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Nitrogen enrichment impacts ecosystems productivity by altering different hierarchical community processes, ranging from responses of individual species traits to species reordering and ultimately species turnover. Most previous studies have focussed on examining the responses of these processes separately, but little has been done to examine how hierarchical community responses are linked and contribute to the changes in productivity.To fill this knowledge gap, we conducted a 4‐year nitrogen addition field experiment to investigate the hierarchical community responses of above‐ground net primary productivity (ANPP) in an alpine meadow ecosystem.We found that the individual species response contributed more to ANPP changes under nitrogen addition, while species reordering and species turnover contributed little over the 4‐year experiment. In particular, plant height was a key trait to ultimately explain and bridge nitrogen addition effects on ANPP across different community levels. Nitrogen addition promoted plant height and growth of individual species, and reordered functional groups by increasing taller grasses and sedges and decreasing shorter forbs and legumes. Species turnover had a weak effect on ANPP changes due to low biomass of both lost and gained species. Finally, taller persistent plants promoted ANPP more with nitrogen addition.Synthesis. This study provides a new perspective on how ecosystem productivity responds to nitrogen enrichment in a hierarchical manner linked to plant height, which is helpful for elucidating complex community processes and establishing direct links between plant traits and ecosystem function. [ABSTRACT FROM AUTHOR]

Climate warming, often accompanied by extreme drought events, could have profound effects on both plant community structure and ecosystem functioning. However, how warming interacts with extreme drought to affect community‐ and ecosystem‐level stability remains a largely open question. Using data from a manipulative experiment with three warming treatments in an alpine meadow that experienced one extreme drought event, we investigated how warming modulates resistance and recovery of community structural and ecosystem functional stability in facing with extreme drought. We found warming decreased resistance and recovery of aboveground net primary productivity (ANPP) and structural resistance but increased resistance and recovery of belowground net primary productivity (BNPP), overall net primary productivity (NPP), and structural recovery. The findings highlight the importance of jointly considering above‐ and belowground processes when evaluating ecosystem stability under global warming and extreme climate events. The stability of dominant species, rather than species richness and species asynchrony, was identified as a key predictor of ecosystem functional resistance and recovery, except for BNPP recovery. In addition, structural resistance of common species contributed strongly to the resistance changes in BNPP and NPP. Importantly, community structural resistance and recovery dominated the resistance and recovery of BNPP and NPP, but not for ANPP, suggesting the different mechanisms underlie the maintenance of stability of above‐ versus belowground productivity. This study is among the first to explain that warming modulates ecosystem stability in the face of extreme drought and lay stress on the need to investigate ecological stability at the community level for a more mechanistic understanding of ecosystem stability in response to climate extremes. [ABSTRACT FROM AUTHOR]

Plant functional traits play important roles in determining plant responses to environmental change and further shaping community composition, but the role of plant stoichiometry remains poorly understood in regulating community responses to continually increased nitrogen (N) deposition.In this paper, we used a 5‐year manipulative experiment with six N addition rates (Nrate) to explore how the N response of different plant functional groups including forbs, grasses, sedges and legumes links to above‐ground plant carbon‐to‐N ratio (C:N, related to N use strategies) in an alpine meadow.We found that plant C:N explained 45.8% and 42.6% of N response variation among functional groups in plant above‐ground net primary productivity (ANPP) and abundance. Over increasing Nrate, functional groups with higher C:N (grasses and sedges) tended to grow better and have greater abundance, yet those with lower C:N (forbs and legumes) were at risk of loss, subsequently mitigating the N‐caused decline in community‐level plant C:N. This was mainly due to that functional groups with higher C:N not only possessed higher N use efficiency but also had greater competitiveness for above‐ground light due to higher plant height. In addition, we also found that plant C:N differences among functional groups remained relatively constant over Nrate, suggesting that the ability of this trait to indicate functional group‐specific N responses was stable.Synthesis. Our results show that the N responses of different herbaceous plants can be well explained by plant C:N. Thus, we suggest the prediction of plant community composition changes under N deposition would be greatly improved by considering this easily measured stoichiometric trait. [ABSTRACT FROM AUTHOR]

