Gross Nitrification Rates and Nitrous Oxide Emissions in an Apple Orchard Soil in Northeast China

A better understanding of nitrogen (N) transformation in agricultural soils is crucial for the development of sustainable and environmental-friendly N fertilizer management and the proposal of effective N2O mitigation strategies. This study aimed: i) to elucidate the seasonal dynamic of gross nitrification rate and N2O emission, ii) to determine the influence of soil conditions on the gross nitrification, and iii) to confirm the relationship between gross nitrification and N2O emissions in the soil of an apple orchard in Yantai, Northeast China. The gross nitrification rates and N2O fluxes were examined from March to October in 2009, 2010, and 2011 using the barometric process separation (BaPS) technique and the static chamber method. During the wet seasons gross nitrification rates were 1.64 times higher than those under dry season conditions. Multiple regression analysis revealed that gross nitrification rates were significantly correlated with soil temperature and soil water-filled pore space (WFPS). The relationship between gross nitrification rates and soil WFPS followed an optimum curve peaking at 60% WFPS. Nitrous oxide fluxes varied widely from March to October and were stimulated by N fertilizer application. Statistically significant positive correlations were found between gross nitrification rates and soil N2O emissions. Further evaluation indicated that gross nitrification contributed significantly to N2O formation during the dry season (about 86%) but to a lesser degree during the wet season (about 51%). Therefore, gross nitrification is a key process for the formation of N2O in soils of apple orchard ecosystems of the geographical region.