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In a locale-scale mountainous forest, changes in slope aspect can cause microclimate heterogeneity, which can impact both soil environment and plant community, and influence the soil microbial communities as well. However, the relationship between the aboveground plant community and belowground soil microbial communities and the change in slope aspect is not well understood. A case study was carried out in a subalpine coniferous forest in western China to investigate the above- and belowground relationship of three slope aspects, the north-facing slope, the flat site and the south-facing slope. The plant community attributes were evaluated by the community survey, considering species diversity and the plant total basal area of trees and shrubs to represent the plant productivity. Soil bacteria were determined based on 16S rRNA gene and soil fungi on a nuclear ribosomal internal transcribed spacer (ITS) using high-throughput Illumina sequencing. The results showed that slope aspects significantly affect the aboveground plant productivity and diversity, and the community composition and structure of both aboveground plants and soil bacteria and soil fungi. Soil bacteria and soil fungi correlated differently with aboveground plant community changes in the slope aspects in terms of diversity and community composition and structure. A structural equation model (SEM) revealed that slope aspects caused by aboveground plant productivity changes correlated significantly with the soil fungal community composition and structure, while the soil bacterial community was rather weakly correlated with the plant community, and its changes in community composition and structure were mainly affected by the soil properties and soil fungal community. Further studies considering plant functional traits, soil microbial functional groups, and seasonal changes may reveal a deeper correlation between the aboveground plants and belowground soil microbials at the local scale in subalpine coniferous forests.

Increasing evidence shows that both abiotic and biotic factors affect species richness and stand biomass in forests, yet the relative and interactive impacts of these factors remain debated in different forest ecosystems. We sampled 55 forest plots (600 m2 per plot) on two subtropical mountains with distinct diversity levels in China to explore the elevational patterns of tree species richness and stand biomass and examined how they were affected by climate, stand structure, and dominance of mycorrhizal types. The tree species richness of both mountains decreased with elevation, while the stand biomass exhibited unimodal or no apparent trends. On both mountains, the tree species richness was strongly shaped by climatic factors, especially the mean annual temperature, whereas the stand biomass was mainly affected by the stand structure. Specifically, on the mountain with higher species richness, both the tree height variation and maximum tree size were strongly correlated with the stand biomass. Meanwhile, on the species-poor mountain with higher elevations, only the maximum tree size correlated with the stand biomass. The dominance of ectomycorrhizal trees also had positive effects on the stand biomass of both mountains. These results suggest that climate, stand structure, and mycorrhizal dominance may jointly drive the decoupling between tree species richness and stand biomass, which should be given more attention in further research and forest management to achieve the climate change mitigation goals.

As one of the main food bamboo species of the giant panda (Ailuropoda melanoleuca), Fargesia denudata is widely distributed in the understory of spruce–fir forests in the mountainous area of southwestern China. However, the driving factors of its biomass and distribution in the forests are still unclear. We conducted a systematic investigation of the tree and shrub layers (including bamboos) of 209 subplots (20 m × 20 m) in a Forest Global Earth Observatory plot, the Wanglang Plot (25.2 ha), to explore the effects of abiotic (topographic and soil characteristics) and biotic (tree density, total basal area (TBA), shrub coverage, etc.) factors on the aboveground biomass of F. denudata (bamboo biomass hereafter). Bamboo biomass averaged 1.17 ton/ha, with a large variation from 0 to 4.88 ton/ha (95% confidence interval) among the 209 subplots. Bamboo biomass increased significantly with elevation, slope and mean diameter at breast height of trees, and decreased significantly with tree density, shrub coverage and soil pH. However, bamboo biomass was not significantly correlated with tree TBA, aspect, soil organic matter or total nitrogen content. The random forest model indicated that topographic factors and biotic factors had greater influences on the bamboo biomass than soil characteristics in general. Specifically, topographic factors mainly affected the bamboo biomass by changing tree density and soil characteristics. Our results can provide valuable guidance for the protection of giant pandas and the management of subalpine spruce–fir forests.