The frequency and severity of extreme weather events are increasing and expected to increase more in the future, together with global change. However, how extreme events and global change factors interactively influence community structures and ecosystem processes is largely unknown. Here, we investigated the responses of the temporal stability and resilience of aboveground net primary productivity (ANPP) of an alpine meadow to an extreme flooding event under different treatments of experimental drought and clipping. We found that ecosystems that were exposed to drought treatments for 3 years significantly decreased the temporal stability of community productivity but increased resilience to flooding, whereas their resistance to or recovery from flooding did not change. Neither clipping nor its interaction with drought altered the responses of these community stability metrics to flooding. Drought treatments significantly decreased plant species richness and asynchrony and dominant species stability, leading to a decrease in temporal stability and an increase in resilience in response to the extreme flooding event. The study also revealed that the change in species asynchrony was the dominant impact pathway determining the responses of resilience and temporal stability to flooding. Our results highlight that alpine grassland that experiences a multiyear drought may aggravate the instability of community productivity to extreme climatic events by reducing species asynchrony. [ABSTRACT FROM AUTHOR]

Aims Terrestrial ecosystem carbon (C) uptake is remarkably regulated by nitrogen (N) availability in the soil. However, the coupling of C and N cycles, as reflected by C:N ratios in different components, has not been well explored in response to climate change. Methods Here, we applied a data assimilation approach to assimilate 14 datasets collected from a warming experiment in an alpine meadow in China into a grassland ecosystem model. We attempted to evaluate how experimental warming affects C and N coupling as indicated by constrained parameters under ambient and warming treatments separately. Important Findings The results showed that warming increased soil N availability with decreased C:N ratio in soil labile C pool, leading to an increase in N uptake by plants. Nonetheless, C input to leaf increased more than N, leading to an increase and a decrease in the C:N ratio in leaf and root, respectively. Litter C:N ratio was decreased due to the increased N immobilization under high soil N availability or warming-accelerated decomposition of litter mass. Warming also increased C:N ratio of slow soil organic matter pool, suggesting a greater soil C sequestration potential. As most models usually use a fixed C:N ratio across different environments, the divergent shifts of C:N ratios under climate warming detected in this study could provide a useful benchmark for model parameterization and benefit models to predict C–N coupled responses to future climate change. [ABSTRACT FROM AUTHOR]

Ecosystem stability is essential to its sustainable functions and services to humanity. Although climate warming is projected to vary from 1 to 5°C by the end of 21st century, how the temporal stability of plant community biomass production responds to different warming scenarios remains unclear.To fill this knowledge gap, we conducted a 6‐year field experiment with three levels of warming treatments (control, +1.5°C, +2.5°C) by using infrared radiators, in an alpine meadow on the Qinghai–Tibet Plateau.We found that low‐level warming (+1.5°C), compared to the control, did not significantly change the temporal stability of plant community biomass production and its underlying causes, including species diversity, compensatory dynamics, mean–variance scaling, biomass temporal stability of plant population (the average of temporal stability of species biomass production of all species in the community) or dominant species. However, high‐level warming (+2.5°C) significantly reduced them. Species diversity was not a significant predictor of temporal stability of plant community biomass production in this species‐rich ecosystem, regardless of the magnitude of warming, while co‐existing species compensatory dynamics and the biomass temporal stability of dominant species determined the response of temporal stability of plant community biomass production to warming.Synthesis. Our results suggest that the responses of plant community biomass temporal stability and its underlying mechanisms to climate warming depend on warming magnitudes. The findings highlight the various responses of ecosystem functions and services to different warming scenarios and imply that ecosystem will fail to maintain and provide stable biomass‐related services for humanity under high‐level climate warming. [ABSTRACT FROM AUTHOR]

Global nitrogen (N) deposition generally reduces ecosystem stability. However, less is known about the responses of ecosystem stability and its driving mechanisms under different N addition gradients. We conducted a four-year N addition experiment in an alpine meadow, using six levels of N addition rates (0, 2, 4, 8, 16, 32 g N m−2 year−1) to examine the effects of N addition on plant community biomass stability and the underlying mechanisms. We found that the stability of ecosystem aboveground net primary productivity (ANPP) decreased linearly with increasing N addition rates, even though it had no effect on plant species richness at low N addition rates and significantly reduced species richness at high N addition rates. The most remarkable finding is that the main mechanism underlying ecosystem stability shifted with N addition rates. The decrease of common species stability contributed most to the reduction of plant community biomass stability under low N addition rates (N0–N4), whereas the decrease of species asynchrony contributed most to the reducing plant community biomass stability under high N addition rates (N8–N32). Our results indicate that species diversity was not a significant predictor of plant community biomass stability in this alpine meadow, which challenges the traditional knowledge. This study highlights the shifts of main mechanism regulating plant community biomass stability under different N addition rates, and suggests that continuous nitrogen deposition in the future may reduce ecosystem stability and potentially impeding the sustainable provision of ecosystem functions and services. [ABSTRACT FROM AUTHOR]

