been developed over the past two decades. Ecological stoichiometry is a study of the mass balance of multiple chemical elements in living systems; it analyzes the constraints and consequences of these mass balances during ecological interactions. All biological entities on the earth have a specific elemental composition and specific elemental requirements, which influence their interactions with other organisms and their abiotic environment in predictable ways. Ecological stoichiometry has been incorporated successfully into many levels of biology from molecular, cellular, organismal and population to ecosystem and globe. At present, the principles of ecological stoichiometry have been broadly applied to research on population dynamics, trophic dynamics, microbial nutrition, host-pathogen interactions, symbiosis, comparative ecosystem analysis, and consumer-driven nutrient cycling. This paper reviews the concepts, research history, principles, and applications of ecological stoichiometry and points out future research hotspots in this dynamic field of study with an aim to promote this discipline of research in China.

Aims It is well documented that nitrogen (N) and phosphorus (P) are the two main growth-limiting nutrients for plants in many natural environments. Plant N:P ratio has proved useful as an indicator of shifts from N (P) to P (N) limitation because it is easily determined and compared. However, little is known about the plant N:P ratio in evergreen broad-leaved forests (EBLF), particularly the pattern along secondary succession. Therefore, our goal was to examine the relationship between the form of nutrient limitation and secondary successional stage by using the N:P ratio of plant leaves (ratio of N to P concentration) as an indicator.
Methods The research was completed in Tiantong National Forest Park (29°52' N, 121°39' E, 200 m elevation), Zhejiang Province, East China. Leaf N and P concentrations of dominant tree species along a secondary succession gradient of EBLF were quantified to provide canopy N:P ratios for different communities. Leaf N and P concentrations of common plant species in a given community were then determined to emphasize the relationships between differences in the N:P ratios among species at each successional stage.
Important finding Shifts in the N:P ratios of species were consistent along the successional series, although the N:P ratios of different species in a given community varied considerably. At the community level, the lowest N:P ratio (7.38) was found in grassland, which was usually considered a primary stage of EBLF succession. Thereafter, the N:P ratio increased to 19.96 in the shrub stage, declined to an average of 14-16 in the mid-stages of succession, including coniferous forest and coniferous-broadleaved mixed forest, and increased at the end stages of succession (e.g. 18.77 in the Schima superba community and 20.13 in the Castanopsis fargesii community). These results suggest that the productivity of vegetation in the Tiantong region is N-limited in the primary stages of succession, N- and P-limited in the mid-succession stages and probably P-limited in the shrub and mature EBLF stages.

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 Little is known about constrained ratios of carbon, nitrogen, and phosphorus (C:N:P) in terrestrial ecosystems. Our objective was to examine the C:N:P stoichiometry and its relationship with N and P resorption in evergreen broad-leaved forests (EBLF), evergreen coniferous forests (CF) and deciduous broad-leaved forests (DF) at the regional scale.

Methods The study was conducted in Tiantong National Forest Park (29°52′ N, 121°39′ E), Zhejiang Province, eastern China. To estimate foliar and litter C:N:P ratios and N and P resorption efficiencies, we quantified the C, N and P concentrations in leaf and litterfall in EBLF, CF and DF. We used type II regression slopes (reduced major axis, RMA) to determine whether C:N:P stoichiometry varied across gradients of forest production and nutrients.

Important findings The C:N:P ratios in EBLF, CF and DF were 758:18:1, 678:14:1 and 338:11:1 in fresh leaves and 777:13:1, 691:14:1 and 567:14:1 in litterfall, respectively. The foliar C:N ratio was highest in CF, intermediate in EBLF and lowest in DF, while the foliar C:P and N:P ratios were highest in EBLF, intermediate in CF and lowest in DF. In contrast, the litterfall C:N and C:P ratios were higher in EBLF than in CF and DF, and there were no significant differences of N:P ratio among forests. The type II regression slope for N vs. P in leaves of overall plants was statistically >1, suggesting an increasing investment of N with increasing of P in fresh leaves. In contrast, the slope for N vs. P in litterfall approximated 1. N resorption in EBLF was significantly higher than in CF and in DF, but the highest P resorption was observed in DF. Although foliar N:P ratios indicated that EBLF was P limited, DF was N limited and CF was both N and P limited, the nutrient resorption efficiency did not respond with relatively high N resorption in EBLF and high P resorption in DF. We concluded that the relative higher resorption of N and P before leaf abscission could be an inherent property of plants, but was not a mechanism thought to have evolved to conserve nutrients in environments with limited N or P supply.

Aims Homeostasis constrains the elemental composition of individual species within narrow bounds no matter the chemical composition of the environment or the resource base. Our objective was to determine the dynamics of leaf stoichiometry during the growth period of plants and the optimum time for stoichiometry study.

Methods We monitored leaf N, P stoichiometry of Scirpus mariqueter, Carex scabrifolia and Phragmites australis, the dominant species in Hangzhou Bay coastal wetlands, at different growth stages from May to October 2007.

Important findings Leaf N, P stoichiometry of the Scirpus, Carex and Phragmites species showed differences: 7.41-17.12, 7.47-13.15 and 6.03-18.09 mg·g^-1 for N, 0.34-2.60, 0.41-1.10 and 0.35-2.04 mg·g^-1 for P, and 7.19-30.63, 11.58-16.81 and 8.62-21.86 for N:P ratios, respectively. The arithmetic means for the three species were (11.69 ± 2.66), (10.17 ± 1.53) and (11.56 ± 3.19) mg·g^-1 for N, (0.93 ± 0.62), (0.74 ± 0.23) and (0.82 ± 0.53) mg·g^-1 for P, and 16.83 ± 8.31, 14.53 ± 3.91 and 16.49 ± 5.51 for N:P, respectively, but there was no significant difference of N, P stoichiometry (p > 0.05). It showed high N, P concentrations at the early stage of growth because of small biomass and then decreased greatly with leaf expansion during the fast growth period, increased as leaf growth became stable and decreased again with leaf senescence. Leaf N:P was low at the early stage of growth and then increased, decreased strongly at the fast growth period, and became stable after leaf maturation.