Relative humidity (RH) cycling during the frequent startup/shutdown (SUSD) process may lead to the generation and propagation of mechanical defects in the polymer electrolyte membrane (PEM) fuel cell. In this study, a 2D finite element model for the evolution of internal defects in PEM fuel cell is established, and the interfacial behavior of the membrane/catalyst layer (CL) is simulated based on the cohesive zone model (CZM), focusing on the RH cycling effect on the membrane/CL delamination evolution. Emphasis is placed on the evolution of membrane/CL delamination when cracks have been generated in the CL. The results show that the existence of the CL crack aggravates the membrane/CL delamination, and the more the position of the CL crack is off-center, the more serious the delamination propagation is. The initial delamination length (IDL) and interface length synergistically affect the interface behavior of membrane/CL. Higher frequency (or shorter period) and greater amplitude of the RH cycle intensify the membrane/CL delamination.

1. Above- and belowground biomass allocation is an essential plant functional trait that reflects plant survival strategies and affects belowground biomass estimation in grasslands. However, due to the difficulty in distinguishing fine and living roots, field-based studies show large uncertainties in estimating the biomass allocation. In addition, how plant density regulates biomass allocation across various species remains poorly addressed. 2. To address this knowledge gap, we conducted greenhouse manipulation experiments to explore patterns of above- and belowground biomass allocation and its regulation by planting density in six common grass species of different functional types (i.e., Cvs C; annuals vs perennials) in China’s temperate grasslands. The six species were Chenopodium glaucum, Cleistogenes squarrosa, Leymus chinensis, Medicago sativa, Setaria viridis, and Stipa grandis. 3. We found that the mean values of root to shoot ratio (R/S) ranged from 0.04 to 0.92 across the six species, with much lower R/S values in annuals than in perennials (C. glaucum and S. viridisvsC. squarrosa, L. chinensis, M. sativa and S. grandis) and in C plants than in C plants (C. squarrosa vs L. chinensis, M. sativa and S. grandis). For C. squarrosa, increasing the planting density decreased the shoot biomass fraction (SMF) but increased the root biomass fraction (RMF) and R/S. In contrast, for the other five species, planting density had nonsignificant effects on the SMF, RMF, and R/S. In addition, the planting density significantly affected the allometric relationships between above- and belowground biomass. 4. Synthesis. Our results suggest that R/S values obtained from field investigations are severely overestimated and that the pattern and density regulation of the biomass allocation vary across species of different functional types. Our findings provide important insights into approximating difficult-to-measure belowground biomass with easier-to-measure aboveground biomass in grassland ecosystems, and also provide important theoretical foundations for grass-based livestock husbandry practices.

China is the largest reforestation country in the world and the accurate estimation of its forest biomass carbon (C) pool is critical for evaluating the country’s C budget and ecosystem services of forests. Although several studies have estimated China’s forest biomass using national forest inventory data, most of them were limited to the period of 2004–2008. In this study, we extended our estimation to the most recent period of 2014–2018. Using datasets of eight inventory periods from 1977 to 2018 and the continuous biomass expansion factor method, we estimated that the total biomass C pool and average biomass C density of Chinese forests increased from 4717 Tg C (1 Tg = 1012 g) in the period of 1977–1981 to 7975 Tg C in the period of 2014–2018 and 38.2 Mg C ha−1 to 45.8 Mg C ha−1 (1 Mg = 106 g), respectively, with a net increase of 3258 Tg C and an annual sink of 88.0 Tg C year−1. Over the recent 10 years (2009–2018) the averaged national biomass C density and C sink showed a respective increase of 5.0 Mg C ha−1 and 91.5 Tg C year−1 much larger than those of 39.6 Mg C ha−1 and 63.3 Tg C year−1 in the period of 1977–2008. These pronouncing increases were largely attributed to afforestation practices, forest age growth, and environmental changes. Our results have documented the importance of ecological restoration practices and provided an essential basis for assessing ecosystem services and achieving China's C neutrality target.