Background and aims: Responses of grassland carbon (C) cycling to changes in precipitation and land use remain a major uncertainty in model prediction of future climate. It remains even far from clear on how does clipping regulate the response sensitivity of grassland C sequestration to precipitation change. Methods: We conducted a manipulative field experiment with five precipitation treatments (1/12P, 1/4P, 1/2P, 3/4P, P, and 5/4P, where P is the growing season precipitation) interacting with clipping treatments in an alpine meadow to examine net ecosystem C uptake and its associated processes in response to clipping and precipitation gradients as well as their interaction. Results: (1) clipping significantly increased soil temperature in 2016 and 2017, while had no effect on soil moisture; (2) clipping also stimulated the net ecosystem productivity (NEP) and gross ecosystem productivity (GEP) during the two growing seasons; (3) although there were no significantly interactive effects of clipping with precipitation changes, clipping increased the response sensitivity of C uptakes to soil moisture changes. NEP increased more with soil moisture under clipping (0.21 μmol m−2 s−1 v/v−1) than unclipping treatments (0.11 μmol m−2 s−1 v/v−1). This was due to the compensatory growth and the positive effect of clipping on soil temperature, which stimulated plant growth and productivity. Conclusions: Our findings suggest that clipping will aggravate grassland's sensitivity to precipitation change. These findings highlight the important role of land use change in regulating ecosystem C sequestration under further climate conditions in an alpine meadow. [ABSTRACT FROM AUTHOR]

The article presents a study which examined the response of aboveground net primary productivity (ANPP) to nitrogen (N) deposition and enrichment in alpine meadows. Also cited are the expected saturation response of ecosystem in N deposition, as well as the data collected in the study, including soil acidification, biodiversity loss, and ammonium toxicity.

Global climate change models predict an increase in the frequency and intensity of extreme droughts, with uncertain ecological impacts across ecosystems. In particular, it is not clear when extreme droughts will elicit extreme ecological responses.For this study, we employed three complementary approaches to explore the relationships between extreme drought and ecosystem responses. First, we used global data mining to evaluate the relationship between extreme gross primary productivity and extreme precipitation from 1980 to 2013. Second, we conducted a meta‐analysis using 132 drought experiments across the globe to assess the response ratios (RRs) of above‐ground net primary productivity (ANPP) to extreme versus non‐extreme drought treatments. Third, we examined community and ecosystem responses in an alpine meadow to a drought gradient experiment, which included five precipitation treatment levels (1/12 P, 1/4 P, 1/2 P, 3/4 P, and P, where P is the growing season precipitation).This study had three key results. In our historical data mining, we found that extreme droughts elicited extreme ecological responses only 15.1% of the time. The meta‐analysis results indicated that there were no significant differences in the RRs of ANPP between the extreme versus non‐extreme drought treatments. The drought gradient experiment results revealed that although the four drought treatments were statistically extreme, only the most extreme drought treatment (1/12 P) significantly reduced ANPP over the 3 years. Meanwhile, species richness and asynchrony were significantly reduced under the 1/12 P treatment, which led to a significant reduction in productivity.Synthesis. These results suggest that extreme ecological responses to extreme drought may be less frequent than previously thought. But when they do occur, extreme ecological responses may be driven by plant community changes such as species asynchrony, species loss or species reordering. Our experimental results highlight the key role of community dynamics in determining the resistance of ecosystem productivity to extreme drought, which should be assessed when predicting ecological responses to climate change. [ABSTRACT FROM AUTHOR]