Aims Nitrogen (N) and phosphorus (P) are two key elements of life and are major limiting nutrients in many ecosystems across the world. The balance of N and P has become the focal point of global change ecology and biogeochemistry, especially as aggravated by atmospheric nitrogen deposition. Although N:P stoichiometry has proved useful in studies of nutrient limitation, biogeochemical cycles, forest succession and degraded land, little is known about it in lower subtropical forest succession. Therefore, our objective is to better understand nutrient controlling factors of plant-soil interaction and reveal interactions of N and P to provide insight and theoretical fundamentals for forest management.

Methods We measured total N and P of organs of dominant species and different soil layers in three forests in Dinghushan Biosphere Reserve, Southern China: pine forest (PF, early successional stage), pine and broad-leaved mixed forest (MF, middle stage) and monsoon evergreen broad-leaved forest (MEBF, advanced stage).

Important findings Soil N content in the 0-10 cm soil layer increased with succession; values in PF, MF and MEBF were 0.440, 0.843 and 1.023 g·kg^-1, respectively. The largest value of P content in the same layer was in MF (0.337 g·kg^-1); the values in PF and MEBF were 0.190 and 0.283 g·kg^-1, respectively. Plant foliage N and P content decreased with succession; the largest values for roots were in MF, and the values in PF equaled those in MEBF. Soil N:P ratio in the 0-10 cm layer significantly increased with succession; 2.3, 2.5 and 3.6, respectively. The N:P ratio of various plant organs also increased with succession, and the value in foliage was close to that in roots; the foliage N:P ratios were 22.7, 25.3 and 29.6, respectively. We discussed the characteristics of N:P ratios in soil and plants of the lower subtropical forest ecosystem, the law of N:P ratios in soil and plants in successional series, and the limiting effect of P on the lower subtropical forest ecosystem.

Aims Our purpose was to study the effects of deposition of nitrogen (N) on plant carbon (C), N, phosphorus (P), N and P nutrient resorption efficiencies, C : N : P stoichiometry and their internal relations on Stipa bungeanaof Loess Plateau natural grassland.

Methods Deposition of N was simulated by N fertilization at four levels. Changes of C, N and P contents were detected, and C : N : P and the N and P nutrient resorption efficiencies were estimated for S. bungeana.

Important findings The C and N contents of leaves and N and P contents of standing litter increased significantly with N addition. However, the P content of leaves and C content of standing litter did not response to N addition. The N and P resorption efficiencies of S.bungeana decreased significantly with N addition. When there was no N addition, N and P resorption efficiencies were highest (60.35% and 71.75%, respectively). Meanwhile, the P resorption efficiency was greater than that of N in same treatment. The C : N ofS. bungeana decreased gradually with N addition, but the N : P and C : P increased with N addition. Values of the N : P were 18.25-29.01. The results showed the Loess Plateau natural grassland was mainly limited by P, and the strength of P limitation was enhanced with N deposition. Higher N and P resorption efficiencies were an important strategy for S. bungeana to survive soil infertility.

Aims Much research is being done on plant nutrients and stoichiometry. Our purpose was to reveal the effects of grazing on plant nutrients and stoichiometry in a typical steppe of Inner Mongolia of China.

Methods We studied nutrient content of C, N and P and their ratio in soil and leaves of dominant plants in three adjacent sites: fenced since 1983 and 1996 and unfenced. We employed the stoichiometric approach and assessed the effects of grazing on spatio-temporal patterns of nutrient cycling between plants and soil in restoration succession of degenerate steppes.

Important findings Both total soil nutrient content and the ratio of the soil total nitrogen and soil total phosphorus (STN:STP) were lower in overgrazed plant communities compared to fenced plant communities at different levels of restoration. Conversely, the ratio of soil organic carbon and STN (SOC:STN) was higher in overgrazed plant communities. The total organic carbon content (TOC) of most plants was higher in fenced communities and lower in grazed communities and was positively correlated with time since community restoration began. However, the content of total nitrogen (TN) and total phosphorus (TP) in plants was higher in fenced communities than that in grazed communities. Both TN and TP correlated negatively with time since restoration began and positively with the degree of degradation due to overgrazing. TP had a larger range in values compared to TN. The stoichiometry ratios of nitrogen and phosphorus (N:P) and carbon and nitrogen (C:N) in leaves were the lowest in grazed communities and correlated negatively with the degree of degradation. These communities had less total N than total P; however, this pattern was reversed in fenced communities, where sometimes both N and P were limiting. We propose that stoichiometry ratios in dominant plant species can serve as indicators of direction of plant succession in this typical steppe.

Aims The nitrogen and phosphorus characteristics of plants represent plant features and responses to environmental factors. Our objectives are to distinguish leaf and litter C : N : P stoichiometric characteristics, nitrogen and phosphorus resorption of trees, and the relationship between stoichiometric ratio and temperature and precipitation for four typical regions in China.

Methods We studied temperate coniferous, subtropical evergreen broad-leaved, tropical monsoon and tropical plantation forest in the Changbaishan, Dinghushan, Xishuangbanna and Qianyanzhou Ecological Stations, respectively. We analyzed leaf and litter C : N : P, N, P and the relation of N, P nutrition limitation at each station.

