The stability of ecosystems determines whether they can sustainably provide key functions and services in the background of global changes. Ecosystem stability, particularly its relation with biodiversity, is one of the central issues in ecology. Whether biodiversity enhances or impairs ecosystem stability has historically aroused much debate. Based on early reviews and studies on different aspects of stability, here we summarized recent advances from three aspects. Firstly, several recent theoretical studies offered novel insights in understanding the multi- dimensionality of stability and the intrinsic link between different stability measures, and we provided an overview on these new insights. Secondly, we reviewed recent empirical and theoretical studies on biodiversity- stability relationships, including those in the context of multidimensional stability. Thirdly, we introduced the recently developed multi-scale stability framework, which provides new opportunity to understand the scaling of stability and extend diversity-stability relations to a multi-scale context. We ended with a discussion on future research questions and directions.

Leaf is one of the important organs of plants that facilitates the exchange of water and air with the surrounding environment. The morphological variation of leaves directly affect the physiological and biochemical processes of plants, which also reflects the adaptive strategies of plants to obtain resources. By focusing on several leaf morphological traits, including leaf size, leaf shape, leaf margin (with or without teeth) and leaf type (i.e. single vs. compound leaf), here, we reviewed the relevant research progresses in this field. We summarized the ecological functions of leaf morphological traits, identified their geographical distribution patterns, and explored the underlying environmental drivers, potential ecological interactions, and their effects on ecosystem functioning. We found that the current studies exploring the distribution and determinants of leaf size and leaf margin states mainly focused on single or specific taxon in local regions. Studies have also explored the genetic mechanisms of leaf morphology development. Leaf traits trade off with other functional traits, and their spatial variation is driven by both temperature and water availability. Leaf morphological traits, especially leaf size, influence water and nutrient cycling, reflect the response of communities to climate change, and can be scaled up to predict ecosystem primary productivity. Further studies should pay attention to combine new approaches to obtain unbiased data with high coverage, to explore the long-term adaptive evolution of leaf morphology, and to generalize the scaling in leaf morphology and its effect on ecosystem functioning. Leaf provides an important perspective to understand how plants respond and adapt to environmental changes. Studying leaf morphological traits provides insight into species fitness, community dynamics and ecosystem functioning, and also improves our understanding of the research progresses made in related fields, including plant community ecology and functional biogeography.

Multi-temporal NOAA/AVHRR NDVI digital images and monthly temperature and precipitation data were obtained for 160 standard observatories across China covering the period 1983 to 1992. The relationships among these were analyzed for trends in temporal and geographic variation with the results that: correlation coefficients declined in a north to south direction across the data and increased from the southeast to the northwest of China. The NDVI changes of the temperate desert, steppe and meadow vegetation regions were strongly influenced by the changes of temperature and precipitation, but the influence of climate was relatively small for some agriculture and forest vegetation types in the southeast of China.

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

Aims Drought effects on terrestrial ecosystems are a key issue in global change research. This study was designed to 1) analyze effects of drought on carbon exchange in a subtropical coniferous plantation; 2) elucidate the sensitivity of carbon exchange to different degree of water deficit and the critical values when the ecosystem converts from carbon sink to source and 3) investigate the main factors that control ecosystem carbon exchange when drought occurs.

Methods The CEVSA2 model, which incorporated several significant modifications based on the CEVSA process-based ecosystem model and has been tested by using eddy covariance observation in different forest ecosystems, was parameterized by using site-specific ecophysiological measurements. Drought scenarios were designed to analyze effects on annual carbon budget and to elucidate the main control factors.

Important findings Drought decreases ecosystem production and carbon exchange significantly. Compared with simulation of no drought effect scenario, the droughts in 2003 and 2004 decrease annual net ecosystem production (NEP) by 63% and 47%, respectively. Ecosystem photosynthesis and respiration respond to drought differently, and the more rapid decrease of gross ecosystem production (GEP) than ecosystem respiration (Re) lead to the decrease of NEP when drought occurs. As daily average vapor pressure deficit (VPD) rises above 1.5 kPa, GEP, Re and NEP begin to decrease; When VPD rises above 2.5 kPa and relative soil water content (RSW; soil water content/saturated soil water content) decreases below 40%, the ecosystem converts from a carbon sink to source. Soil water deficit, which is the main factor controlling the ecosystem carbon exchange, accounts for 46% to the decrease of total annual NEP in 2003 and 2004, and atmospheric drought accounts for only 4%.

As the increasing pressure caused by climatic changes and human activities, the structure and function of terrestrial ecosystems are undergoing dramatic changes. Understanding how ecosystem processes change at large spatial-temporal scales is crucial for dealing with the threats and challenges posed by global climate change. Traditional field survey method can obtain accurate plot-level ecosystem observations, but it is difficult to be used to address large-scale ecosystem patterns and processes because of spatial and temporal discontinuities. Compared to traditional field survey methods, remote sensing has the advantages of real-time acquisition, repeated monitoring and multi spatial-temporal scales, which can compensate for the shortcomings of traditional field observation methods. Remote sensing can be used to identify the type and characteristic of ground objects, and extract key ecosystem parameters, energy flow and material circulation through retrieving the information contained by electromagnetic signals. Remote sensing data have become an indispensable data source in ecological studies, especially at the ecosystem, landscape, regional or global scales. With the emergence of new remote sensing sensors (e.g., light detection and ranging, and solar-induced chlorophyll fluorescence) and near-surface remote sensing platforms (e.g., unmanned aerial vehicle and backpack), remote sensing is entering the three-dimensional era and the observation platform become more diverse. These three-dimensional, multi-source and time-series remote sensing data bring new opportunities to fully understand ecosystem processes across different spatial scales. This paper reviews the advances of the application of remote sensing in terrestrial ecosystem studies. Specifically, this study focuses on the derivation of biological factors from remote sensing data, including vegetation types, structures, functions and biodiversity of terrestrial ecosystems. We also summarize the current status of the remote sensing technology in ecosystem studies and suggest the future opportunities of ecosystem monitoring in China.

