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.

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.

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

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

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.

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.

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.

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.

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.

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.

With the growth of human population and the development of human society, land use and land cover change (LUCC) is inevitable. LUCC not only has a far-reaching impact on the elements, structure and functions of ecosystems, but also has a feedback effect on global climate change. Scientific research has been carried out on the processes of land use change, the driving mechanisms, and the possible ecological and environmental effects in various aspects. This paper reviews the research progress on the impacts of land use change on climate, soil, biogeochemical cycle, biodiversity and regional ecological environment, and puts forward the prospects for the frontier research. With the continuous development of new technologies, scholars will focus more on the prediction of the future development, rationality and adaptability of LUCC in the context of future global change, providing basic information and theoretical basis for sustainable development.

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

Aims Plant biomass accounts for the main part of grassland productivity. The productivity of grassland regarded as one of important ecosystem function is always co-limited by nitrogen and water availability, therefore, how grasslands respond to atmosphic nitrogen (N) addition and precipitation increasing need to be systematically and quantitatively evaluated at different climate conditions and temporal scales.
Methods To investigate the impact of nitrogen addition and precipitation increasing on grassland biomass over climate conditions and temproal scales, a meta-analysis was conducted based on 46 papers that were published during 1990-2017 involving 1 350 observations.
Important findings Results showed that: (1) N addtion, precipitation increasing and the combinations of these two treatments significantly increased the aboveground biomass (37%, 41%, 104%), total biomass (32%, 23%, 60%) and the ratio of aboveground biomass to belowground biomass (29%, 25%, 46%) in grassland ecosystem. Belowground biomass showed no response to single N addtion, but could be significantly enhanced together with increaseing precipitation; (2) The response of grassland biomass under these N addtion and the increasing of precipitation showed obvious spatial pattern under different climate conditions. The N addition tended to increase more aboveground biomass, total biomass and the ratio of aboveground biomass to belowground biomass under high sites with high mean annual air temperature (MAT) and mean annual precipitation (MAP) while precipitation increasing tended to simulate more belowground biomass and total biomass under low MAT and MAP sites; (3) In addition, the response of grassland biomass under these two global change index showed obvious temporal pattern. With the increase of duration of N addition, the belowgound biomass tended to decrease, while the aboveground biomass, total biomass and the ratio of aboveground biomass to belowground biomass tended to increase under N addition. With the increase of duration of precipitation manipulation, the total biomass showed no response to precipitation increasing, while aboveground biomass, belowground biomass and the ratio of aboveground biomass to belowground biomass tended to be enhanced. The results indicated that aboveground biomass was more likely to be enhanced than belowground biomass under N addition or precipitation increasing in the long term.

Rising ground-level ozone (O₃) is currently an essential environmental issue in the world, especially in China. While research on the effects of O₃ on leaf photosynthetic gas exchange, plant growth and biomass has received a lot of attention, ecosystem-scale studies are however scarce and subject to great uncertainties. This article combs trends and hotpots of ground-level O₃ concentration and its effects on plants and ecosystems over the past 40 years. Research techniques and assessment methods for studying the ecological effects of ozone pollution are covered. The most important advances on the impacts of elevated ozone on terrestrial ecosystem are reviewed: plant response mechanisms, effects on grain yield, crop quality, carbon sequestration capacity, community structure and below-ground processes of different terrestrial ecosystems. Finally, regional risk assessment of the O₃ pollution is discussed. Considering the main knowledge gaps, future research should focus on belowground ecosystem response to elevated O₃ and should also incorporate O₃ and multi-factor experiments using Free-Air Ozone Concentration Elevation (FACE) system. More attention should also be paid on food security, establishment of Asian ozone network, standardization of risk assessment approach, and exploration of ecological measures to reduce the negative effects of O₃ pollution. This review can help to promote more studies on the ecological effects of ground-level O₃ pollution.

