Sewage irrigation increased methane and nitrous oxide emissions from rice paddies in southeast China

Greenhouse gas emissions from rice paddies under sewage irrigation deserve much attention since domestic sewage effluents are increasingly used for agriculture in developing countries. A field experiment was conducted to simultaneously measure methane (CH4) and nitrous oxide (N2O) emissions from rice (Oryza sativa L.) paddies under sewage and unpolluted river water irrigation in southeast China. The rice paddies were under a local typical water regime, which was characterized by flooding-midseason drainage-reflooding-moist intermittent irrigation but without water logging. Relative to unpolluted river water irrigation, sewage irrigation significantly increased CH4 and N2O emissions from rice paddies. Seasonal fluxes of CH4 averaged 1.51 Mg m(-2) h(-1) for the plots irrigated by river water and 1.92 Mg m(-2) h(-1) for the plots irrigated by sewage. In contrast with river water irrigation, sewage irrigation increased CH4 by 27% and 33% for paddy plots with and without chemical N addition, respectively. Under sewage irrigation, seasonal fluxes of N2O-N averaged 26.79 mu g m(-2) h(-1) for the plots without N application and 74.07 mu g m(-1) h(-1) for the plots applied at the rate of 200 kg N ha(-1). Relative to river water irrigation, sewage irrigation increased N2O by 68% and 170% for the plots with and without N application, respectively. The direct emission factor of fertilizer N for N2O was estimated to be 0.71% for the rice paddies under sewage irrigation and 0.52% for the plots irrigated by river water. Besides direct N2O emissions, N input by sewage irrigation induced substantial indirect N2O emission from rice paddies. The results of the net GWPs from CH4 and N2O indicate that sewage irrigation would intensify the radiative forcing of rice paddies with midseason drainage and moist irrigation. (C) 2008 Elsevier B.V. All rights reserved.