Global change has exerted profound impacts on ecosystem function, such as variations in plant productivity and imbalances in nutrient cycling. Previous studies mostly focused on the impacts of global change on individual functions. However, ecosystems have multiple functions, known as ecosystem multifunctionality (EMF), such that the evaluation based on a single functionality is inappropriate to reflect the overall performance of ecosystems due to the occurrence of trade-offs or synergies among the differential functions. This imposes limitation to our understanding of the effects of global change on ecosystems. Since the initial quantitative study of EMF by Hector and Bagchi in 2007, this field of research has undergone rapid development and the environmental impacts on EMF have received wide attention with intensification of global change. In order to gain systematic understanding of the progress in EMF studies, we conducted a bibliometric analysis for the period 2007-2020 based on CNKI and ISI Web of Science databases. This paper provides a brief description of the development in EMF research and summary of studies concerning the impacts of land use change, warming, changes in precipitation, and nitrogen deposition on EMF. We raised six issues of further attention in future studies of EMF in the context of global change, including (1) requirement of consensus in EMF indices and evaluation method; (2) consideration on the interactive effects among different factors on EMF; (3) elucidation of EMF responses to global change across various temporal scales; (4) understanding of the relationships between multi-dimensional, multi-scale biodiversity and EMF; (5) understanding of the relationships between multiple trophic diversity and EMF; and (6) understanding of the relationships between root functional traits and EMF.

Atmospheric nitrogen deposition has increased in the last several decades due to anthropogenic activities and global changes. Increasing nitrogen deposition has become an important factor regulating carbon cycle in grassland ecosystems. Litter decomposition, a key process of carbon and nutrient cycling in terrestrial ecosystems, is the main source of soil carbon pool and the basis of soil fertility maintenance. Elevated nitrogen deposition could affect litter decomposition by raising soil nitrogen availability, increasing the quantity and quality of litter inputs, and altering soil microorganism and soil conditions. Litter decomposition are complex biological, physical and chemical processes, which were affected by abiotic, biological factors and their interactions. The effects of nitrogen deposition on litter decomposition and the underlying mechanisms were discussed in this paper, including the aspactes of soil nitrogen availability, litter production, litter quality, microclimate, soil microorganism and enzyme activities. The main research contents, directions, methods and existing problems of litter decomposition in grasslands were discussed. We also discussed the prospect of future directions to study the interaction and feedback between nitrogen deposition and grassland ecosystem carbon cycling process.

Nitrogen (N) and phosphorus (P) inputs induced by anthropogenic activities and atmospheric N and P deposition have largely increased the availability of soil N and P in terrestrial ecosystems, which have considerably affected terrestrial carbon cycling processes. Tree growth and productivity in forest ecosystems play an important role in global carbon cycling, and determine the magnitude and direction of terrestrial carbon sequestration. Currently, a large number of field manipulation experiments have been conducted to investigate the effects of N and/or P addition on tree growth and forest productivity, but the results from these studies were inconsistent. Such inconsistent results might be affected by multiple factors, including biological, environmental and experimental variables. Here, we reviewed the present research status of the effects of N and P addition on tree growth and forest productivity in forest ecosystems based on three aspects, including the number of publications and experiments with field N and P addition, and the global distributions of these experiments. Then, we summarized the methods for assessing tree growth and forest productivity at ecosystem level in forest ecosystems, including relative growth rate and absolute increment. According to the related results, we reviewed the regulating factors that affect tree growth and productivity, and the potential mechanisms for such factors, including climate, tree size and stand age, plant functional traits (including type of tree-associated mycorrhizal fungi, N-fixation property of trees, and conservative and acquisitive functional traits), plant-microbe interaction, ambient nutrient (i.e., N and P) deposition rate, and experimental variables. Finally, we summarized the current studies, and pointed out five aspects that are urgently needed to provide further insights in future studies, including the physiological mechanism of how tree growth responds to N and P addition, the tradeoff and allocation among growth of various parts of tree under N and P addition, the role of plant functional traits in regulating and predicting the responses of tree growth to N and P addition, how the competition among trees regulates the responses of tree growth to N and P addition, and conducting long-term and coordinated distributed field experiments investigating the effects of N and P addition on tree growth and forest productivity at the global scale.

Over the recent decade, biodiversity and ecosystem multifunctionality (BEMF) has aroused as an emerging reserach hotspot in the filed of biodiversity and ecosystem functioning. Ecosystem multifunctionality is defined as the capacity of an ecosystem to provide multiple ecosystem functions simulateneously, it has received broad consideration by community and ecosystem ecologists. In this study, we first conducted a literature review of the research history in biodiversity and ecosystem multifunctionality. Next, we summarized the major trends in biodiversity and ecosystem multifunctionality research including the impacts of biodiversity dimensions, global change drivers and spatial-temporal scales on ecosystem multifunctionality. We reviewed the new research methods and research directions emerged in the field. We also defined a new concept, i.e., ecosystem multiserviceability (EMS) based on the distinction between ecosystem functions and ecosystem services. Finally, we briefly summarized the limitations in current research of biodiversity and ecosystem multifunctionality/multiserviceability (BEMF/BEMS) and presented the outlook for future study.

