Continuous Sub-Irrigation with Treated Municipal Wastewater for Protein-Rich Rice Production with Reduced Emissions of CH 4 and N 2 O

Herein, we introduce continuous sub-irrigation with treated municipal wastewater (TWW) as a novel cultivation system to promote resource recycling and cost-effective forage rice production in Japan. However, both TWW irrigation and forage rice cultivation were previously considered to intensify CH 4 and N 2 O emissions. In the present study, therefore, we evaluate the emissions of greenhouse gases (GHGs) and yielding capacity of forage rice between conventional cultivation and continuous sub-irrigation systems employing different water supply rates. Results indicated that continuous sub-irrigation with TWW resulted in high rice yields (10.4–11 t ha −1 ) with superior protein content (11.3–12.8%) compared with conventional cultivation (8.6 t ha −1 and 9.2%, respectively). All TWW irrigation systems considerably reduced CH 4 emissions, while higher continuous supply rates significantly increased N 2 O emissions compared with the conventional cultivation. Only the continuous irrigation regime employing suitable supply rates at appropriate timings to meet the N demand of rice plants decreased both CH 4 and N 2 O emissions by 84% and 28%, respectively. Overall, continuous sub-irrigation with TWW provides high yields of protein-rich forage rice without the need for synthetic fertilisers and effectively mitigated GHG emissions from paddy fields.