Saturated N2O emission rates occur above the nitrogen deposition level predicted for the semi-arid grasslands of Inner Mongolia, China

Nitrous oxide (N2O) is one of the most important greenhouse gases emitted by the semi-arid grasslands of Northern China. The majority of previous studies focused on nitrogen (N) deposition and its impacts on N2O emission rates in this region, while the mechanisms and controls of N2O emission and the evidence for a balance point following increasing N deposition remained unclear. In this study, we investigated during 2013-2015 the environmental and plant controls over N2O emission rates within a long-term N addition experiment with nitrogen levels of 1, 2, 4, 8, 16, 32, 64 g N m(-2) yr(-1), respectively, by using an in situ static chamber method. The results showed that N2O emission rates increased with increasing nitrogen addition rates. The emission rates showed significantly positive linear correlations with soil temperature, NO3--N, NH4+-N, inorganic N, microbial biomass carbon, microbial biomass N, total soluble N, below-ground plant biomass and above-ground plant biomass, but a significantly negative correlation with soil pH. A structural equation modeling analysis showed that N addition affected in particular the pH value and subsequently N cycling and soil N2O emission. Meteorological factors impacted N2O emission rates through affecting soil environment and N cycling processes. The formulated N2O emission balance hypothesis was supported and the balance point at which the N2O emission rate became saturated was somewhere between 32 and 64 g N m(-2) yr(-1). The plant N usage efficiency was highest when the rate of N addition was 2 g N m(-2) yr(-1). The increased knowledge of environmental and plant control over soil N2O emission provides a better understanding of N cycling within the semi-arid temperate grassland ecosystem and will be fundamental for quantifying N2O budgets at various scales.