Microorganisms govern soil nutrient cycling. It is therefore critical to understand their responses to human-induced increases in N and P inputs. We investigated microbial community composition, biomass, functional gene abundance, and enzyme activities in response to 10-year N and P addition in a primary tropical montane forest, and we explored the drivers behind these effects. Fungi were more sensitive to nutrient addition than bacteria, and the fungal community shift was mainly driven by P availability. N addition aggravated P limitation, to which microbes responded by increasing the abundance of P cycling functional genes and phosphatase activity. In contrast, P addition alleviated P deficiency, and thus P cycling functional gene abundance and phosphatase activity decreased. The shift of microbial community composition, changes in functional genes involved in P cycling, and phosphatase activity were mainly driven by P addition, which also induced the alteration of soil stoichiometry (C/P and N/P). Eliminating P deficiency through fertilization accelerated C cycling by increasing the activity of C degradation enzymes. The abundances of C and P functional genes were positively correlated, indicating the intensive coupling of C and P cycling in P-limited forest soil. In summary, a long-term fertilization experiment demonstrated that soil microorganisms could adapt to induced environmental changes in soil nutrient stoichiometry, not only through shifts of microbial community composition and functional gene abundances, but also through the regulation of enzyme production. The response of the microbial community to N and P imbalance and effects of the microbial community on soil nutrient cycling should be incorporated into the ecosystem biogeochemical model.

Purpose The local microclimate of different slope aspects in the same area can not only impact soil environment and plant community but also affect soil microbial community. However, the relationship between aboveground plant communities and belowground soil microbial communities on various slope aspects has not been well understood.Methods We investigated the above- and belowground relationship on different slope aspects and explored how soil properties influence this relationship. Plant community attributes were evaluated by plant species richness and plant total basal area. Soil microbial community was assessed based on both 16S rRNA and ITS rRNA, using High-throughput Illumina sequencing. Results There was no significant correlation between plant richness and soil bacterial community composition on the north slope, but there was a positive correlation on the south slope and a significantly negative correlation on the flat site. There was a significantly negative correlation between soil fungal community composition and plant total basal area, which did not change with the slope aspect. In addition, there was no significant correlation between plant community species richness and soil microbial species richness.Conclusions In subalpine coniferous forests, the relationship between plant-soil bacteria varies with slope aspect, but the plant-soil fungi relationship is relatively consistent across different slope aspects. These results can improve our understanding of the relationship between plant and soil microorganisms in forest ecosystems under microtopographic changes and have important implications for the conservation of biodiversity and forest management in subalpine coniferous forests.

Alien species invasions are considered to be one of the multiple key drivers that trigger ecological regime shifts in ecosystem structure and function. Predicting population collapse and understanding the self-reinforced feedback mechanisms that erode the resilience of native species are two major challenges in invasion ecology. However, to date, empirical evidence of species invasion-induced regime shifts remains scarce, and the minimum required data to generate early warning signals (EWSs) before critical transitions remains unclear, despite its importance for ecosystem management. By combining 80-year (i.e., 1936–2016) fishery data from Lake Qiandao and a theoretical competition model, we provide evidence that relative to nutrient enrichment and climate change, stocking alien fish species of silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobtilis) is the most important driver leading to the collapse of local fish populations. Detectable EWSs indicated by the variance, autocorrelation and composite resilience indicators require at least ten years of data before an abrupt decline, demonstrating that a long-term ecological monitoring program is necessary for providing insight into resilience dynamics. A structural equation model (SEM) suggests that the combined effects of eutrophication, high water levels and carp stocking-induced water clarity decline may play an important role in the reduced fitness of local fish populations, which subsequently increase the carrying capacity of alien fish populations. Our results demonstrate that stocking alien carps result in competitive exclusion of local fish populations, especially in the current context of water eutrophication and global climate changes. Furthermore, quantifying the minimum required time series length with detectable EWS across various ecosystems should be further studied given that this information is critical for flexible management policies to take actions to avert the harmful transitions.