In recent years, there has been increasing concern about the impacts of drought stress on terrestrial ecosystem productivity and the carbon cycle in the context of global change. In this paper, we have reviewed recent progresses in understanding how drought stress affects terrestrial ecosystem processes and how ecosystems adapt to increasing drought stress. Drought stress could cause terrestrial ecosystems to act as a carbon source to the atmosphere by decreasing terrestrial gross primary productivity. Drought stress also results in a reduction of both autotrophic and heterotrophic respiration. Drought often associates with high rates of fire intensity, plant mortality and disease, which could lead to a large reduction of terrestrial ecosystem productivity. However, plant and ecosystem respond to drought dress in a complex way. There are three adaptation strategies that plants can live with a drought condition: 1) some plants adjust their growing season to avoid drought stress; 2) some other plants modify their internal mechanism to counter drought stress; 3) the other plants hold some physiological properties to tolerate drought stress. Experimental and modeling investigations of how ecosystems respond to drought and associated stresses are clearly needed in the future research.

Comprehensively understanding the mechanisms underlying the formation of ecosystem services is a prerequisite for maintaining the sustainable supply of ecosystem services. Plant functional traits directly participate in a variety of ecosystem processes, which in turn affect the supply of ecosystem services. Revealing the relationship between plant functional traits and ecosystem services is an important way to understand the formation mechanism of ecosystem services. Based on a systematic literature review, 86 papers on plant functional properties and ecosystem services were retrieved in the Web of Science database, and data for 466 pairs of plant functional traits and ecosystem services and 83 plant functional traits were collected. The current status of research on the relationship between plant functional traits and ecosystem services was revealed. Moreover, the main plant functional traits that affect different ecosystem services and their mechanisms underlying their impacts were also demonstrated. The results show that the research on the relationship between plant functional traits and ecosystem services mostly focuses on natural ecosystems such as grasslands and forests. Most of these studies focus on ecosystem products providing and supporting services, including biomass, net primary productivity, and soil fertility. Based on the impacts of plant functional traits on different ecosystem services, the plant functional traits can be clustered into five categories: soil-conservation-related traits, water-cycle-related traits, ecosystem- multifunction- related traits, product-providing-related traits, and pollination-biocontrol-related traits. The impacts of climate change, human activities, and variations in spatial and temporal scales on the relationship between plant functional traits and ecosystem services need to be further explored.

Aims Our objective was to determine the effects of changes in global pattern of precipitation on soil respiration and the controlling factors. Methods Data were collected from literature on precipitation manipulation experiments globally and a meta-analysis was conducted to synthesize the responses of soil respiration to changes in precipitation regimes. Important findings We found that an increased precipitation stimulated soil respiration while a decreased precipitation suppressed it. When changes in rainfall were normalized to the average treatment level (41% of the current annual precipitation), the level of increases in soil respiration with increased precipitation (49%) were higher than that of decreases with decreased precipitation (21%), showing an asymmetric responses of soil respiration to increases and decreases in precipitation. Soil moisture occurred as the most predominant factor driving the changes in soil respiration under altered precipitation. Changes in soil moisture affected soil respiration directly and indiscreetly by changing aboveground/belowground net primary productivity and microbial biomass carbon, which collectively contributed 98% of variations in soil respiration. In addition, the responses of soil respiration to altered precipitation varied with background temperature and precipitation. The sensitivity of soil respiration increased with local mean annual temperature when precipitation was reduced, while remaining unchanged when precipitation was increased. Meanwhile, the sensitivity of soil respiration to either increases or decreases in precipitation decreased with increasing local mean annual precipitation. Under future altered precipitation regimes, the sensitivity of soil respiration to changes in precipitation is likely dependent of local environment conditions.

Globally elevated temperatures and changed precipitation distributions may lead to deficits of fresh water that reduce crop yields and degrade natural ecosystems. Plant carbon (C) and nitrogen (N) metabolism and its abiotic environmental regulation are responsible for net primary productivity and plant nutrient status. We review the relationship between C and N and regulation by environmental factors such as temperature, water moisture and CO₂ enrichment at multiple levels of plant organization, including molecule, tissue, organ, whole plant and ecosystem. For cereal crops including wheat and rice, grain N mainly includes: 1) N reallocated in vegetative organs before anthesis, and 2) N absorbed from soil after anthesis. Their proportions depend on the activity and size of grains as an N sink and species and cultivars, affecting the grain yield and quality. Leaf N level can explain 45%-75% of leaf photosynthesis, and 71%-88% of leaf N can be allocated into protein, with Rubisco, the key enzyme for photosynthesis, accounting for 30%-50% of total leaf soluble protein, making it the protein using most N. Furthermore, the N proportions among the photosynthetic organs and the ratio between soluble sugar and starch may be associated with the Rubisco gene. Therefore, plant N level may be assessed by photosynthetic capacity.
Many studies have demonstrated that drought can promote C allocation to below-ground parts of plants, increasing root:shoot biomass ratio. There is, however, evidence that this enhancement of roots due to moderate drought can be negated by severe drought. On the other hand, drought also increases N concentration in sink organs, such as wheat grains, and decreases mature leaf N concentration, decreasing leaf net photosynthetic rate. However, high temperature does not significantly increase C allocation to roots, but may decrease leaf N concentration and affect Rubisco level. Thus, a decline of photosynthetic capacity induced by above optimal temperature, particularly at night, may be ascribed to an adverse effect on photosystem Ⅱ (PSⅡ). Generally, elevated CO₂ dilutes plant tissue N, leading to a lower C:N ratio that may come from the effect on Rubisco expression. Whole ecosystem N allocation and cycle can be affected by elevated CO₂, thereby changing ecosystem structure and function. The interaction between severe drought and high temperature can lead to a decrease in leaf N, reducing plant C-fixing ability, depending on the time and severity of stresses. Under high CO₂ concentration and drought, the C allocation into below-ground parts can be enhanced and the C:N ratio may increase. The interaction between elevated CO₂ and high temperature can alter plant tissue N allocation, with elevated CO₂ increasing CO₂ site activity of Rubisco and decreasing N investment in photosynthetic apparatus, but high temperature increasing O₂ site activity of Rubisco and increasing N investment. Nevertheless, under global change conditions, the combined effects from various stress factors are complex, and may include both positive and negative relationships. Research is urgently needed to 1) elucidate plant C and N allocation models from molecular to ecosystem levels; 2) address synergistic effects of multiple environmental stresses; 3) predict C and N allocation based on different global change scenarios; 4) quantify the threshold for change in C and N allocation in response to global change; and 5) strengthen knowledge of the key role of C and N allocation in agricultural and forest productivity and conservation of natural ecosystems.

