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

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