High atmospheric nitrogen (N) deposition and low soil phosphorus (P) availability occur simultaneously in tropical areas, and thus tropical plants need to adapt nutrient-use strategies to maintain growth and survival. Therefore, identifying the adaptative strategies of tropical plants at different successional stages under low soil P availability is indispensable. Here, we separately investigated foliar traits, photosynthetic characteristics, and P fractions of 8 species in the primary and secondary tropical forests after 10 years of N and P fertilization. P addition increased foliar P concentrations and deceased N:P ratio in the primary forest and secondary forest. The foliar photosynthetic rates did not significantly respond to nutrient additions, and the foliar photosynthetic P-use efficiency (PPUE) reduced under the P addition in the primary forest. In contrast, the foliar photosynthetic rates and photosynthetic nitrogen (N)-use efficiency (PNUE) were enhanced with nutrient additions in the secondary forest. The allocations of foliar nucleic acid P and residual P were reduced by P addition in the primary forest, whereas the allocation of metabolic P was enhanced and the allocation of residual P was reduced by P addition in the secondary forest. Additionally, a higher proportion of structural P was found in the primary forest, and a higher proportion of metabolic P was observed in the secondary forest. Interesting, structural equation model analysis revealed that the plants decreased the allocation of foliar nucleic acid P and increased the allocation of structural P in the primary forest, thereby reducing photosynthetic rates. Whereas the plants enhanced photosynthetic rates by promoting PPUE and the allocation of foliar metabolic P in the secondary forest. Our findings highlighted tropical plants at different successional stages can reasonably allocate foliar P to regulate photosynthetic rates and acclimate to low P environments.

Top-down cascade effects are among the most important mechanisms underlying community structure and abundance dynamics in aquatic and terrestrial ecosystems worldwide. A current challenge is understanding the factors controlling trophic cascade strength under global environmental changes. Here, we synthesized 161 global sites to analyze how multiple factors influence consumer-resource interactions with fish in freshwater ecosystems. Fish have a profound negative effect on zooplankton and water clarity but positive effects on primary producers and water nutrients. Furthermore, fish trophic levels can modify the strength of trophic cascades, but an even number of food chain length does not have a negative effect on primary producers in real ecosystems. Eutrophication, warming, and predator abundance strengthen the trophic cascade effects on phytoplankton, suggesting that top-down control will be increasingly important under future global environmental changes. We found no influence or even an increasing trophic cascade strength (e.g., phytoplankton) with increasing latitude, which does not support the widespread view that the trophic cascade strength increases closer to the equator. With increasing temporal and spatial scales, the experimental duration has an accumulative effect, whereas the experimental size is not associated with the trophic cascade strength. Taken together, eutrophication, warming, temporal scale, and predator trophic level and abundance are pivotal to understanding the impacts of multiple environmental factors on the trophic cascade strength. Future studies should stress the possible synergistic effect of multiple factors on the food web structure and dynamics.

Lakes have played a critical role in providing water and ecosystem services for people and other organisms in China for millennia. However, accelerating climate change and economic boom have resulted in unprecedented changes in these valuable lakes. Using Landsat images covering the entity of the country, we explored the changes in China’s lakes and the associated driving forces over the last 30 years (i.e. mid-1980s to 2015). We discovered that China’s lakes have changed with divergent regional trends: in the sparsely populated Tibetan Plateau, lakes are abundant and the lake area has increased dramatically from 38 596 to 46 831 km2 (i.e. increased by 8235 km2, or 21.3%), whereas, in the densely populated northern and eastern regions, lakes are relatively scarce and the lake area has decreased from 36 659 to 33 657 km2 (i.e. decreased by 3002 km2, or 8.2%). In particular, severe lake decreases occurred in the Mongolia-Xinjiang Plateau and the Eastern Plain (−2151 km2). Statistical analyses indicated that climate was the most important factor controlling lake changes in the Tibetan Plateau, the Yun-Gui Plateau and the Northeast Plain. However, the strength of climatic control on lake changes was low in the Eastern Plain and the Mongolia-Xinjiang Plateau, where human activities, e.g. impoldering, irrigation and mining, have caused serious impacts on lakes. Further lake changes will exacerbate regional imbalances between lake resources and population distribution, and thus may increase the risk of water-resource crises in China.