Aims Climatic change has and will continue to decrease summer precipitation in the Dongling Mountain area of Beijing, China. Decreased precipitation impacts trees and hence temperate forest vegetation. Experimental studies suggested that the effects of decreasing summer precipitation on forest were closely related to species-specific characteristics during drought. Our major goals were to project the impact of decreasing summer precipitation on forest dynamics in this region and to analyze long-term consequences of tree-species specific drought response of the temperate forest ecosystem.
Methods We used LPJ-GUESS dynamic vegetation model coupled with different water uptake strategies to investigate drought effects on trees and forests in this temperate region of China.
Important findings Increases in net primary productivity (NPP) and carbon biomass of the predicted area under future climate conditions of increased temperature and elevated CO₂ concentration were independent of summer precipitation. This suggests that precipitation will not be the limiting factor in this area. However, tree diversity strongly depended on the drought response that we assumed. Drought-sensitive tree species (e.g., Juglans mandshurica) were not influenced by long-term drought, whereas the carbon biomass of the most drought-tolerant species (i.e., Quercus liaotungensis) would decrease in the future. Moreover, tree-species specific drought response will affect the water cycle of the temperate forest, including evapotranspiration. Our findings of the species-specific drought response should be considered in future ecosystem models.

Aims Droughts are common in arid and semiarid regions and affect the capacity of carbon sequestration of grassland ecosystems by influencing the process of ecosystem carbon cycling. We analyzed the continuous measurements of ecosystem CO₂ fluxes during three growing seasons (May-September) over a Leymus chinensis steppe in Inner Mongolia in order to examine the effect of drought stress on carbon accumulation of this grassland ecosystem.

Methods We used the eddy covariance technique to measure CO₂ fluxes during the 2004-2006 growing seasons. Only 126 and 215 mm precipitation fell during the 2005 and 2006 growing seasons, respectively, far less than normal (in 2004, 364 mm); therefore, the steppe was in an extreme drought condition.

Important findings Maxima for gross primary productivity (GPP) and ecosystem respiration (R_e) were 4.89 and 1.99 g C·m^-2·d^-1, respectively, in the 2004 growing season (normal year). However, in drought years, GPP and R_e were 1.53-3.01 and 1.38-1.77 g C·m^-2·d^-1, respectively. GPP and R_e in the drought years decreased by 68% and 11%, respectively, compared with the normal year. Accumulated GPP and R_e were 294 and 180 g C·m^-2, respectively, during the growing season in 2004 and 102-123 and 132-158 g C·m^-2, respectively, in drought years. Accumulated GPP and R_e decreased 58%-65% and 12%-27%, respectively, in drought years compared with those of the normal year. The slope of the curve in the sensitivity for R_e to T_s (Vant’Hoff type) reached its maximum at θ = 0.16-0.17 m³·m^-3; below or above this value of θ, the sensitivity of R_e to T_s decreases. GPP and R_e decline under drought stress conditions, with GPP having a larger decline. Long-term and continuous drought reduced C-accumulation and resulted in the steppe ecosystem switching from a carbon sink in typical years to a carbon source in drought years.

The response and feedback of ecosystems to global change is a scientific frontier in ecosystem ecology, which combines macro- and micro-level studies across multidisciplines. It focuses on the responses of ecosystem structure and function to global change, and its objective is to achieve sustainable use of ecosystem services. Based on the review of previous studies, we summarized the major progress and main achievements in this field and made an outlook for future challenges. According to the research content and object, this special issue systematically reviewed the effects of different global change factors, including increasing atmospheric CO₂ and O₃ concentration, global warming, precipitation change, increasing nitrogen deposition and land use change, on terrestrial plant ecophysiology, community structure, and ecosystem functions, and global change impacts on marine ecosystems. It mainly discussed the changes in biogeochemical cycles and biodiversity under global change, and clarified the mechanisms underlying feedback between ecosystem and climate change. The study of this research area could provide theoretical basis for the construction of global change adaptation strategies.

