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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]

• N addition enhanced GEP and NEP only when volumetric soil moisture exceeded 15%. • GEP and NEP showed saturating responses to N addition only at high soil moisture. • N effects on C exchange were mediated by changes in species composition. • Temperature and soil moisture interactively regulate N effects on NEP, ER and GEP. Primary productivity in terrestrial ecosystems can be significantly altered by the predicted increases in nitrogen (N) deposition, but it is still unclear how N deposition influences the carbon (C) exchange processes especially in dryland ecosystems. In this study, a 3-year experiment with two types of fertilizers and five N addition levels was conducted in a semiarid steppe of Erguna, Inner Mongolia to assess the effects of exogenous N input on net ecosystem productivity (NEP), gross ecosystem productivity (GEP) and ecosystem respiration (ER). Our results showed that enhanced N input significantly increased NEP and GEP when the volumetric soil water content (VWC) was greater than 15% (v/v%), but had trivial and inconsistent effects when VWC was less than that threshold. Moreover, the NEP and GEP increased significantly with increasing N addition but leveled off when the N input reached 10 g N m–2 year–1 and the VWC was above 15%. However, the ER showed a significant increase with N addition rates, regardless of soil water content. Our main findings are as follows. First, the ecosystem C exchange can be significantly enhanced by addition of N only when soil moisture exceeds certain threshold, suggesting co-limitation of water and N in the grassland ecosystem that we studied. Second, in addition to concurrent consideration of water, temperature and N as well as their interactions also need to be accounted for. Third, the frequency of days with the VWC being higher than 15% accounted only around one third of days during the three growing seasons. Taken together, our findings imply that the expected increases of carbon sequestration should be minimal under N deposition in the Inner Mongolia grassland ecosystems. [ABSTRACT FROM AUTHOR]

• Wheat ET increased significantly and maize ET decreased significantly. • The high mean annual ET occurred in the well-irrigated area and humid region. • Human activities controlled ET trend in wheat season. • Climate change dominated ET trend in maize season. Accurate evapotranspiration (ET) estimation is important for understanding hydrological cycle and water resources management in the cropland. Based on eight flux sites within the North China Plain (NCP) and the surrounding area, which were integrated together for the first time, we applied support vector regression method to develop ET dataset for the cropland in NCP from 1982 to 2015 with 1/12° spatial resolution and eight-day temporal interval. The mean annual ET over cropland within NCP was 575 mm yr−1 with a non-significant increasing trend. In wheat season, the seasonal ET increased significantly at the rate of 1.28 mmyr−2 with a mean value of 238 mmyr−1; in maize season, the mean value of seasonal ET was 221 mmyr−1 with a decreasing trend of −0.95 mmyr−2. The comparatively high mean annual ET values occurred primarily in the well irrigated area near the Taihang Mountains, irrigation districts along the Yellow River and the humid region. The attribution analysis which combined multiple regression with the first difference method revealed that human activities dominated ET trend at annual scale as well as in wheat season, with relative contributions of 52% and 56%, respectively. While in maize season, ET trend was mostly controlled by climate change with relative contribution of 77%. Among the climate factors, in wheat season, the significantly increasing air temperature was the principal climatic cause of ET increase which was partly offset by the significantly decreasing wind speed; while in maize season, the significant ET decline was primarily resulted from the significantly decreasing net radiation and wind speed. Compared with our ET dataset, the four widely used global or national ET datasets significantly underestimated ET in wheat season, especially in the extensively irrigated areas, suggesting that local observations are crucial for regional studies. [ABSTRACT FROM AUTHOR]