Impact of 13-years of nitrogen addition on nitrous oxide and methane fluxes and ecosystem respiration in a temperate grassland.

Nitrogen (N) fertilizer application and atmospheric N deposition will profoundly affect greenhouse gas (GHGs) emissions, especially nitrous oxide (NO) and methane (CH) fluxes and ecosystem respiration (R, i.e. CO emissions). However, the impacts of long-term N inputs and the often associated N-induced soil acidification on GHG fluxes in arid and semi-arid ecosystems, especially temperate grasslands, are still uncertain. An in situ experiment was conducted to investigate the effect of long-term (13-years) N addition on NO and CH fluxes and R from a temperate grassland in Inner Mongolia, northeast China, from April 2017 to October 2018. Soil pH values in the 0-5 cm layer receiving 120 (N) and 240 (N) kg N ha decreased from 7.12 to 4.37 and 4.18, respectively, after 13 years of N inputs. Soil CH uptake was significantly reduced, but NO emission was enhanced significantly by N addition. However, N addition had no impact on R. Structural Equation Modeling indicated that soil NH-N content was the dominant control of NO emissions, but with less effect of the decreasing pH. In contrast, CH uptake was generally controlled by soil pH and NO-N content, and R by forb biomass. The measured changes in NO and CH fluxes and R from temperate grassland will have a profoundly impact on climate change.