Grazing amplifies the stimulating effects of N addition on N2O emissions in a temperate meadow steppe

Abstract

Nitrogen (N) addition typically increases nitrous oxide (N2O) emissions, but the magnitude of this effect depends on land use type and intensity. Grazing has an important influence on grassland N dynamics, but it is not clear to what extent grazing may interact with N addition in affecting grassland N2O emissions. We investigated the mechanisms underlying the individual and interactive effects of N addition and cattle grazing (control (CK), N addition (N), grazing (G), and N addition plus grazing (N + G) treatments) on N2O emissions in a temperate meadow steppe by measuring N2O fluxes, weather factors, vegetation parameters and soil properties from October 2017 to September 2018. The studied temperate meadow steppe was a source of N2O emissions during the experimental period. Approximately 80 % of the annual N2O emissions were released during the plant growing season. The highest N2O emission was detected in the N + G plots, which indicated the grazing treatment amplified the stimulating effects of N addition on N2O emissions. The results from structural equation model (SEM) suggested that N addition enhanced N2O emission through increases in NH4+-N concentration and soil potential net nitrification rate (Nnitri); grazing promoted N2O emissions by increasing substrate availability directly via excreta or indirectly through trampling-associated soil aggregate disruption and substrate release. N2O emissions increased most strongly with N addition and grazing due to the synergetic effects between animal trampling, animal excreta and available N on the processes of nitrification and denitrification. Overall, our results suggest that the interactive effects of grassland management and global change factors on N2O emissions need to be considered for the accurate assessment of grassland greenhouse gas emissions.