Disentangling the relative response sensitivity of soil autotrophic (Ra) and heterotrophic respiration (Rh) to nitrogen (N) enrichment is pivotal for evaluating soil carbon (C) storage and stability in the scenario of intensified N deposition. However, the mechanisms underlying differential sensitivities of Ra and Rh and relative contribution of Rh to soil respiration (Rs) with increasing N deposition remain elusive.A manipulative field experiment with multi‐level N addition rates was conducted over 3 years (2015–2017) in an alpine meadow to explore the relative impact of N enrichment on Ra and Rh and the response of Rh/Rs ratio to the gradient of N addition.Soil respiration components had different sensitivities to N enrichment, with Ra decreasing more than Rh, leading to a higher Rh/Rs ratio as a function of increasing N addition rates. Ra and Rh decreased nonlinearly as N addition rates increased, with a critical load of 8 g N m−2 year−1 above which N enrichment significantly inhibited them. Ra and Rh were controlled by different abiotic and biotic factors, and the regulation of controlling factors on soil respiration components varied over time. N‐induced reduction in the relative abundance of forb significantly affected Ra, and this effect was mainly evident in the second and third years. Nitrogen enrichment significantly changed Rh in the third year, and the decreased Rh under high doses of N addition could be attributed to the changes in microbial biomass C, soil substrate quality and microbial composition.Our study highlights the leading role of Ra in regulating Rs responses to N enrichment and the enhancement of Rh/Rs ratio with increasing N addition. We also emphasize that N‐induced shifts in plant community composition play a vital role in regulating Ra instead of Rh. The changing drivers of Ra and Rh with time suggests that long‐term experiments with multiple levels of N addition are further needed to test the nonlinear responses and underlying mechanisms of soil respiration components in face to aggravating N deposition. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Changes in precipitation have the potential to cause dramatic changes in ecosystem carbon (C) cycling; however, it remains unclear whether different components of the net ecosystem exchange (NEE) (e.g., C uptake vs. release, plant vs. microbe respiration, above‐ground vs. below‐ground plant respiration) have similar or differential sensitivity to precipitation gradients.We conducted a manipulative field experiment (from 2015 to 2017) with six precipitation treatments, including 1/12 annual precipitation (P), 1/4 P, 1/2 P, 3/4 P, P and 5/4 P in an alpine meadow to investigate the responses of the NEE components.Over the 3 years, all C fluxes showed a nonlinear response to the precipitation gradients, except for root respiration. The most extreme drought treatment (1/12 P) caused strong reductions in NEE by 15.57%, gross primary productivity by 17.26% and ecosystem respiration by 19.05%, in contrast to the control. Plant respiration was more sensitive to precipitation change than microbe respiration, and above‐ground plant respiration was more susceptible than below‐ground respiration. Structural equation models revealed that the response of C fluxes under precipitation changes was primarily due to changes in the soil water content and above‐ground net primary productivity.Our findings indicate that future precipitation changes, particularly extreme drought, will decrease ecosystem C fluxes with different magnitudes, leading to a consequent reduction of NEE. These emergent ecosystem properties are essential for the improved elucidation of carbon cycle dynamics and benchmarking models, to predict ecosystem responses to precipitation changes. A plain language summary is available for this article. [ABSTRACT FROM AUTHOR]

• CUE had different sensitivity to clipping and warming. • Clipping significantly increased CUE due to the stronger positive effects of clipping on GEP than on ER. • Warming had a minor effect on CUE attributed to the synchronized increase in GEP and ER. Ecosystem carbon use efficiency (CUE) is a critical variable in quantifying the efficiency and capacity of an ecosystem to sequestrate carbon (C) from the atmosphere. However, how CUE responds to land-use change and climate warming remains elusive and lacks empirical evidence. We implemented a field manipulative experiment to examine the effects of clipping and warming on CUE in an alpine meadow on the Qinghai-Tibetan Plateau over 7 years. The results showed that clipping significantly increased CUE by 7.1%, which was mainly attributed to the stronger positive effects of clipping on gross ecosystem productivity (GEP, 13.0%) than on ecosystem respiration (ER, 5.3%). In contrast, warming had a minor effect on CUE due to the synchronized increase in GEP (8.8%) and ER (7.3%). There was no significant interactive effect of clipping and warming on CUE. Moreover, the dominance of grasses played a pivotal role in controlling the responses of CUE to clipping and warming by increasing ecosystem C uptakes but decreasing microbial decomposition. Soil temperature and above-ground net primary productivity also contributed to the changes in CUE under clipping and warming. These results suggest that clipping once a year at the end of the growing season could increase the efficiency of this alpine meadow for C sequestration, while climate warming could not change ecosystem C use efficiency. We highlight that the dominance of grasses is an emerging property in determining CUE under land-use change and climate warming in grasslands. [ABSTRACT FROM AUTHOR]

• R h and R a are highly sensitive to warming and clipping. • Increased allocation of C toward below-ground cause the increasing in R a and R s. • Due to the changes in ANPP/BNPP and ST, warming effect vary largely with years. • Plant C allocation is an emerging property in determining R s under warming. Global warming and land-use change have profound impacts on soil respiration (R s), with consequent changes in carbon exchange between terrestrial ecosystem and atmosphere and its feedback to climate change. However, it is not clear how soil respiration and its components (heterotrophic respiration, R h ; and autotrophic respiration, R a) respond to long-term warming and clipping in alpine meadow, and what are the controlling factors for the responses of soil respiration. To answer these questions, we conducted a field experiment of warming and clipping in an alpine meadow on the Qinghai-Tibet Plateau from 2014 to 2019. Infrared heaters were used to simulate climate warming and plots were clipped once a year to mimic hay or biofuel harvest. The results showed that warming decreased R h by 5.06% but stimulated R a by 10.08% across years, while clipping enhanced R s , R h and R a by 12.31%, 18.65% and 1.29%, respectively, and marginally interacted with warming in impacting R a across the years. Furthermore, the warming effect on soil respiration and its components varied largely over years, which was mostly attributable to the changes in the ratio of above-ground to below-ground net primary productivity (ANPP/BNPP, A/B) and soil temperature. Increased allocation of photosynthate toward below-ground caused the gradual increase in R a and R s under long-term warming. While soil temperature changes explained the variation of warming effects on R h across years. The results demonstrated that soil respiration and its components in the alpine meadow are highly sensitive to climate warming and hay harvest but with large interannual variations in their responses. We highlight that plant carbon allocation is an emerging property in determining soil respiration under climate warming. [ABSTRACT FROM AUTHOR]

