Responses of N₂O and CH₄ fluxes to fertilizer nitrogen addition rates in an irrigated wheat-maize cropping system in northern China

Abstract. Model and field studies generally posit that when the application rates of nitrogen fertilizer exceed crop needs, nitrous oxide (N₂O) emissions will increase nonlinearly, though linear responses are also extensively reported by field studies. We conducted year-round measurements of crop yield, N₂O and methane (CH₄) fluxes for treatments of six fertilizer nitrogen levels (0, 135, 270, 430, 650 and 850 kg N ha⁻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 N₂O emissions to fertilizer rates well; therefore, the calculated N₂O 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 kg N ha⁻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 kg N ha⁻¹ yr⁻¹, the crop yields increased only 3 ∼ 15% (0.5 ∼ 2.1 t ha⁻¹ yr⁻¹), but cumulative N₂O emissions increased 35 ∼ 115% (0.9 ∼ 3.0 kg N ha⁻¹ yr⁻¹). We recommend 270 kg N ha⁻¹ yr⁻¹ as the locally optimum fertilizer rate. Considering the nitrogen inputs by fertilization (270 kg N ha⁻¹ yr⁻¹), irrigation (4.3 ± 0.2 kg N ha⁻¹ yr⁻¹) and deposition (wet deposition: 30.5 ± 1.5 kg N ha⁻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 N₂O emissions (>3.51 kg N ha⁻¹ yr⁻¹) of the local farmers' practice (fertilizer rate > 430 kg N ha⁻¹ yr⁻¹).