Important findings Leaf C : N : P in temperate needle and broad-leaved mixed, subtropical evergreen broad-leaved, tropical rain and subtropical plantation forests were 321 : 13 : 1, 561 : 22 : 1, 442 : 19 : 1 and 728 : 18 : 1, respectively. Litter C : N : P of the four forest types were 552 : 14 : 1, 1 305 : 35 : 1, 723 : 24 : 1, 1 950 : 27 : 1, respectively. The C : N of evergreen coniferous forest is higher than in evergreen broad-leaved and deciduous broad-leaved forests, but C : P has no relationship with forest type. Leaf N : P was highest in evergreen broad-leaved forest and lowest in deciduous broad-leaved forest. Plant N : P has a linear relation with latitude and mean monthly temperature, but neither N or P concentration has such a relationship. Plant at high latitude are easily limited by N, those in low latitude are easily limited by P, but results show that plants limited by N or P don’t have higher N or P resorption. Stoichiometric ratios of leaf and litter are consistent, but environmental factors have different effects on different kinds of plant.

Aims Leaf N and P stoichiometry has been widely studied at the species level in both aquatic and terrestrial ecosystems, however, it lacks research at the community level. Since the ecological stoichiometric characteristics could play important roles in connecting different levels of ecological studies and former studies mainly focused on the individual level, in this study, we try to figure out the pattern of foliar N and P at the community level of grassland ecosystems in Qinghai-Tibetan Plateau. Additionally, we also try to find out the relationships between community level leaf N, P and site climate factors.

Methods Leaf samples were collected from 47 research sites in Qinghai-Tibetan Plateau at the end of the growing season yearly from 2006 to 2008. We measured the leaf N concentrations by using an elemental analyzer and the leaf P concentration based on a molybdate/stannous chloride method. Climate data of annual mean temperature and annual mean precipitation (65 national standard stations) between 2006 and 2008 were used to interpolate into gridded data with a resolution of 1 km × 1 km through the tchebycheffian spline function.

Important findings Leaf N, P concentrations and N:P ratios at the community level over the southern part of Qinghai-Tibetan Plateau were 23.2 mg·g^-1, 1.7 mg·g^-1 and 13.5, respectively. Significant inter-annual differences were presented in leaf N, P concentrations and N:P ratios. Mean annual temperature was strongly correlated with leaf N, P and N:P ratios. Besides, the correlations between climate factors and leaf N, P, N:P ratios were generally consistent with the previous results found at the global scale. Our results suggest that the high variation in leaf P concentration and its strong correlation with environmental factors reveal that, to some extent, stoichiometric traits at the community level are adaptive to local environmental conditions.

Aims Plant or biomass stoichiometry can be used to distinguish biological entities (genes, cells, organisms, etc.) based on element composition. Our objective was to determine the stoichiometry characteristics and examine nutrient limitation in evergreen broad-leaved forest, coniferous and broad-leaved mixed forest and coniferous forest.

Methods We determined C, N, P stoichiometry of leaves of 19 dominant trees of 16 taxa in three forest types at the Pearl River Delta Forest Ecosystem Research Station, Guangdong Province, South China.

Important findings Leaf stoichiometry showed large variations: C ranged from 434 to 537 mg·g^-1, N from 6.8 to 23.0 mg·g^-1, P from 0.56 to 2.10 mg·g^-1, C:N from 21.22 to 70.74, C:P from 227.14 to 844.64 and N:P from 5.26 to 20.91. Leaf N, P, C:N and C:P were linearly correlated (p < 0.01). Leaf C, C:P and N:P (weighted average ± standard deviation: (517.85 ± 35.96), (727.47 ± 231.52) and (15.71 ± 3.76) mg·g^-1, respectively) were the highest in coniferous forest, followed by mixed forest (509.47 ± 19.38, 553.01 ± 152.32 and 10.93 ± 1.89, respectively) and evergreen broad-leaved forest (481.59 ± 18.35, 412.19 ± 200.91 and 9.46 ± 4.28, respectively), and a reverse sequence was detected for leaf P content. The sequence for N content was coniferous forest ((12.20 ± 5.65) mg·g^-1) > evergreen broad-leaved forest ((11.50 ± 4.24) mg·g^-1) > mixed forest ((10.51 ± 5.22) mg·g^-1) and for C:N was mixed forest (51.35 ± 13.65) > coniferous forest (47.40 ± 15.85) > evergreen broad-leaved forest (45.59 ± 14.70), and higher nutrient use efficiency was discovered in three forest types. Several evergreen broad-leaved trees and evergreen broad-leaved forest had shortages of N.

Aims Understanding the stoichiometry of nutrient elements of plants growing in phosphorus-enriched areas can help characterize plant differentiation and guide ecological restoration in different biogeochemical environments. The Lake Dianchi watershed of southwestern China has P-enriched soils, and its main plant species may illustrate the relationship between plant ecological traits and the environment. Our aim was to test whether different plant life forms living at different P levels in this area have different patterns of leaf nutrient stoichiometry.
Methods We collected leaf samples from 75 adult plants and soil samples from their root-zones in P-enriched areas and reference sites within the watershed. We determined N, P and K contents of leaves and total P contents of soil samples and calculated element ratios.
Important findings The arithmetic means of leaf C, N and K were 441.42, 16.17 and 13.57 mg·g^-1, respectively, and the geometric mean of leaf P was 1.92 mg·g^-1. Significant correlations among leaf C, N, P and K were observed in all plant species. Higher P and K contents were observed in plants growing in higher P areas, but higher N/P and K/P were observed in lower P sites. Leaf nutrient concentration was significantly higher in herbaceous plants than in woody plants, but there was no difference in leaf nutrient concentrations between trees and shrubs. Leaf N/P and K/P were correlated negatively with soil P content. Results suggested that plant growth and vegetation development in the Lake Dianchi watershed were limited by low soil N contents and plant growth enhanced by N addition should be important for vegetation resilience and prevention of non-point source pollution in the process of ecosystem restoration.