Aims Over the past twenty years, most biodiversity and ecosystem functioning (BEF) research has focused on the effects of species diversity on single or just a few ecosystem functions. However, ecosystems are primarily valued for their ability to maintain multiple functions and services simultaneously (i.e. multifunctionality here- after). This paper first introduced the constantly perfected concept of “multifunctionality”, and then tried to make some modifications to the current mainstream quantitative method in order to evaluate the multifunctionality of grassland communities with the management of clipping, enclosure and grazing in Inner Mongolia, investigating the relationship between the multifunctionality and species diversity. Methods In free grazing grassland, four sites were set and each site was divided into two parts to conduct enclosure and clipping management respectively. After seven years, 15 quadrats (1 m × 1 m) were established for each type of management in each site (total 60 quadrats for each type) using the regular arrangement method; as a control, we also established 20 quadrats (two sites) in grazing grassland. For each quadrat, we carried out plants census and collected soil mixture sample, measuring 16 soil variables, and then calculated the biodiversity indices and multifunctionality index (M-index) by means of factor analysis. Important findings The results showed that M-indexes by the two evaluation methods were strongly correlated at both quadrat and site scale, suggesting that our modified method was reliable. Over-grazed communities had the lowest biodiversity indices and their most soil indicators were also low, showing obvious degradation features. Enclosure and clipping communities (seven years) had higher biodiversity and better soil indicators. The rank of M-indexes was clipping community (0.2178) > enclosure community (0.0704) > grazing community (-0.8031). The vegetation was distributed mainly along the gradients of water and fertility. Among the biodiversity indices, evenness (Pielou) index and richness (Margelf) index were most strongly correlated with multifunctionality, and their explanatory power (R²) for M-index were higher at site scale (R² = 0.5921, p = 0.0093; R² = 0.7499, p = 0.0007) than at quadrat scale (R² = 0.1871, p < 0.0001; R² = 0.1601, p < 0.0001), indicating study scale played an important role in the determinants of multifunctionality. At both quadrat and site scales, M-indexes is a linear positive function with species evenness and a hump-shaped function of species richness. Therefore, in contrast to enclosure, clipping was more conducive to maintain the ecosystem multifunctionality in this region, and the ecosystem with moderate specie richness, where these species are evenly distributed might have better multifunctionality.

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

Massive fossil fuel burning and the rapid urbanization have caused significant increases in atmospheric carbon dioxide (CO₂) and ozone (O₃) concentrations. The increased gas concentration has significant impacts on the structure and function of terrestrial plants and ecosystems. Rising CO₂ concentration increased the plant growth and productivity, while elevated O₃ decreased grain yield and carbon sequestration capacity. The Free-Air Concentration Enrichment (FACE) is one kind of facility closest to the natural conditions for simulating effects of rising atmospheric gas concentration on ecosystems. FACE has been widely used in various ecosystems and provides key basis to understand the ecological progress in response to global change and parameters for risk assessment in terrestrial ecosystem models. In this paper, CO₂/O₃-FACE facility around world and their technology are reviewed. The advantages and disadvantages of the design of each FACE in different terrestrial ecosystems were discussed. The current status of global FACE facility and progress in research achievements are also introduced. Furthermore, the problems in running current FACE and the frontiers of scientific questions are also highlighted.

Global atmospheric circulations are greatly affected by anthropogenic activities. Several atmospheric circulation models predict that the frequencies of extreme rainfall events and extreme droughts will increase in the future. Water is one of the most limiting resources for growth and development of plants in arid and semi-arid ecosystems. Furthermore, grassland ecosystems have been proven to be very sensitive to changing precipitation regimes. However, our understanding on the effects of extreme climatic events on the structure and functioning of grassland ecosystems is inadequate. By far, the definitions of extreme climatic events are still inconsistent. Therefore, based on analyses of the definitions of extreme climatic events and research methods in literature, we synthesize the effects of extreme rainfall events and extreme droughts on soil water and nutrient availability, individual plant development and physiological characteristics, community structure, ecosystem productivity and carbon cycling. In addition, we put forward five scientific questions on research concerning the impacts of extreme climatic events and identify two key issues on manipulative precipitation experiments to help with understanding the mechanisms on how grassland ecosystems respond to extreme climatic events in the context of global change.