Changes of diffuse radiation resulting from global changes, especially atmospheric composition changes, influence terrestrial ecosystem productivity and the carbon budget. We review the effects of diffuse radiation on terrestrial ecosystem productivity and carbon budget, including controls and methods of estimating diffuse radiation, processes and mechanisms of diffuse radiation effects on canopy light-use efficiency (LUE), terrestrial ecosystem productivity and carbon budget. Suggested future research tasks are study of 1) responses of leaf photosynthesis to diffuse radiation at different temporal and spatial scales; 2) effects of diffuse radiation and its interaction with other environmental factors on photosynthesis and modeling; 3) effects of diffuse radiation and its interaction with other environmental factors on soil respiration; 4) adaptation of plants and terrestrial ecosystems to diffuse radiation and its interaction with other environmental factors; and 5) response processes and mechanisms of terrestrial ecosystem productivity and carbon budget to diffuse radiation and its interaction with other environmental factors.

Aims Forest litter is both a large source of CO₂ released from terrestrial ecosystems to the atmosphere and a critical sink of carbon and nutrients for plant growth. Studying dynamics of forest litter and its response to temperature and precipitation changes can improve our understanding of forest carbon and nutrient cycles and their interactions with projected climate change. Our objective was to examine the stocks and chemical quality of forest litter in natural birch (Betula platyphylla) forests that vary in both annual mean temperature (AMT) and annual mean precipitation (AMP).

Methods During July and August 2008, we measured the standing stocks and concentrations of carbon, nitrogen and phosphorus and organic fractions (extractives, acid soluble fraction (AS) and acid insoluble fraction (AIF)) of three litter layers (L1: slightly decomposed layer, L2: half-decomposed layer and L3: humus layer) in the forest floor of 12 birch forests in Inner Mongolia, China.

Important findings Along the decomposition gradient (i.e., from L1 to L3), nitrogen and phosphorus concentrations increased, AS concentration decreased, AIF concentration increased but extractives did not show significant change. Temperature and precipitation did not have significant effects on carbon fractions but at sites where AMT was higher, carbon stocks in L3 layer were higher, probably because of greater accumulation at higher-temperature sites as a result of higher litter production but similar decomposition rate compared to lower-temperature sites. These findings indicate that the litter layer (particularly the L3 layer) is an important carbon and nutrient pool at the ecosystem scale and future increases in temperature without concurrent increases in precipitation may enhance litter accumulation in these natural birch forests.

Soil organic carbon (SOC) pool is the largest carbon pool in terrestrial ecosystems and plays an important role in regulating the global carbon cycle and climate change. The inputs of nitrogen (N) and phosphorus (P) induced by anthropogenic activities and atmospheric deposition of N and P increase the availabilities of N and P in terrestrial ecosystems, which in turn will have important impacts on SOC dynamics via regulating plant growth and microbial activity. At present, many field-manipulation experiments regarding the effects of N addition and/or P addition on the dynamics of SOC have been conducted worldwide, and some breakthroughs and progress have been made, but a systematic and comprehensive review and summary of them is still lacking. By taking the effects of N addition and/or P addition on the inputs and outputs of soil carbon as the starting point, we systematically reviewed the effects of N addition and/or P addition on SOC and the potential mechanisms from three aspects: the size, fraction and molecular composition of SOC. According to the results of previous studies, N addition, P addition, and combined N and P (N + P) addition generally stimulate the size of SOC pool. The stimulation effect of N is caused by the decreased carbon outputs from microbial decomposition and/or the enhanced carbon inputs of plant above- and/or below-ground under N addition. However, the stimulation effect of P may be dominated by the enhanced carbon inputs of plant above- and/or below-ground under P addition. As for the fractions of SOC separated by particle-size or density fractionation, N addition promotes both labile fractions (particulate organic carbon or light fraction carbon) and stable fractions (mineral-associated organic carbon or heavy fraction carbon) of SOC, but reduces the proportion of stable carbon fractions to total SOC. In addition, the effects of N addition on the molecular composition of SOC are complex and diverse, and are regulated by environmental and experimental factors such as soil N availability, N addition rate, and N fertilizer form. Compared with N addition, studies on the effects of P addition and N + P addition on the fraction and molecular composition of SOC are very limited, and the associated mechanisms for the effects of P addition and N + P addition on these variables are still unclear. To improve our understanding, we propose four aspects of studies that need to be strengthened in the future, including the effects of P addition on SOC in different types of ecosystems (especially tropical forests), the role and relative contribution of plants and microorganisms in regulating the changes of SOC and its fractions under N addition and/or P addition, the effects of long-term N addition and/or P addition and their interactions on SOC, and the effects of N addition and/or P addition on SOC in deep soils (below 20 cm).

Stable isotope technique has been widely used in ecology research with the increasing concern on global change. Our objectives are to better understand the impacts of nitrogen addition and other environment changes on the nitrogen cycling of terrestrial ecosystem, predict the consequent changes in environmental conditions, and provide a reference for policy making to help ensure the sustainable development of terrestrial ecosystems. Based on the relationship between nitrogen (N) isotope composition (δ ¹⁵N) in ecosystem N status and soil N cycle, we summarized the effects and mechanisms of N input and other environment changes on δ ¹⁵N of plant and soil. Most studies show significant positive relationships between N input and δ ¹⁵N values of plant and soil. Higher N input increases soil N availability, which leads to ¹⁵N enrichment in soil because of mass discrimination during soil N cycling processes. Foliar δ ¹⁵N also will be higher as plants take up the relatively ¹⁵N-enriched soil available N. ¹⁵N natural abundance can be a useful tool for assessing nitrogen saturation and N cycling.

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.

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

Aims Linear models have been widely used to examine the impacts of climatic factors on plant phenology, although the relationship between phenology and climate could be nonlinear. Based on survival analysis, robust nonlinear models were empirically developed to examine the phenological changes in relation to air temperature and precipitation for the grasslands in China and individual woody plants in Europe.

