Methane and nitrous oxide emissions from paddy fields with no fertilizer use under continuous irrigation with treated municipal wastewater.

Treated wastewater (TWW) irrigation has been recommended as an environmentally friendly agricultural practice and has been applied in many countries for decades. The effects of wastewater irrigation on rice yield and quality, as well as on the environment, with particular focus on greenhouse gas emissions from paddy fields with municipal wastewater irrigation, have gained substantial attention. In this study, bench-scale experiments were conducted in two cultivation seasons where seedlings of Bekoaoba, a large-grain high-yield rice variety, were transplanted and irrigated with TWW without fertilization. A control experiment was performed to simulate the cultivation conditions of normal paddy fields. The study aimed to quantify the effects of TWW irrigation on rice yield and quality, in addition to CH4 and N2O emissions. The highest rice yield (10.4 t ha-1) and protein content in brown rice (13.8%) was achieved when the soil was repeatedly subjected to bottom-to-top TWW irrigation without any synthetic fertilizer. Bottom-to-top TWW irrigation decreased CH4 emissions by up to 95.6% when compared with tap water irrigation, whereas bottom-to-top and top-to-top TWW irrigation increased N2O emissions by 5 and 15 times, respectively. Bottom-to-top irrigation of TWW could be considered a promising solution for reducing greenhouse gas emissions as TWW irrigation resulted in a lower combined global warming potential than tap water irrigation. Further, bottom-to-top irrigation of TWW produced less CH4 and N2O than top-to-top irrigation.