Reclamation intensifies the positive effects of warming on NO emission in an alpine meadow.

Climatic warming can alter grassland nitrous oxide (NO) emissions due to soil property alterations. However, how the reclamation affect grassland NO flux under warming conditions remains unclear in alpine meadow ecosystems. We conducted a long-term manipulative warming experiment in a natural alpine meadow and a cultivated grassland on the Qinghai-Tibetan Plateau to explore the separate and interactive effects of warming and reclamation on the soil NO emission flux. NO fluxes were measured under four treatments including control (CK), warming (W), reclamation (R) and warming under reclamation (WR) from August 2018 to July 2019. We measured the content of soil C, N nutrients and 5 enzymatic activities in 2018 and 2019. Correlation analysis and structural equation modeling were used to clarify how soil N availability and soil enzyme activities affect NO emission. Our results indicated that compared to the ambient conditions for the growing and non-growing seasons, soil NO flux was significantly increased 59.1% and 152.0% by warming and 28.4% and 142.4% by reclamation, respectively. Compared with W, WR significantly increased NO flux by 18.9% and 81.1% during the growing and non-growing seasons, respectively. Soil moisture was negatively correlated to enzymatic activity and NO flux. Both warming and reclamation promoted soil nitrification by increasing related enzymatic activities that acted to increase the NO flux. Reclamation resulted in a greater sensitivity of the activity of ammonia monooxygenase and hydroxylamine oxidoreductase to warming, thus enhancing the effects of warming on increasing the NO flux. Our research indicated that reclamation can additionally increase the effects of warming on NO emissions for alpine meadows. Therefore, excessive expansion of arable land should be avoided, and new reclamation sites should be planned scientifically, as warming is expected to intensify in the future.