Methods Three datasets were used in our survival analysis: two datasets of the remotely-sensed vegetation phenology for grasslands in Nei Mongol grasslands and meadows in Qinghai-Xizang Plateau, and a dataset of the phenological observations of individual woody plants in Europe. Monte Carlo simulations were performed to estimate model parameters in our survival analysis.

Important findings The survival analysis appeared to be a powerful tool in modeling the nonlinear changes in green-up date (GUD) to the climatic factors. The analyses showed that both spring temperature and precipitation are significantly correlated with the GUD in the semi-arid grasslands in Nei Mongol. For Qinghai-Xizang Plateau and Europe, the spring temperature seemed highly correlated with GUD, while the correlation was weak with the higher Holdridge aridity index. The survival model predicted that the GUD in the three regions would be advanced by 1-6 days with an increase in temperature of 1 °C. A combined increase in spring temperature and precipitation would lead to nonlinear responses, suggesting the need for developing nonlinear models. Our empirical exercise in this study demonstrated that the survival analysis could offer an alternative tool for predicting plant phenology under the changing climate.

In this article, I first briefly introduce the concept of “Carbon Neutrality”, and then discuss the vital role of ecosystem carbon sinks in achieving the carbon neutrality target. It is assertive that any efforts to achieve the carbon neutrality target depend unavoidably on both reducing carbon emissions and enhancing carbon sequestrations. There are four key factors in reducing carbon emissions, including lowering energy consumption in human activities, restructuring fossil energy consumptions especially decreasing coal consumption, promoting energy use efficiency, and developing clean and low-carbon energy. Enhancing carbon sequestration relies inclusively on restoration, construction, and better management of the ecosystems. Benefited from enhanced vegetation growth and ecological engineering practice, Chinese terrestrial ecosystem has acted and will continue to act the significant role in the carbon sequestration. To improve the ecosystem carbon sequestration, I propose the “three-optimization principles”, i.e., optimal ecosystem arrangement, optimal species setting, and optimal ecosystem management. In addition, I also state some viewpoints on potential problems and challenges in the “post-carbon neutrality” era. It may be crucial to proactively and rationally think about the possibilities of declining global vegetation productivity and relevant new environmental issues caused by a decrease in the CO₂ concentration rising in the era.

Aims As the second largest C flux between the atmosphere and terrestrial ecosystems, soil respiration plays a vital role in regulating atmosphere CO₂ concentration. Therefore, understanding the response of soil respiration to the increasing nitrogen deposition is urgently needed for prediction of future climate change. However, it is still unclear how nitrogen deposition influences soil respiration of shrubland in subtropical China. Our objectives were to explore the effects of different levels of nitrogen fertilization on soil respiration, root biomass increment, and litter biomass, and to analyze the relationships between soil respiration and soil temperature and moisture.
Methods From January 2013 to September 2014, we conducted a short-term simulated nitrogen deposition experiment in the Rhododendron simsii shrubland of Dawei Mountain, located in Hunan Province, southern China. Four levels of nitrogen addition treatments (each level with three replicates) were established: control (CK, no nitrogen addition), low nitrogen addition (LN, 2 g·m^-2·a^-1), medium nitrogen addition (MN, 5 g·m^-2·a^-1) and high nitrogen addition (HN, 10 g·m^-2·a^-1). Soil respiration was measured by LI-8100 soil CO₂ efflux system. At the same time, we measured root biomass increment and litter biomass in each plot.
Important findings Soil respiration exhibited a strong seasonal pattern, with the highest rates found in summer and the lowest rates in winter. Annual accumulative soil respiration rate in the CK, LN, MN and HN was (2.37 ± 0.39), (2.79 ± 0.42), (2.26 ± 0.38) and (2.30 ± 0.36) kg CO₂·m^-2, respectively. Annual mean soil respiration rate in the CK, LN, MN and HN was (1.71 ± 0.28), (2.01 ± 0.30), (1.63 ± 0.27) and (1.66 ± 0.26) μmol CO₂·m^-2·s^-1, respectively, and it was 17.25% higher in the LN treatment compared with CK (p = 0.06). The root biomass increment was increased by LN, MN, and HN treatments by 18.36%, 36.49% and 61.63%, respectively, compared to CK. The litter biomass was increased by LN, MN, and HN treatments by 35.87%, 22.17% and 15.35%, respectively, compared with CK. Soil respiration exhibited a significant exponential relationship with soil temperature (p < 0.01, R² is 0.77 to 0.82) and a significant linear relationship with soil moisture at the depth of 5 cm (p < 0.05, R² is 0.10 to 0.15). The temperature sensitivity (Q₁₀) value of CK, LN, MN and HN plots was 3.96, 3.60, 3.71 and 3.51, respectively. These results suggested that nitrogen addition promoted plant growth and decreased the temperature sensitivity of soil respiration. The increase of root biomass under N addition may be an important reason for the change of soil respiration in the study area.