Above- and belowground biomass allocation is an essential plant functional trait that reflects plant survival strategies and affects belowground carbon pool estimation in grasslands. However, due to the difficulty of distinguishing living and dead roots, estimation of biomass allocation from field-based studies currently show large uncertainties. In addition, the dependence of biomass allocation on plant species, functional type as well as plant density remains poorly addressed. Here, we conducted greenhouse manipulation experiments to study above- and belowground biomass allocation and its density regulation for six common grassland species with different functional types (i.e., C3 vs C4; annuals vs perennials) from temperate China. To explore the density regulation on the biomass allocation, we used five density levels: 25, 100, 225, 400, and 625 plant m-2. We found that mean root to shoot ratio (R/S) values ranged from 0.04 to 0.92 across the six species, much lower than those obtained in previous field studies. We also found much lower R/S values in annuals than in perennials (C. glaucum and S. viridis vs C. squarrosa, L. chinensis, M. sativa and S. grandis) and in C4 plants than in C3 plants (C. squarrosa vs L. chinensis, M. sativa and S. grandis). In addition to S. grandis, plant density had significant effects on the shoot and root biomass fraction and R/S for the other five species. Plant density also affected the allometric relationships between above- and belowground biomass significantly. Our results suggest that R/S values obtained from field investigations may be severely overestimated and that R/S values vary largely across species with different functional types. Our findings provide novel insights into approximating the difficult-to-measure belowground living biomass in grasslands, and highlight that species composition and intraspecific competition will regulate belowground carbon estimation.

Atmospheric nitrogen (N) deposition has a significant influence on soil organic carbon (SOC) accumulation in forest ecosystems. Microbial residues, as by-products of microbial anabolism, account for a significant fraction of soil C pools. However, how N deposition affects the accumulation of soil microbial residues in different forest biomes remains unclear. Here, we investigated the effects of six/seven-year N additions on microbial residues (amino sugar biomarkers) in eight forests from tropical to boreal zone in eastern China. Our results showed a minor change in the soil microbial residue concentrations but a significant change in the contribution of microbial residue-C to SOC after N addition. The contribution of fungal residue-C to SOC decreased under low N addition (50 kg N ha−1 yr−1) in the tropical secondary forest (−19%), but increased under high N addition (100 kg N ha−1 yr−1) in the temperate Korean pine mixed forest (+21%). The contribution of bacterial residue-C to SOC increased under the high N addition in the subtropical Castanopsis carlesii forest (+26%) and under the low N addition in the temperate birch forest (+38%), respectively. The responses of microbial residue-C in SOC to N addition depended on the changes in soil total N concentration and fungi to bacteria ratio under N addition and climate. Taken together, these findings provide the experimental evidence that N addition diversely regulates the formation and composition of microbial-derived C in SOC in forest ecosystems.

