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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]

• 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]