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

Aims Climate warming strongly influences reproductive phenology of plants in alpine and arctic ecosystems. Here we focus on phenological shifts caused by warming in a typical alpine meadow on the Qinghai-Xizang Plateau. Our objective was to explore phenological responses of alpine plant species to experimental warming. Methods Passive warming was achieved using open-top chambers (OTCs). The treatments included control (C), and four levels of warming (T1, T2, T3, T4). We selected Kobresia pygmaea, Potentilla saundersiana, Potentilla cuneata, Stipa purpurea, Festuca coelestis and Youngia simulatrix as the focal species. Plant phenology was scored every 3-5 days in the growing season. The reproductive phenology phases of each species were estimated through fitting the phenological scores to the Richards function. Important findings Under soil water stress caused by warming, most plants in the alpine meadow advanced or delayed their reproductive events. As a result, warming significantly delayed phenological development of K. pygmaea. Warming significantly advanced reproductive phenology of P. saundersiana, S. purpurea and F. coelestis, but not of P. cuneata and Y. simulatrix. In addition, warming significantly shortened the average flowering duration of alpine plant species. The potentially warmer and drier growing seasons under climate change may shift the reproductive phenology of the alpine systems in similar pattern.

Recent advances in solar-induced chlorophyll fluorescence (SIF), which is a complement to optical remote sensing based on greenness observation, have made it possible to monitor the photosynthesis of plants in terrestrial ecosystems using state-of-the-art technologies. With the rapid development of tower-based, unmanned aerial vehicle (UAV), airborne and space-borne SIF observation technology and improving understanding of SIF mechanism, SIF is providing essential data support and mechanism understanding for the estimation of biological traits and gross primary production of terrestrial ecosystem, early detection of abiotic stress, extraction of photosynthetic phenology and monitoring of transpiration. In this review, we first introduce the fundamental theory, the observation systems and technologies and the retrieval method of SIF. Then, we review the applications of SIF in terrestrial ecosystem monitoring. Finally, we propose a roadmap of activities to facilitate future directions and discuss critical emerging applications of SIF in terrestrial ecosystem monitoring that can benefit from cross-disciplinary expertise.

Aims The decomposition of plant litter is a complex process mediated by biotic and abiotic factors. However, litter decomposition and its controlling factors are still controversial and unclear in arid and semi-arid ecosystems. In arid lands, precipitation has inconsistent effects on litter decomposition and nitrogen dynamics. Our objectives were to: (1) examine litter decomposition and nitrogen dynamics in litter with water additions at different seasons and (2) determine the factors critical to surface litter decomposition in arid lands.
Methods We used the litter-bag method to investigate leaf decomposition of Eremurus inderiensis and Erodium oxyrrhynchum and stem decomposition of Erodium oxyrrhynchum and Seriphidium santolinum in China’s Gurbantunggut Desert. We placed litterbags filled with those litters on soil surface in October 2009. We added snow from December to March of the next year and water from June to August. Litterbags were collected in April, July and October of 2010 and in April and July of 2011. Mass loss, carbon, nitrogen and phosphorus content, and decomposition rates of litter were analyzed at each decomposition stage. In addition, soil water content at 0-10 cm soil depth was measured at 10-day intervals from April to November.
Important findings The mass loss of different litters fit the exponential decay model (R²> 0.90). After 637 days of decomposition, no significant differences were observed among natural precipitation, snow addition and water addition treatments, and the mass remaining for leaves of Eremurus inderiensis and Erodium oxyrrhynchum and stems of Erodium oxyrrhynchum and Seriphidium santolinum with natural precipitation were 40.59%, 35.50%, 36.00% and 63.96%, respectively. The mass remaining was positively related to nitrogen remaining, which meant the litter nitrogen loss was faster than mass loss. Correlation analysis showed that decay rates were positively related to initial nitrogen content and inversely related to initial C/N. Initial C/N could explain 71% of the variation in decomposition rate. Results suggest that water addition in different seasons will not promote decomposition of surface litters, and initial litter chemical composition is critical to surface litter decomposition in the Gurbantunggut Desert.

Under intensifying human activities and climate change, spatiotemporal changes in ecosystem composition and structure are becoming increasingly drastic and intricate, and there are trends of degradation in many ecosystems. An improved understanding of ecosystem dynamics and their underlying mechanisms in the context of global change can not only help resolve fundamental theoretical questions in ecology, but can also inform applied issues in ecosystem restoration and conservation. Here, we review different models of ecosystem dynamics (gradual continuum, threshold/regime shift, and stochastic) and conceptualize the mechanisms by which biotic interactions can potentially modulate ecosystem dynamics. We then synthesize the state of understanding how biotic interactions regulate secondary succession, regime shift, and species range shift—ecosystem dynamics subject to intense recent investigation. We further discuss results from studies that applied theories on biotic interactions in ecosystem restoration and conservation. We show that there is a growing body of research revealing 1) that multiple types of biotic interactions, such as competition, facilitation (including mutualism), and trophic interactions, can drive or substantially alter the patterns, directions, and rates of ecosystem change at various spatiotemporal scales, and 2) that managing biotic interactions is likely to greatly enhance the performance of ecosystem restoration and conservation. To move forward, we highlight that further research is needed to better understand how the impacts of biotic interactions on ecosystem dynamics vary spatially and temporally, how biotic interactions modulate ecosystem dynamics under multiple anthropogenic disturbances, and how best to manage biotic interactions to optimize ecosystem conservation and restoration.