Background and aims Plant elemental composition is of fundamental importance for plant growth and metabolic functions. However, knowledge of how multi-elemental stoichiometry responds to varying nitrogen (N) and phosphorus (P) availabilities remains limited. Methods We conducted experimental manipulations with nine repeat experiments to investigate the effects of N and P supply on the concentrations and variability of six elements, carbon (C), N, P, potassium (K), calcium (Ca) and magnesium (Mg), in leaves of Arabidopsis thaliana. Key results N supply increased the concentrations of N, K and Mg, decreased the concentration of P, but exerted little influence on the concentrations of C and Ca in green leaves. P supply increased the concentrations of P and Ca, decreased the concentration of C, initially increased and then decreased the concentration of K, but showed little influence on the concentrations of N and Mg in green leaves. Multivariate patterns among the concentrations of these six elements in green leaves was influenced by the type of nutrient supply (i.e. N or P). Elemental variability decreased with increasing elemental concentrations in green leaves at the intraspecific level, supporting the Stability of Limiting Elements Hypothesis that was originally proposed from a meta-analysis of pooled data across species or communities. Compared with green leaves, the senesced leaves showed greater variability in C, N, P, K and Mg concentrations but lower variability in Ca concentration. Conclusions N and P supplies exerted differential influences on the concentrations of C, N, P, K, Ca and Mg in green leaves. The specific C content should be considered when assessing C cycling under global nutrient changes. Stage-dependent patterns of leaf stoichiometric homeostasis differed among elements with various chemical characteristics. These findings can help to improve our understanding of plant eco-physiological responses and acclimation under global nutrient changes from the stoichiometric perspective of multiple elements.

Rapid increase of global nitrogen (N) deposition has greatly altered carbon cycles and functioning of forest ecosystems. Previous studies have focused on changes in carbon dynamics of temperate and subtropical forests through N enrichment experiments; however, the effects of N deposition on tree growth remain inconsistent, especially in tropical forests. Here, we conducted a five-year N addition experiment (0 and 50kgNha-1yr-1) in a tropical montane rain forest in Hainan Island, China, to explore the effects of enhanced N deposition on growth of trees. We also set phosphorus (P) treatment (50kgPha-1yr-1) and N+P treatment (50kgNha-1yr-1+50kgPha-1yr-1) to examine potential P limitation driven by N deposition. Our results showed that N addition has not significantly influenced tree growth, while P addition significantly increased the relative growth rate of small (diameter at breast height, DBH≤10cm) and medium (10

Abstract. Plant carbon (C) content is one of the most important plant traits and is critical in the assessment of global C cycle and ecological stoichiometry. However, the global variation in plant C content remains poorly understood. We conducted a global analysis of the plant C content by synthesizing data from 4318 species to provide specific values of C content and to assess their variation across plant organs and life forms. Our results showed that C content varied markedly across plant organs. Plant organ C content ranged from 45.01 % in reproductive organs to 47.88 % in stems at global scales, which were significantly lower than a canonical value of 50 % that has been widely employed in previous studies. Plant C content in leaves was higher than that in roots. Across life forms, woody plants exhibited higher C content than herbaceous plants. Conifers, relative to broad-leaved woody species, had higher C content in roots, leaves and stems. Plant C content tended to decrease with the increasing latitude. The life form explained more variation of the C content than climate due to plant structural requirements. Our findings suggest that specific C content values from different organs and life forms may be more suitable to evaluate global vegetation C stock and plant ecological stoichiometry.

Soil extracellular enzymes play a key role in mediating a range of forest ecosystem functions (i.e., carbon and nutrients cycling and biological productivity), particularly in the face of atmospheric N deposition that has been increasing at an unprecedented rate globally. However, most studies have focused only on surface soils in a single ecosystem. In this study, we aimed to determine whether the effect of simulated N deposition on the activities and ratios of soil enzymes changes with soil depth across six forest ecosystems in eastern China. We collected soil samples from three blocks×four soil depths (0-10cm, 10-20cm, 20-40cm and 40-60cm)×three N treatment levels (control, 50 and 100kgNha-1year-1) at each of the six forest ecosystems. We measured the activities of seven soil enzymes involved in C-, N- and P-cycling. We found that 4-5years of N addition had no significant effect on the activities and ratios of these enzymes in most cases. The interactions among N addition, site and soil depth on soil enzyme activities were not significant, except that acid phosphatase activity showed site-specific responses to N addition. Our findings suggest that the activities of soil enzymes involved in C- and N-cycling generally do not track simulated N deposition in the six forest ecosystems. Further work on plant, soil and microbial characteristics is needed to better understand the mechanisms of soil enzyme activities in response to N deposition in forest ecosystems.

