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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 subirrigation 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. Cultivation of forage rice (Oryza sativa L.) has been promoted by the Japanese government to reduce the cost of domestic animal husbandry by reducing the use of imported feedstuffs, which can be of unstable supply and highly priced depending on global markets 1. As a result, there has been a recent increase in forage rice cultivation. However, this has been accompanied by high levels of N fertiliser use to ensure high-yield production, which might lead to inefficient N use and considerable N loss to the environment 2. Effluents from wastewater treatment plants (WWTPs) contain high concentrations of organic and inorganic nutrients beneficial for plant growth and development. Thus, reusing these effluents for agricultural irrigation has major advantages for crop production and environmental management 3. Paddy rice cultivation generally demands large amounts of irrigation water and synthetic fertilisers, and thus, would greatly benefit from recycling water and valuable nutrients from WWTPs. To promote forage rice production and establish an effective resource circulation model for the management of agricultural water, we developed new cultivation systems, and treated municipal wastewater (TWW) was effectively reused in paddy fields to produce high yields of forage rice without applying synthetic fertilisers 1,4,5. The reuse of wastewater or TWW for rice cultivation has been intensively investigated and is widely practiced owing to its undeniable benefits. For instance, prior studies have demonstrated that rice grain yield from fields irrigated with treated wastewater could be 35-55% higher than that from groundwater-irrigated fields 6. Furthermore, reusing wastewater could lower the...

To obtain a high rice yield and quality for animal feed without synthetic fertilizers, an experiment with bench-scale apparatus was conducted by applying continuous irrigation with treated municipal wastewater (TWW). Uniform rice seedlings of a high-yield variety (Oryza sativa L., cv. Bekoaoba) were transplanted in five treatments to examine different TWW irrigation directions (“bottom-to-top” and “top-to-top” irrigation) and fertilization practices (with and without P-synthetic fertilizers) as well as one control that simulated the irrigation and fertilization management of normal paddy fields. The highest rice yield (14.1 t ha−1), shoot dry mass (12.9 t ha−1), and protein content in brown rice (14.6%) were achieved using bottom-to-top irrigation, although synthetic fertilizers were not applied. In addition, this subsurface irrigation system could contribute to environmental protection by removing 85–90% of nitrogen from TWW more effectively than the top-to-top irrigation, which showed a removal efficiency of approximately 63%. No accumulation of heavy metals (Fe, Mn, Cu, Zn, Cd, Ni, Pb, Cr, and As) in the paddy soils was observed after TWW irrigation for five months, and the contents of these metals in the harvested brown rice were lower than the permissible limits recommended by international standards. A microbial fuel cell system (MFC) was installed in the cultivation system using graphite-felt electrodes to test the capacity of electricity generation; however, the electricity output was much lower than that reported in normal paddy fields. Bottom-to-top irrigation with TWW can be considered a potential practice to meet both water and nutrient demand for rice cultivation in order to achieve a very high yield and nutritional quality of cultivated rice without necessitating the application of synthetic fertilizers.

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