Responses of net ecosystem carbon budget and net global warming potential to long-term nitrogen deposition in a temperate grassland

Abstract

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 (Re) was observed, except at low LN concentration (30 kg N ha−1 yr−1). Annual N2O flux was significantly increased, and the CH4 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 CO2 ha−1yr−1, respectively, and significantly decreased with increasing LN application. The offset effect of N2O 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.