Nitrogen (N) and phosphorus (P) are essential components of the basic cell structure of plants. In particular, leaf N and P concentrations and their stoichiometric relationship largely determine the photosynthesis, growth, reproduction, and ecophysiological processes of plants. As important leaf functional traits, leaf N and P concentrations and their stoichiometric relationship play vital roles in indicating plant nutrient‐use strategies and their evolution in terrestrial ecosystems. They also influence physiological and ecological processes in leaves (e.g., growth rate and energy metabolism) and productivity (e.g., net primary production and net ecosystem production) at ecosystem level. However, the lack of a comprehensive data set containing paired leaf N and P concentration records has distinctly limited research on nutrient stoichiometry and leaf functional traits. Here, we provide a global database of paired records of leaf N and P concentrations. A total of 11,354 individual records were acquired spanning 1,291 sites worldwide, including 201 families, 1,265 genera, and 3,227 species. The records span a latitudinal range of 45.28 °S to 68.35 °N and a longitudinal range of 155.5 °W to 168.0 °E. The variables provided for each individual record are (1) geographical location (longitude, latitude, and altitude); (2) matched leaf N and P concentrations and N:P ratio; (3) taxonomic information (family, genera, and species); (4) life form (angiosperm/gymnosperm, monocotyledonous/dicotyledonous and woody plants/herbaceous plants; note that woody plants were further divided into coniferous, deciduous broad‐leaved, and evergreen broad‐leaved woody species and that herbaceous plants were further divided into annual and perennial species); (5) mean annual temperature (MAT) and mean annual precipitation (MAP); and (6) soil N and P concentrations and pH value in some records. To date, this database is the world's largest database of paired leaf N and P concentrations, which contains matched information of geographical location, environmental factors, and taxa. We believe that the database will play a fundamental and crucial part of ecological stoichiometric studies. There are no copyright restrictions. When using this database, we kindly request that you cite this article, respecting all the authors’ hard work during sample collection and data compilation.

Stem carbon (C) content is widely used to present tree C content to estimate forest C stocks. However, size- and age-dependent changes in tree stem C content are still unclear. Based on 576 tree size (expressed by diameter at breast height (DBH) and biomass), age and C content data, our results showed that C content varied significantly among organs, and the mean value of C content for bark, branch, leaf, reproductive organ, root and stem was 48.4 %, 49.2 %, 49.6 %, 50.1 %, 48.8 %, 49.7 %, respectively. C content of stem was significantly correlated with that of leaf, branch and root, while showed no relationship with that of bark and reproductive organ. With the increasing tree size and age, stem C content showed increasing trends. Using stem C content as tree C content could produce an error of -2.49%−5.87% in the estimations of forest C stock. Thus, it is necessary to consider tree organ C content of stand in estimating forest C stock.

Soil carbon (C) stabilization has become an important topic in recent years in the context of global climate change. Increasing evidence suggests that microbial residues represent a significant fraction of persistent soil C pools. However, patterns and determinants of soil microbial residues across different biomes remain poorly understood. Here, by combining field investigations from tropical to boreal forests in eastern China with a meta-analysis across global forest biomes, we assessed how biotic and abiotic factors governed the geographic pattern of soil microbial residues indicated by amino sugars. We found that microbial residue concentrations in soils increased significantly from tropical to boreal forests in China and across the globe, which was mainly driven by temperature and soil nitrogen (N). Compared with bacterial residues, fungal residues preferentially accumulated at high latitudes. However, a significant negative relationship existed between soil C/N ratio and total residue-C/SOC in all examined forest soils, indicating a greater contribution of microbial residue-C to SOC in tropical forests than in boreal forests. These findings demonstrate that climate and soil play an important role in microbial-mediated soil C stabilization from boreal to tropical forests.

