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

Abstract

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 (CH 4) and nitrous oxide (N 2O) 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 CH 4 and N 2O emissions from rice paddies. Seasonal fluxes of CH 4 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 CH 4 by 27% and 33% for paddy plots with and without chemical N addition, respectively. Under sewage irrigation, seasonal fluxes of N 2O-N averaged 26.79 μg m −2 h −1 for the plots without N application and 74.07 μg m −2 h −1 for the plots applied at the rate of 200 kg N ha −1. Relative to river water irrigation, sewage irrigation increased N 2O by 68% and 170% for the plots with and without N application, respectively. The direct emission factor of fertilizer N for N 2O 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 N 2O emissions, N input by sewage irrigation induced substantial indirect N 2O emission from rice paddies. The results of the net GWPs from CH 4 and N 2O indicate that sewage irrigation would intensify the radiative forcing of rice paddies with midseason drainage and moist irrigation.