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 Our objectives were to determine the feasibility of estimating nitrogen content in fresh and dry wheat leaves using near-infrared (NIR) spectroscopy and chemometrics and to establish the near-infrared model for estimating nitrogen content in wheat leaves in order to lay a foundation for wheat nitrogen management.

Methods We conducted three field experiments with different years, wheat varieties and nitrogen rates and determined time-course near-infrared absorbance spectroscopy and total nitrogen content from fresh and dry wheat leaves. The methods of partial least squares (PLS), back-propagation neural network (BPNN) and wavelet neural network (WNN) were used to establish the calibration models, and a dataset selected at random was used to evaluate the established models.

Important findings Near infrared calibration models based on PLS, BPNN and WNN could be used to estimate nitrogen content in wheat leaves with high precision and stable performance, especially WNN. The validation results showed that the root mean square errors of prediction (RMSEP) for the power model are 0.147, 0.101 and 0.094, respectively, while those for the fresh leaves model are 0.216, 0.175 and 0.169, respectively. The correlation coefficients (R²) for all models are >0.84. Therefore, near-infrared spectrometry can be an efficient method to estimate the nitrogen nutrition of crops.

AimsThe carbon (C), nitrogen (N) and phosphorus (P) stoichiometry (C:N:P) of soil profoundly influences the growth, community structure, biomass C:N:P stoichiometry, and metabolism in microbes. However, the relationships between soil and microbes in the C:N:P stoichiometry and their temporal dynamics during ecosystem succession are poorly understood. The aim of this study was to determine the temporal patterns of soil and microbial C:N:P stoichiometry and their relationships during ecosystem succession.MethodsAn extensive literature search was conducted and data were compiled for 19 age sequences of successional ecosystems, including 13 forest ecosystems and 6 grassland ecosystems, from 18 studies published up to May 2016. Meta-analyses were performed to examine the sequential changes in 18 variables that were associated with soil and microbial C, N and P contents and the stoichiometry. Important findings (1) There was no consistent temporal pattern in soil C:N along the successional stages, whereas the soil C:P and N:P increased with succession; the slopes of the linear relationships between soil C:N:P stoichiometry and successional age were negatively correlated with the initial content of the soil organic C within given chronosequence. (2) There was no consistent temporal pattern in microbial C:N:P stoichiometry along the successional stages. (3) The fraction of microbial biomass C in soil organic C (qMBC), the fraction of microbial biomass N in soil total N, and the fraction of microbial biomass P in soil total P all increased significantly with succession, in consistency with the theory of succession that ecosystem biomass per unit resource increases with succession. (4) The qMBC decreased with increases in the values of soil C:N, C:P, or N:P, as well as the stoichiometric imbalances in C:N, C:P, and N:P between soil and microbes (i.e., ratios of soil C:N, C:P, and N:P to microbial biomass C:N, C:P, and N:P, respectively). The C:N, C:P, and N:P stoichiometric imbalances explained 37%-57% variations in the qMBC, about 7-17 times more than that explainable by the successional age, illustrating the importance of soil-microbial C:N:P stoichiometry in shaping the successional dynamics in qMBC. In summary, our study highlights the importance of the theories of ecosystem succession and stoichiometry in soil microbial studies, and suggests that appropriately applying macro-ecological theories in microbial studies may improve our understanding on microbial ecological processes.

Aims Leaf carbon (C), nitrogen (N), and phosphorus (P) concentrations and their stoichiometry can provide a basis for plant nutrient status and element limitation. Our objective was to explore variations of leaf C:N:P stoichiometry in plants of different growth forms.

Methods We analyzed leaf C, N, and P concentrations in three graminoids (Eriophorum vaginatum, Carex globularis, Deyeuxia angustifolia), five deciduous shrubs (Betula fruticosa, Salix myrtilloides, Salix rosmarinifolia, Vaccinium vitis-idaea, Vaccinium uliginosum), and three evergreen shrubs (Ledum palustre, Chamaedaphne calyculata, Rhododendron capitatum) across 18 peatland sites in the Da Hinggan Ling, northeastern China.

Important findings (1) Leaf C, N, and P concentrations were higher, and the leaf C:N, C:P, and N:P values were lower, in deciduous and evergreen shrubs than in graminoids, indicating that plants of different growth forms had different nutrient utilization strategies. Shrubs had higher C, N and P storage and lower N and P use efficiency than graminoids. (2) Leaf N:P values in Deyeuxia angustifolia and R. capitatum were less than 10, and their leaf N concentrations were lower than the global mean leaf N concentration, indicating that those species were limited by N more than other plants. (3) The sampling sites explained 12.8%-40.8% of the variations in leaf C, N, and P stoichiometry, and plant species explained 9.3%-25.5%. (4) Graminoids had greater inter-site coefficient of variance (CV) values in leaf C, N, and P variables than deciduous and evergreen shrubs, indicating greater sensitive to site factors. (4) The inter-species CV values in leaf N were greater in graminoids than in deciduous and evergreen shrubs, and the inter-species CV values in leaf P were greater in deciduous shrubs than in graminoids and evergreen shrubs, indicating greater physiological differentiation in N and P use strategies in graminoids and deciduous shrubs than in evergreen shrubs.

Aims Leaf nitrogen (N) and phosphorus (P) and N : P stoichiometry have been studied intensively in different regions in China. Songnen grassland is a natural region. Its dominant vegetation is meadow, which is determined by soil properties, and its flora is complex. Our objective was to find the stoichiometric patterns for this region.
Methods Leaf samples of 80 herbaceous species were collected in Songnen grassland in August 2008. We determined leaf N, P, and N : P on both a mass and an area basis and tested the differences according to plant life forms and functional groups.
Important findings Leaf N and P concentrations were (24.2 ± 0.96) and (2.0 ± 0.10) mg·g^-1 on a mass basis and (13.0 ± 0.54) and (1.0 ± 0.05) mg·cm ^-2 on an area basis, respectively. N : P was (13.0 ± 0.39). Plant growth was limited by N in Songnen grassland. The concentration of leaf N and P and coefficient of variation were higher in annual plants than in other life forms on a mass basis, and there were no significant differences of leaf N concentration on an area basis and N : P between different life forms. Leaf N concentrations both on mass and area bases and N : P of legumes were higher than in other functional groups. There was no significant difference in the leaf P concentrations on an area basis among different life forms or functional groups. Our findings indicate that appropriately increasing the proportion of legume plants would improve both the yield and quality of primary productivity in Songnen grassland.