Biomass allocations between aboveground and belowground organs provide pivotal information for connecting aboveground productivity and belowground carbon sequestration. As accurate measurement of belowground biomass is essential for determining the biomass allocation, we first reviewed the methods in quantifying belowground biomass and their merits. We then presented the major advances on plant biomass allocations between aboveground and belowground organs, as well as the potential drivers such as precipitation, warming, atmospheric CO₂ concentration, and nitrogen deposition. We finally provided a list of challenges in studying belowground biomass allocation for the future. This review has important implications for studies on carbon cycling in grassland ecosystems under the changing climate.

Aims Nitrogen use efficiency (NUE) is a key functional trait in plants, which closely relates to ecosystem functions. However, it is still unclear about the regional patterns and affecting factors of plant NUE.
Methods This study quantified leaf and root NUE in 139 grassland plant species and explored their relationships with environmental factors and plant functional groups across 82 sampling sites in Nei Mongol and Qinghai-‌Xizang Plateau.
Important findings 1) We found that leaf NUE (53 g·g ^-1) in meadow steppe was significantly greater than those in alpine meadow (46 g·g ^-1), desert steppe (41 g·g ^-1) and typical steppe (39 g·g ^-1). Root NUE (108 g·g ^-1) in alpine meadow was higher than those in other ecosystems. 2) Leaf NUE was more sensitive to temperature than root NUE, but with increasing drought index they all showed a significant decrease. 3) Leaf and root NUE in forbs were significantly lower than sedges and grasses. In addition, leaf and root NUE of legume were 48% and 60% lower than those of non-legume, respectively. 4) Plant NUE did not show any significant relationship with soil nitrogen content. Overall, there was significant difference between leaf and root NUE in their spatial patterns in the Nei Mongol and Qinghai-Xizang Plateau grasslands. The main impacting factors were plant functional group and drought index. The findings are helpful for better understanding the mechanisms underlying the variation of grassland productivity in China, and also provide more scientific basis for grassland management.

Global change has already posed a serious threat to different freshwater ecosystems by raising water temperatures, changing precipitation patterns and water flow conditions, enhancing species invasion, and increasing extreme events. In order to identify the major works carried out and highlights of the outcomes of research in freshwater ecology in the context of global change, we conducted literature search and analysis of papers published during 1900-2018 via Web of Science. In this review, the major researches in freshwater ecology in the context of global change are categorized into: (1) the effects of various global change factors on individuals, populations, communities and ecosystems; (2) changes in biogeochemical cycles of ecosystems in the process of global change; and (3) adaptation strategies of freshwater ecosystems to global changes. Over the past 10-15 years, research in freshwater ecosystems and global change progressed rapidly and showed breakthroughs in the following aspects: (1) elucidated the response processes and mechanisms of the structure and function of freshwater ecosystems to global climate change, in particular rising water temperatures; (2) revealed that freshwater ecosystems (wetlands, lakes, rivers, etc.) are important components of the global carbon cycle, such that under the influence of global change factors organic carbon burial decreased and mineralization rate increased. In future research, it is necessary to strengthen the systematic observations and integration of the total elements of freshwater ecosystems, to conduct research on carbon transport and transformation processes mediated by the river-connected multi-systems, and to strengthen basic theoretical research for uncovering the adaptation mechanisms of freshwater ecosystems to global change.

Since the industrial revolution, marine ecosystems have faced unprecedented stress caused by increasing temperature and atmospheric CO₂ concentration as a result of anthropogenic activities. In this review, we analyzed the domestic and international research status about impacts of global change on marine ecosystems by bibliometrics, briefly introduced the history of the research on marine ecosystems under global change, and reviewed the main progress in studies about the effects of global change on key processes of marine primary production, focusing on the impacts of ocean warming, ocean acidification, and eutrophication and hypoxia. We also summarized the major issues in current studies and proposed future research directions in the field.