Seasonal variations in N2 and N2O emissions from a wheat–maize cropping system

The N losses via denitrification and the N2/N2O emission ratio are highly uncertain, mainly due to methodological difficulties concerning measurement of N2 emissions. Here, we report on seasonal measurements of N2O and N2 emissions from the top soil of a winter wheat–summer maize double-cropping system in the North China Plain. A novel double cylinder soil core method was used to measure N2O and N2 fluxes in situ from a 20-year field experiment with four N fertilizer treatments (0, 200, 400, and 600 kg N ha−1 year−1) and 16 sampling occasions over a 1-year period. The N2-free helium-oxygen atmosphere in the inter-layer between the two cylinders acted as a buffer against N2 diffusion from the atmosphere into the soil core. Total N2O emissions were 0.44, 1.31, 2.19, and 2.23 kg N ha−1 year−1, and total N2 emissions were 8.3, 20.0, 22.5, and 24.7 kg N ha−1 year−1 from the N0, N200, N400, and N600 treatments, respectively. The N2/N2O emission ratio ranged from 2 to 136, indicating that N2 was the dominant gas. The N2/N2O ratio was much higher in summer than in winter, and was inversely related to fertilizer N application. Fluxes of N2 were statistically related to soil temperature and concentrations of NO3−-N and DOC in soil extracts, while N2O fluxes were related to soil water content and concentrations of NO3−-N and exchangeable NH4+ in soil extracts. The N2/N2O ratio was related to soil temperature and exchangeable NH4+ and NO3−-N concentrations. Our study provides new insights on variations of the N2/N2O emission ratio in agroecosystems, and shows the importance of seasonal measurements.