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 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.

What would be the impact of external nitrogen additions on soil carbon, an issue still under debating, as reported experimental results were either positive, negtive or neutral. Several factors may be related to these seemingly controversial results: differences in ecosystem types and soil properties, soil carbon detection methods, soil depths, and contents of soil labile and recalcitrant carbon that affect the responses to nitrogen additions, all could cause discrepancies and variations in carbon sequestration. The several processes that contribute to enhance soil organic carbon storage include increasing litter input, decresing soil carbon output, particularly, by supressed decomposition of recalcitrant carbon, promoting soil humifiction and formation of recalcitrant carbon storage. However, there are still many uncertainties associated with these issues. To improve our understanding, the research about carbon in deep soil layers, dissolved organic carbon leaching and accumulation, and the effect of labile and recalcitrant soil C ratios on N addition responses, should be further investigated in the future studies.

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.

Aims Organ stoichiometric characteristics are the bridge that connects environment and plant organ traits. The relationships among environment, organ stoichiometric characteristics and organ traits reveal mechanisms of environmental effects on plant organ traits and make it possible to regulate plant traits. Our objective was try to find the relationships among soil and leaf nitrogen (N), phosphonium (P) stoichiometric characteristics and leaf chlorophyll content for Oligostachyum lubricum.
Methods Total N, P concentrations of the original soil in pots were 421.76 and 37.35 mg·kg^-1, respectively, and the original soil was treated as the control (1N1P). Total experimental N, P concentration were two, three and four times as high as the control. Different N, P levels were combined into 10 combinations (2N2P, 2N3P, 2N4P, 3N2P, 3N3P, 3N4P, 4N2P, 4N3P, 4N4P and 1N1P) and every combination except the control was achieved by adding different amounts of NH₄NO₃ and NH₄H₂PO₄. Leaf samples were collected from ramets of O. lubricum after grown in the pot soil with different N, P level combinations for 45 days. Leaf total N concentration was determined by employing the Kjeldahl method and leaf total P concentration by the acid melt-molybdenum stibium anti-color method. Leaf chlorophyll concentrations were measured based on acetone-ethanol mixture (1 : 1) extraction method. Soil and leaf total N, P concentrations were expressed as mg·kg^-1DW.
Important findings Soil total N concentration was significantly positively correlated with leaf total N concentration and leaf N : P ratio, whereas soil total P concentration had no significant correlation with leaf total P concentration and leaf N : P ratio. Leaf N : P ratio increased with the increasing of soil N : P ratio, and the rate of increase of soil N : P ratio was faster than that of leaf N : P ratio. At the same soil condition, leaf N : P ratio of ramets growing in soil with 2N2P and 3N3P had no significant difference, but the both were higher than the control (1N1P) and lower than that growing in soil with 4N4P. Leaf N : P was the main factor that affected leaf chlorophyll content. Results suggested that soil total N concentration had more effect on leaf N, P stoichiometric characteristics than soil total P. Sufficient supply of soil total N lead to the luxury uptake of N by leaves of O. lubricum. The growth of O. lubricum was limited by low soil total N concentration before N and P addition.

The objectives of this study were to characterize the C:N:P stoichiometry of the “plant-litter-soil” continuum and to better understand nutrient cycling and stability mechanisms in karst forest ecosystems in Southwest China. Three representative forest sites were selected for each of the primary and secondary communities (28 years of natural restoration) in Northwest Guangxi, and measurements were made on carbon (C), nitrogen (N), and phosphorus (P) contents in plants, litter and soils. Compared with other regions, the plants in karst forest ecosystems had relatively lower C content and higher N content, with a lower C:N ratio in consistency with the characteristics of plants. After 28 years of natural recovery, N and P absorption in secondary forests were at a relatively stable state compared with the primary forest communities. The values of N:P ratio varied from a range of 16-19 in the primary forest communities to 17-19 in the secondary forest communities, without apparent difference in the mean vale between the two contrasting community types. Soil organic C, N and P in karst forests occurred primarily in the top 0-10 cm soil layer, at 92.0 mg·g^-1 C, 6.35 mg·g^-1 N, and 1.5 mg·g^-1 P, respectively. In contrast, the nutrient utilization efficiency and nutrient resorption rate were lower in karst forest plants than in other plant types, with karst forest plants exhibiting a relatively rapid nutrient turnover rate. The N resorption rate was lower, and the P resorption higher, in the primary forest communities than in the secondary forest communities, indicating that the higher N deficiency and lower P deficiency of the primary forest communities compared with the secondary forest communities. Determination of the C:N:P stoichiometric characteristics in the plant-litter-soil continuum in this study provides a scientific guidance for restoration of the vulnerable karst ecosystem in Southwest China.

