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Nitrogen (N) fertilizer application and atmospheric N deposition will profoundly affect greenhouse gas (GHGs) emissions, especially nitrous oxide (N 2 O) and methane (CH 4) fluxes and ecosystem respiration (R e , i.e. CO 2 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 N 2 O and CH 4 fluxes and R e 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 120) and 240 (N 240) kg N ha−1 decreased from 7.12 to 4.37 and 4.18, respectively, after 13 years of N inputs. Soil CH 4 uptake was significantly reduced, but N 2 O emission was enhanced significantly by N addition. However, N addition had no impact on R e. Structural Equation Modeling indicated that soil NH 4 +-N content was the dominant control of N 2 O emissions, but with less effect of the decreasing pH. In contrast, CH 4 uptake was generally controlled by soil pH and NO 3 −-N content, and R e by forb biomass. The measured changes in N 2 O and CH 4 fluxes and R e from temperate grassland will have a profoundly impact on climate change. An in situ experiment was conducted to investigate the effect of long-term (13-years) increased N addition on N 2 O and CH 4 fluxes and R e 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 120) and 240 (N 240) kg N ha−1 decreased from 7.12 to 4.37 and 4.18, respectively, after 13 years of N inputs. Soil CH 4 uptake was significantly reduced, but N 2 O emission was enhanced significantly by N addition. However, N addition had no impact on R e. Structural Equation Modeling indicated that soil NH 4 +-N content was the dominant control of N 2 O emissions, but with less effect of the decreasing pH. In contrast, CH 4 uptake was generally controlled by soil pH and NO 3 −-N content, and R e by forb biomass. The measured changes in N 2 O and CH 4 fluxes and R e from temperate grassland will have a profoundly impact on climate change. Image 1 • We studied the impacts of long-term N addition on a temperate grassland. • CH 4 uptake was significantly reduced by long-term N addition. • N 2 O emission was controlled mainly by soil ammonium N rather than soil pH. • CH 4 uptake was affected mainly by soil pH and nitrate N content. • Ecosystem respiration was impacted mainly by production of forbs. Long-term increased N addition significantly reduced CH 4 uptake and enhanced N 2 O emissions, but had no significant impact on ecosystem respiration in temperate grassland. [ABSTRACT FROM AUTHOR]

• Soil carbon sequestration was significantly increased by long-term N addition. • NECB and NGWP were significantly reduced by long-term N addition. • The offset effect of N 2 O emissions to ecosystem C uptake was decreased by LN addition. Grassland systems are important terrestrial carbon sinks and have great potential for carbon (C) sequestration. Increased atmospheric nitrogen (N) deposition has profoundly affected C balance and greenhouse gas emissions in grassland systems. However, the effects of long-term nitrogen (LN) deposition on net ecosystem carbon budget (NECB) and net global warming potential (NGWP) in grassland systems are still not clearly understood. A field experiment was conducted to test the effect of LN addition (0, 30, 60, 120 and 240 kg N ha−1 yr−1) on NECB and NGWP in a temperate grassland area in Inner Mongolia, China. LN addition significantly increased soil organic carbon density (SOCD) and C sequestration in surface soil (0–30 cm) with the increase of N addition rate from 2005 to 2018. In contrast, a decrease in ecosystem respiration (R e) was observed, except at low LN concentration (30 kg N ha−1 yr−1). Annual N 2 O flux was significantly increased, and the CH 4 sink was significantly decreased by LN addition. NECB and NGWP were relatively weak in this temperate grassland, ranging from −627.29 ± 198.81 and −232.87 ± 23.11 kg CO 2 ha−1yr−1, respectively, and significantly decreased with increasing LN application. The offset effect of N 2 O emission to ecosystem C uptake decreased significantly with increased LN addition, and was stable at high LN addition. This was related to the increase in soil C sequestration due to plant C uptake. These results indicate that LN addition significantly decreased the NECB and NGWP of this grassland. Increased long-term N deposition significantly enhanced soil C sequestration, which has important implications for mitigating climate warming. [ABSTRACT FROM AUTHOR]