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 The relationships between temporal changes in vegetation growth and climate change tend to be asymmetric. Considering the temporal effects of climate factors on vegetation growth can provide important scientific basis for accurately understanding vegetation-climate relationships and predicting the dynamic responses of vegetation to global climate change. Methods Based on the MODIS normalized difference vegetation index (NDVI), climate, and vegetation type data, this study investigated the temporal effects of climate factors on vegetation growth and the dominant factors influencing vegetation growth on the Qingzang Plateau through establishing four temporal effects equations between climate and vegetation NDVI. Important findings Among the four temporal effects, models considering both time lag and accumulation effects had the highest explanation degree (47%). Compared with model without considering temporal effect, the explanation power of the time lag and accumulation effects on vegetation would increase by 4%-18%. Vegetation dynamics on more than 43% of the Qingzang Plateau was dominated by the combined effects of time lag and accumulation, followed by the area only affected by time accumulation or lag effects, and the area without time effect. The partial correlation coefficient between NDVI and precipitation (0.25 ± 0.56) was overall higher than it between NDVI and temperature (0.08 ± 0.6). The areas dominated by the precipitation were mainly distributed on the northeast and southwest of the Qingzang Plateau with an area ratio of 40.1%, whereas the areas dominated by temperature were largely distributed on the center and southeast of the Qingzang Plateau with an area ratio of 29.7%. These research results can provide basic judgments for the relationships between vegetation growth and climate on the Qingzang Plateau.

Aims Temperate forest is one of the most important components of the global forests and main carbon pools. Nitrogen (N) is considered as the limiting nutrient for the forest growth. However, the heterogeneities in plant species and stem sizes were largely ignored in previous researches on the effects of N addition on plant growth. Quercus wutaishanica is one of the most common and dominant tree species in the temperate forests in North China. In this study, we investigated the responses of growth of trees and forests to N addition in the Quercus wutaishanica forests in Mt. Dongling in Beijing.
Methods We conducted a 7-year N fertilization experiment in Quercus wutaishanica forests in Mt. Dongling, Beijing, since 2011. The N addition was conducted at three treatment levels, i.e., 0 kg hm ^-2·a ^-1, 50 kg hm ^-2·a ^-1 and 100 kg hm ^-2·a ^-1. Nitrogen was added at the beginning of each month from May to October each year. We used electronic vernier caliper to measure tree growth rate for each year. All trees were divided into three groups based on their diameter at breast height (DBH), namely small trees (DBH = 3-10 cm), median trees (DBH =10-20 cm) and large trees (DBH > 20 cm). Particularly, we considered growth at species level for all Quercus wutaishanica and the growth at community level for all tree species in the stands.
Important findings (1) At species level, N addition enhanced the growth rate of Q. wutaishanica. (2) At community level, the growth rate showed no difference among different N addition treatments. (3) Small trees were restrained, while median and large Q. wutaishanica trees were not significantly influenced, by the N addition.

Aims Plant roots play a critical role in the uptake of nutrients, and nitrogen (N) absorption is considered as the first step and a pivotal process in N metabolism of plants. Our objective was to better understand the absorption of two major inorganic N forms (NH₄⁺ and NO₃^-) in subalpine coniferous forests under global warming Methods Experimental warming using infrared heater was applied to two dominant species in subalpine coniferous forests of Sichuan, China, Picea asperata and Abies fargesiivar. faxoniana. The non-invasive micromeasurement technology was used to investigate the effects of warming on the uptake rates of NH₄⁺ and NO₃^- and the potential interactions between these two ions.Important findings Results showed that the maximal net root uptake of NH₄⁺ and NO₃^-occurred at a distance of 17-18 mm and 17 mm from root tips, respectively for P. asperata. and at a distance of 11 mm and 11.5 mm from root tips respectively for A. fargesiivar. faxoniana. Experimental warming elevated the uptake rates of NH₄⁺ and NO₃^- in both species, but the interactions between NH₄⁺ and NO₃^- differed between the two species. While NO₃^- uptake was inhibited in the presence of NH₄⁺ for both P. asperataand A. fargesiivar. faxoniana, net NH₄⁺ uptake was promoted by NO₃^- supply only in P. asperata roots under experimental warming.

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.

Characterizing ecosystem responses to past, present and future changes in atmopsheric carbon dioxide (CO₂) concentration is critical for understanding and predicting the consequences of global change over evolutionary and ecological timescales. Over the past two decades, CO₂ studies have provided great insights into the effects of rising CO₂ concentration on plant growth and productivity, carbon-nitrogen turnover, the formation of progressive nitrogen limitation (PNL) in ecosystems, and the interaction between elevated CO₂ concentration and other envrionmental factors (O₃ pollution, N deposition, warming and drought). However, scaling CO₂ effects across wide spatial and temporal scales, especially at belowground part, has many uncertainties. Here we explore major research areas and hotspots of CO₂ studies on plants and ecosystems from 1990 to 2018, and review the development of manipulated experiments in the field of elevated CO₂ impacts. In detail, we discussed the states of art in five international frontiers research directions: crop yield and quality, carbon fixation, water use efficiency, ecosystem nitrogen use and soil microorganism. Finally, we identify several topics and research outlooks to facilitate further developments in the field of CO₂ effects on ecosystems.

As an important compartment of the Earthʼs surface, terrestrial ecosystems act as a vital harbor for human survival and development. Climate change significantly increased the frequency, intensity and duration of drought since the beginning of the 21st century, which have marked impact on ecosystems, leading to serious restriction or even threat on the sustainable development of human beings. Therefore, developing integrative research on effects of drought on terrestrial ecosystems and assessing the associated ecological risk are impressive in global change field. This study reviewed the effects of drought on plant physiological and ecological processes, biogeochemical cycles, biodiversity, and ecosystem structure and functions in terrestrial ecosystems, and discussed current hotspot issues in this field as well as deeply analyzing the existing problems and the potential development direction. This study aims to provide some suggestions for the future observation, manipulative experiments, and modeling prediction on effects of drought on terrestrial ecosystems, and offer new insights to enhance risk assessment and management under drought.