Aims The stoichiometric characteristics of carbon (C), nitrogen (N) and phosphorus (P) in plant organism is vital to understand plant adaptation to environment. In particular, the correlations of elemental stoichiometric characteristics between leaf and fine root could provide insights into the interaction and balance among the plant elements, nutrient use strategies and plant response to global change.Methods We measured C, N, P contents and C:N, C:P, N:P in leaves and fine roots of 60 dominant plants in Horqin sandy land. The 60 plant species were classified into five life forms and two categories such as perennial forb, annual forb, perennial grass, annual grass, shrub, legume, and non-legume. We statistically analyzed the differences and correlations of C, N and P stoichiometry either between fine root and leaf or among five life forms.Important findings The average C, N and P concentrations in leaves of 60 plant species in Horqin sandy land are 424.20 mg·g^-1, 25.60 mg·g^-1 and 2.10 mg·g^-1, respectively. In fine roots, the corresponding element concentrations are 434.03 mg·g^-1, 13.54 mg·g^-1, 1.13 mg·g^-1. N and P concentrations in leaf are approximately twice as high as averages in fine root. Furthermore, similar N:P between leaf and fine root indicates conservative characteristic of elemental stoichiometry in plant organism, suggesting that nutrients distribution is proportional between aboveground and underground of plants. There are significant difference of C, N, P, C:N, C:P and N:P in leaf and root among five life forms. N and P in forb and C:N and C:P in grass are averagely higher than those in other life forms. N:P in annual forb and grass, however, are lower than those in other life forms. C, N in legume are higher than those in non-legume, while C:N in legume is lower than in non-legume. These results imply that nutrient use strategies are significantly different among plant life forms. Correlations analysis showed that N and P in leaf or fine root positively correlated, but C and N, C and P in fine root negatively correlated, suggesting coupling relationship among C, N and P in leaf and fine root. Subsequently, we detected positively significant correlations in C, N, P and their ratios between leaf and fine root, suggesting proportional distribution of photosynthate and nutrient between aboveground and underground during plant growth. Generally, these results supplied fundamental data to understand mass turnover and nutrients cycling of leaves and roots in sand land.

Aims Suaeda salsa is a typical species in coastal wetlands, and understanding change in its stoichiometric characteristics would help to assess its health status and target conservation efforts. We investigated which nutrient factor restricts its growth and proposed theories for protecting and managing coastal wetland by comparing the C, N and P stoichiometric characteristics of S. salsa in different growth periods.
Methods We collected S. salsa leaves in different growth phases from June to November 2010 in Yancheng coastal wetlands, Jiangsu Province. The C, N and P contents of the leaves were measured. Data were analyzed by correlation analysis between N content and C:N and P content and C:P. N content and P content were also analyzed.
Important findings Leaf C content of S. salsa had significant differences among three different growth phases, with the lowest in the growth phase and the highest in the decline phase. Leaf N content in the decline phase is significantly lower than in the mature and growth phases, and no significant difference of leaf P content was found. C:N and C:P were gradually increasing in the growth period while N:P showed a gradually decreasing trend. Correlation analysis indicated that C:N and C:P were negatively correlated with corresponding N, P content in three different phases. N content was positively linearly correlated with P content, indicating consistent demand of N and P by S. salsa. Furthermore, N is a restrictive factor for S. salsa in coastal wetlands during its growth and development.

Aims Our objectives were to investigate differences in nutrient resorption between different plant organs (leaf and branch), among plants with different life spans (one-year old, two-year old and senesced), and under different duration of nitrogen (N) deposition treatments in a Chinese fir (Cunninghamia lanceolata) plantation.

Methods The long-term N deposition experiment was conducted in a 12-year-old fir plantation of subtropical China. N deposition treatment was initiated in January 2004 until now, up-going 14 years. N deposition were designed at 4 levels of 0, 60, 120, and 240 kg·hm ^-2·a ^-1, indicated as N0, N1, N2, and N3, respectively, with 3 replicates for each treatment. The solution of CO(NH₂)₂was sprayed on the forest floor each month. In the study, we measured N and phosphorus (P) concentrations and analyzed the pattern of nutrient resorption of mature and senescing leaves and branches. The different responses of needles N and P resorption after 7- and 14-year N deposition treatments were also compared.

Important findings After 14 years of N deposition, (1) during the senescing process, leaf and branch C, N, and P content gradually decreased with increasing treatment duration, with higher content in leaf than in branch. N content decreased in the order of one-year old green leaf > two-year old green leaf > senescent leaf > one-year old living branch > two-year old living branch > senescent branch, and N3 > N2 > N1 > N0, with C:N showing the opposite trend. Senescent organs had higher C:N, N:P, and C:P than mature living organs. N deposition increased N, N:P, and C:P of mature living organs (except for the two-year old green leaf), while decreased P and C:N. (2) N resorption efficiency (RE_N) and P resorption efficiency (RE_P) of leaves and branches decreased gradually with increasing life span. RE_P was typically higher in leaf and branch than RE_N. Leaf had lower RE_N (28.12%) than branch (30.00%), but higher RE_P (45.82%) than branch (30.42%). A highly significant linear correlation existed between N:P and RE_N:RE_P in leaves and branches. (3) RE_N decreased but RE_P increased with the treatment duration of N deposition. The longer experimental duration (14 years) reduced RE_N by 9.85%, 3.17%, 11.71% under N1, N2, and N3 treatments, respectively, and increased RE_P by 71.98%, 42.25%, 9.60%, respectively, than the shorter treatment duration (7 years). In summary, the responses of essential nutrients resorption efficiency for different plant organs and life span varied with the levels and duration of N deposition treatment. RE_N:RE_P in leaf and branch were mostly driven by N:P of leaf and branch. The results highlight that nutrients resorption is significantly influenced by long-term N deposition.

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.