• Alleviation of light limitation increased species richness under N enrichment. • Alleviation of light limitation stimulated NEP under N enrichment. • The promotion of species richness by weakening light limitation stimulated NEP. Global nitrogen (N) deposition generally reduces plant biodiversity and changes ecosystem carbon (C) cycle. Light limitation caused by accumulated litter under N enrichment is proposed as a main reason for the decreases of biodiversity and ecosystem C sequestration. However, mechanisms underlying this light limitation effect are not well understood, especially under different N input rates. We conducted a field manipulation experiment with N addition as the primary factor nested with litter manipulation as the secondary factor in an alpine meadow in 2017, measured ecosystem C fluxes and investigated the plant communities about 10 days after the N addition treatment, to examine the responses of biodiversity and ecosystem C sequestration to litter manipulation and N addition. Our results showed that alleviation of light limitation by removing litter increased plant species richness under N enrichment, which principally caused by higher species gains and lower losses. Common and rare species contributed most of the increased species as light limitation was relieved. N addition had no significant impact on net ecosystem productivity (NEP), whereas alleviation of light limitation and the subsequent increase in plant coverage and density under N enrichment stimulated gross primary productivity (GPP) but did not affect ecosystem respiration (ER), causing positive responses of NEP. This study highlights the promoting effect of the enhanced light availability through litter removal on species diversity and ecosystem C sequestration under N enrichment. These findings provide new insights into the mechanisms underlying light limitation influence on biodiversity and ecosystem C sequestration under N enrichment. We suggest that grassland management by removing the accumulated litter is an adaptive way to enhance light availability, mitigate diversity loss and thus increase C sequestration under eutrophication. [ABSTRACT FROM AUTHOR]

• Warming increased CH 4 uptake and decreased N 2 O emission on the annual basis. • Clipping alone did not affect CH 4 and N 2 O fluxes. • Warming had a large effect on CH 4 uptake in the growing season. • Warming had a large effect on N 2 O emission in the nongrowing season. • Warming shifted ecosystem from a net source to a net sink of CH 4 and N 2 O fluxes. Methane (CH 4) and nitrous oxide (N 2 O) are two greenhouse gases with much more warming potential than carbon dioxide (CO 2). However, there have been less studies on their responses to climate warming and land use practices, such as hay harvest in grasslands. Especially, their fluxes are not well estimated during the nongrowing season. In this study, we investigated year-round (August 2015–August 2016) continuous measurements of CH 4 and N 2 O fluxes in response to simulated warming, clipping (as a mimic of hay harvest), and their interaction in an alpine meadow on the Qinghai-Tibetan Plateau. Compared to the control, warming alone and in combination with clipping significantly increased CH 4 uptake by 42% and 51%, respectively, on the annual basis. Warming alone also significantly decreased year-round N 2 O emission by 57% relative to that under control. However, clipping alone did not affect CH 4 and N 2 O fluxes during the study period, and no significant interactive effect of clipping and warming was detected. Furthermore, warming had larger effects on CH 4 uptake but smaller effects on N 2 O emission in the growing than nongrowing season. We also found that the responses in CH 4 and N 2 O fluxes to different treatments were regulated by changes in soil temperature and moisture. Based on sustained global warming potential approach and expressed as CO 2 -equvalents, the ecosystem switched from a net source of these two gases in the control (1.2 g CO 2 -eq m-2) to a net sink in warming (-11.3 g CO 2 -eq m-2) and its combination with clipping (-9.9 g CO 2 -eq m-2). The findings highlight the importance of understanding greenhouse gas fluxes in the nongrowing season and suggest the increase of CH 4 uptake and reduction in N 2 O emission under climate warming will benefit ecosystem feedback and help mitigate climate change. [ABSTRACT FROM AUTHOR]