Microbial residue retention is critical for soil organic carbon (SOC) accumulation in forests. However, the changes in soil microbial residues under atmospheric nitrogen (N) and phosphorus (P) deposition in tropical forests remain unclear. Here, we measured soil phospholipid fatty acids, enzyme activity, and amino sugars (biomarkers of microbial residues) in a seven-year N and P addition experiment in a secondary tropical montane rainforest. Our results showed that five years of nutrient additions showed no significant effect on microbial community structure but the effect became significant at the seventh year. In N addition plots, a decrease in the arbuscular mycorrhizal fungal biomass resulted in declines in the contribution of fungal residues to SOC and the ratio of fungal to bacterial residues. The fungi:bacteria ratio increased in P and N&P addition plots while the fungal residue:bacterial residue ratio was decreased by P addition, which may be related to an imbalance in microbial residue decomposition. Both extracellular enzyme activities and SOC showed minor changes under nutrient additions. Our results suggest that changes in microbial community structure and enzyme activity induced by N and P deposition may alter the accumulation and composition of microbial residues in SOC in N-rich tropical forests.

China has been experiencing a rapid increase in nitrogen (N) deposition due to intensified anthropogenic N emissions since the late 1970s. By synthesizing experimental and observational data taken from literature, we reviewed the responses of China's forests to increasing N deposition over time, with a focus on soil biogeochemical properties and acidification, plant nutrient stoichiometry, understory biodiversity, forest growth, and carbon (C) sequestration. Nitrogen deposition generally increased soil N availability and soil N leaching and decreased soil pH in China's forests. Consequently, microbial biomass C and microbial biomass N were both decreased, especially in subtropical forests. Nitrogen deposition increased the leaf N concentration and phosphorus resorption efficiency, which might induce nutrient imbalances in the forest ecosystems. Although experimental N addition might not affect plant species richness in the overstorey, it did significantly alter species composition of understory plants. Increased N stimulated tree growth in temperate forests, but this effect was weak in subtropical and tropical forests. Soil respiration in temperate forests was non-linearly responsive to N additions, with an increase at dosages of 60 kg N ha−1 yr−1. However, it was consistently decreased by increased N inputs in subtropical and tropical forests. In light of future trends in the composition (e.g., reduced N vs. oxidized N) and the loads of N deposition in China, further research on the effects of N deposition on forest ecosystems will have critical implications for the management strategies of China's forests.

Increasing nitrogen (N) deposition has aroused large concerns because of its potential negative effects on forest ecosystems. Although microorganisms play a vital role in ecosystem carbon (C) and nutrient cycling, the effect of N deposition on soil microbiota still remains unclear. In this study, we investigated the responses of microbial biomass C (MBC) and N (MBN) and microbial community composition to 4–5 years of experimentally simulated N deposition in temperate needle-leaf forests and subtropical evergreen broadleaf forests in eastern China, using chloroform fumigation extraction and phospholipid fatty acid (PLFA) methods. We found idiosyncratic effects of N addition on microbial biomass in these two types of forest ecosystems. In the subtropical forests, N addition showed a significant negative effect on microbial biomass and community composition, while the effect of N addition was not significant in the temperate forests. The N addition decreased MBC, MBN, arbuscular mycorrhizal fungi, and the F/B ratio (ratio of fungi to bacteria biomass) in the subtropical forests, likely due to a decreased soil pH and changes in the plant community composition. These results showed that microbial biomass and community composition in subtropical forests, compared with the temperate forests, were sensitive to N deposition. Our findings suggest that N deposition may have negative influence on soil microorganisms and potentially alter carbon and nutrient cycling in subtropical forests, rather than in temperate forests.