The survival and growth strategies, community structure and functions of microbial decomposers vary with substrate stoichiometry, which profoundly influences substrate decomposition, turnover, and hence the carbon and nutrient cycles of terrestrial ecosystems. It is crucial to understand the relationships among microbial metabolism, community structure and ecosystem processes of terrestrial ecosystems and their responses and feedbacks to global changes. In this review, we first introduced the significance of microbial decomposers in the carbon, nitrogen, and phosphorus cycles of terrestrial ecosystems from perspectives of ecological stoichiometry and metabolic theories. Then we synthesized four potential mechanisms of microbial response and control on substrate stoichiometric variations, i.e., through (1) modifying microbial stoichiometry, (2) shifting microbial community structure, (3) producing extracellular enzymes to acquire limiting resources, and (4) changing microbial carbon, nitrogen, and phosphor use efficiencies. Finally, we proposed three research directions in this field: (1) to comprehensively explore various microbial mechanisms in response to changes in substrate stoichiometry and the relative importance of these mechanisms; (2) to examine influences of global changes on microbial-driven cycles of carbon, nitrogen, and phosphorus; and (3) to explore spatiotemporal changes in the strategies of microbial adaptation to changes in the substrate stoichiometry.

Aims Leaf is the organ of plant photosynthesis, and it is important to understand the drivers for the variations of leaf nitrogen (N) and phosphorus (P) stoichiometry along geographical and climatic gradients. Here we aimed to explore: 1) the changes in leaf nitrogen (N) and phosphorus (P) stoichiometry of woody plants along an altitudinal gradient in Changbai Mountain, and 2) the relative contribution of climate, plant characteristics, and phylogeny to the changes in leaf N, P concentration and N:P.

Methods We measured leaf N and P concentration, and N:P of 48 woody species in 14 plots along an altitudinal gradient in Changbai Mountain. General linear models (GLMs) and variation partitioning analyses were used to explain leaf N and P stoichiometry with parameters related to plant functional groups, climate and phylogenetic groups.

Important findings Leaf N concentration and N:P decreased with altitude, and were positively correlated with mean annual temperature (MAT) but negatively correlated with mean annual precipitation (MAP). Leaf P concentration had no significant relationship with altitude. Leaf N, P concentration and N:P were significantly different among different plant functional groups. Leaf N concentration of shrubs was higher than that of trees, but leaf P concentration and N:P did not differ significantly between shrubs and trees. Leaf N, P concentration and N:P of broad-leaved plants were significantly higher than those of coniferous plants. Similarly, leaf N, P concentration and N:P of deciduous plants were significantly higher than those of evergreen plants. The effects of climate on leaf N, P concentration and N:P were significant but very weak, with MAT and MAP together explaining 1.50%-2.98% of the variations. Phylogenetic groups explained 30.36%-54.38% of the variations, suggesting the critical effect of phylogeny. Our study also showed that the synergistic effect of climate and species composition (phylogeny) on leaf N and P stoichiometry was neglectable, at least along the altitude gradient of Changbai Mountain. More comparative studies between altitudinal and latitudinal gradients are needed to better understand the drivers of the changes in leaf stoichiometry along an environmental gradient.

Nitrogen (N), phosphorus (P) and their stoichiometry play pivotal roles in plant structure and functions, development and ecological strategies in terrestrial ecosystems due to their coupling with each other and their irreplaceability. Plant N and P can be influenced by biotic and abiotic factors, such as individual traits, climate change and human disturbance, and it is those factors that determine the plant community composition and structure that finally affect the ecosystem processes. According to previous studies, there is an allometric relationship between N and P. Relationships between plant N and P depend on the soil nutrient condition and species plasticity in N and P. Understanding the relationships between plant N and P in major ecological gradients can further our knowledge about vegetation restoration, succession, biodiversity, ecosystem trophic structure and biogeochemical cycles. This information could help predict potential changes in terrestrial ecosystems in response to future climate change. We review recent advances in the influencing factors and mechanism of stoichiometry in order to improve understanding of plant responses to global change.

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.

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 Our objective was to explore seasonal variations of leaf C:N:P stoichiometry in plants with the same growth form.
Methods We chose six shrubs in the desert of the Alxa Plateau in north-central China (Zygophyllum xanthoxylum, Nitraria tangutorum, Reaumuria soongorica, Ceratoides lateens, Oxytropis aciphylla and Ammopiptanthus mongolicus) and observed their phenological stages from May to October 2010. Leaf samples were collected during this period, and leaf C, N and P contents and C:N:P stoichiometry were monitored.
Important findings Seasonal dynamics of leaf C, N and P contents and C:N, C:P and N:P mass ratio in the six shrubs were species-specifics, and the variation of leaf C, N and P and C:N, C:P and N:P mass ratios in different species were also dramatically different. Based on variation analysis among different seasons within species, there were less seasonal dynamics in C and N contents and C:N mass ratio than the other three parameters including P contents and C:P and N:P mass ratios. The range of values of coefficient of variation (CV) for C and N contents and N:P mass ratio was 0.60%-10.20%, 6.09%-20.50% and 5.87%-18.78%, respectively. For the other three parameters, the range of CV values for P content was 16.43%-43.43%, and C:P and N:P mass ratios were 8.48%-31.95% and 11.86%-40.73%, respectively. With the comprehensive analysis based on the total variation (resulting from two factors: season and species) for each parameter in these six shrubs, the rank of CV for each parameter was P (28.85%) > C:P (25.02%) > N:P (22.18%) > N (14.22%) > C:N (12.48%) > C (4.62%). Factorial analysis of variation for each parameter, with sampling date (season) and species as independent variables, showed that leaf C and N contents and leaf C:N, C:P and N:P mass ratios were mainly determined by plant species. For leaf P contents, it was the sampling date (season).

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.

