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• 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]