Responses of N_2O and CH_4 fluxes to fertilizer nitrogen addition rates in an irrigated wheat-maize cropping system in northern China

摘要

Model and field studies generally posit that when the application rates of nitrogen fertilizer exceed crop needs, nitrous oxide (N_2O) emissions will increase nonlinearly, though linear responses are also extensively reported by field studies. We conducted year-round measurements of crop yield, N_2O and methane (CH_4) fluxes for treatments of six fertilizer nitrogen levels (0, 135, 270, 430, 650 and 850 kgNha~(-1) yr~(-1) in the form of urea) in a typical irrigated wheat-maize rotation field in northern China. Linear models characterized the responses of cumulative N_2O emissions to fertilizer rates well; therefore, the calculated N_2O emission factors of 0.17±0.02 %, 0.73±0.05% and 0.49±0.02% for the wheat season, maize season and annual scale, respectively, were appropriate for the different fertilizer rates. The cumulative CH4 uptake by the soil tended to be enhanced at higher fertilizer rates (>350 kgNha~(-1)) in the maize season whereas no effect was observed for the wheat season. When the annual fertilizer rates increased from 270 to 430, from 270 to 650, and from 270 to 850 kgNha~(-1) yr~(-1), the crop yields increased only 3～15% (0.5～2.1 t ha~(-1) yr~(-1)), but cumulative N_2O emissions increased 35～115% (0.9～3.0 kgNha~(-1) yr~(-1)). We recommend 270 kgNha~(-1) yr~(-1)as the locally optimum fertilizer rate. Considering the nitrogen inputs by fertilization (270 kgNha~(-1) yr~(-1)), irrigation (4.3±0.2 kgNha~(-1) yr~(-1)) and deposition (wet deposition: 30.5±1.5 kgNha~(-1) yr~(-1)), the slightly positive soil nitrogen balance could maintain the current crop yield (～13.8 t ha~(-1) yr~(-1)) and reduce the present high N_2O emissions (>3.51 kgNha~(-1) yr~(-1)) of the local farmers’ practice (fertilizer rate>430 kgNha~(-1) yr~(-1)).