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 Stoichiometric homeostasis is an important mechanism in maintaining ecosystem structure, function, and stability. The invasion of exotic species, Spartina alterniflora, has largely threatened the structure and function of native ecosystems in the Minjiang River estuarine wetland. However, how S. alterniflora invasion affect plant stoichiometric homeostasis is largely unknown. This could enhance our understanding on wetland ecosystem stability and expand the applications of ecological stoichiometry theory.
Methods Nitrogen (N) and phosphorus (P) contents of plant organs and soils in the S. alterniflora, Cyperus malaccensis var. brevifolius, and S. alterniflora-C. malaccensis var. brevifolius mixture were measured, and the homeostatic index (H) was calculated according to the stoichiometric homeostasis theory.
Important findings Our results showed that the invasion of S. alterniflora significantly increased soil N:P ratio (p < 0.05), but did not affect soil N or P contents. The N and P contents of leaf and stem were the highest for S. alterniflora, and those of the stem were the highest for C. malaccensis var. brevifolius. At the ecosystem level, the average of homeostatic index (H) of N (H_N, 25.31) was larger than those of P (H_P, 10.33) and N:P (H_N:P, 2.50). At the organ level, root H_N was significantly larger than stem H_N (p < 0.05) and sheath H_N:P was greater than root H_N:P (p < 0.05), while there was no significant difference for H_P among root, stem, leaf, and sheath (p > 0.05). As for species, root H_N of S. alterniflora was significantly larger than that of C. malaccensis var. brevifolius in the mixture community (p < 0.05). In the monoculture, stem H_N:P of S. alterniflora was significantly higher than that of C. malaccensis var. brevifolius (p < 0.05). Furthermore, root H_N, leaf H_N and sheath H_N of S. alterniflora in the mixed community was significantly larger than that of S. alterniflora in the monoculture (p < 0.05), suggesting that S. alterniflora invasions increased their stoichiometric homeostasis. Meanwhile, the stoichiometric homeostasis of invasive and native plants were influenced by multiple factors, such as nutrients, organs, vegetation, and invasion. However, larger homeostasis was found in S. alterniflora than in C. malaccensis var. brevifolius in some particular organs either in mixture or monoculture communities. Therefore, the successful invasion of S. alterniflora may result from higher homeostatic index than the native species, C. malaccensis var. brevifolius.

Exudation measurements focus exclusively on total exudate carbon (C) fluxes without considering how root-derived nitrogen (N) inputs and variable exudate stoichiometries may influence microbial activity and biogeochemical cycles. As a result, the biogeochemical consequences of exudate stoichiometry for soil C-nutrient couplings and feedbacks to environmental changes remain largely unknown. Our objective is to explore to what extent N availability modifies soil microbial processes and the dynamics of soil carbon pool induced by labile C.

We conducted a 50-day laboratory incubation experiment by addition of simulated exudates varying in C:N to two coniferous forest soils: a natural forest and Picea asperata plantation. The five exudate addition treatments are C alone, N alone, and combinations of C and three N levels (C:N ratio of 10, 50 and 100).

The addition of labile C exudates decreased soil total C for both natural forest and the plantation by stimulating soil organic matter (SOM) mineralization (i.e. greater priming effect), while the addition of N decreased total C. The decreased soil total C induced by exogenous labile C addition was greater in the plantation than that in the natural forest. The influence of exudate additions produced no significant influence on labile and recalcitrant carbon pools at either soil. The addition of labile C exudate decreased the total phospholipid fatty acid (PLFA), actinomycetic, bacterial and fungal PLFA for the natural forest, but increased them in the plantation. Moreover, the microbial community composition (i.e. the value of bacterial PLFA:fungi PLFA) varied greatly among the treatments. These results indicate that both root-derived N inputs and soil N availability co-regulate the direction and magnitude of priming effects on SOM decomposition by controlling the activity and the relative abundance of bacterial and fungal. Our results provide additional evidences toward a robust theoretical foundation for better understanding the ecological consequences of exudate stoichiometry on soil C cycling in forests.

：Aims Stoichiometric ratios of carbon (C), nitrogen (N) and phosphorus (P) are important characteristics of the ecological processes and functions. Studies on population ecological stoichiometry can refine the content of flora chemometrics, determine the limited nutrient, and provide data for process-based modeling over large scale. Phyllostachys edulis is an important forest type, whose area accounts for 74% of total bamboo forest area in Southern China. However, little is known about the ecological stoichiometric in P. edulis. This study aimed to reveal C:N, C:P and N:P stoichiometry characteristics of the “plant-soil-litter” continuum and to provide a better understanding nutrient cycling and stability mechanisms in P. edulis forest in China. Methods The data were collected from the published literature containing C、N、P content in leaf or surface soil (0-20 cm) or littefall in P. edulis forests. Important findings 1) The leaf C, N, P content were estimated at 478.30 mg·g^-1, 22.20 mg·g^-1, 1.90 mg·g^-1 in P. edulis, and the corresponding C: N, C: P and N: P were 26.80, 299.60 and 14.40, respectively. Soil C, N, and P content in 0-20 cm were 21.53 mg·g^-1, 1.66 mg·g^-1, 0.41 mg·g^-1, with ratios of 14.20 for C:N, 66.74 for C:P and 4.28 for N:P. The C, N and P contents were 438.49 mg·g^-1, 13.39 mg·g^-1, 0.86 mg·g^-1 for litterfall, with the litter C:N, C:P and N:P being 25.53, 665.67, 22.55, respectively. 2) In the plant-soil-litter system in P. edulis forest, leaf had higher C:N, litter had higher C:P and N:P, while soil were the lowest. The N, P resorption rate was 39.68% and 54.74%, indicating that P. edulis forest growth and development was constrained by P or by both of N and P in China. 3) N content and N:P in leaf showed a tendency to increase with latitude, while the C:N of leaf declined with latitude. N:P of leaf increased with longitude, but the P content and the C:N of leaf showed a opposite trend. C: N of soil increased with longitude, whereas the N content of soil declined longitude. The N content of litter declined with longitude. 4) The leaf N content was negatively correlated with mean annual temperature and mean annual precipitation, but being more sensitive to temperature than precipitation. The positive correlations between N content and latitude support “Temperature-Plant Physiological” hypothesis, reflecting an adaptive strategy to environmental conditions.