Aiming to promote low-cost production of protein-rich forage rice and resource recycling from wastewater treatment plants, a pot experiment was conducted to assess the possibility to substitute mineral fertilizers with composted sewage sludge (CSS) with/without top-dressing with treated municipal wastewater (TWW). Results indicated that a basal application of CSS at 2.6 g N pot⁻¹ replaced conventional mineral fertilization of 1.3 g N pot⁻¹ to produce comparable yields with the same rice protein content, although there might be a risk of increased As concentration in rice grains. Interestingly, CSS application at a reasonable dose of 1.3 g N pot⁻¹, followed by a topdressing with TWW resulted in 27% higher yield and 25% superior rice protein content relative to the mineral fertilization, with no risk of heavy metal(loid) accumulation in grains and in paddy soils. Here we demonstrated an appealing fertilization practice with zero use of mineral fertilizers in paddy rice cultivation, expectedly contributing towards sustainable rice farming and animal husbandry in Japan.

Herein, we examined emissions of CH₄ and the community structures of methanogenic archaea and methanotrophic bacteria in paddy soils subjected to a novel irrigation system, namely continuous sub-irrigation with treated wastewater (TWW). This system has recently been developed by our group to effectively reuse TWW for the cultivation of protein-rich rice. The results showed that, despite not using mineral fertilisers, the wastewater reuse system produced a rice yield comparable to that of a conventional cultivation practice and reduced CH₄ emissions from paddy fields by 80%. Continuous sub-irrigation with TWW significantly inhibited the growth of methanogens in the lower soil layer during the reproductive stage of rice plants, which was strongly consistent with the effective CH₄ mitigation, resulting in a vast reduction in the abundance of methanotrophs in the upper soil layer. The compositions of the examined microbial communities were not particularly affected by the studied cultivation practices. Overall, this study demonstrated that continuous sub-irrigation with TWW was an effective method to produce high rice yield and simultaneously reduce CH₄ emissions from paddy fields, and it also highlighted the potential underlying microbial mechanisms of the greenhouse gas mitigation.

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 CH₄ and N₂O 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 CH₄ emissions, while higher continuous supply rates significantly increased N₂O 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 CH₄ and N₂O 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.

The use of composted sewage sludge (CSS) and centrate as alternatives to synthetic fertilizers in rice cultivation holds great promise. This study aims to determine the effects of varying doses and timings of centrate derived from anaerobically digested sewage sludge on rice yield, nutrient quality, and soil fertility when applied as a topdressing to rice fields fertilized with CSS. At the panicle initiation (PI) stage, 100, 300, and 500 kg N ha^-1 of centrate topdressing (CT100, CT300, and CT500, respectively) was applied. In addition, different topdressing timings at a total dose of 500 kg N ha^-1 were evaluated, including a two-split application (40% at active tillering (AT) and 60% at PI; CT500S2) and a three-split application (40% at AT + 40% at PI + 20% at heading; CT500S3). At a rate of 160 kg N ha^-1, CSS was used as a base fertilizer in all treatments. A control treatment received synthetic fertilizers at a rate of 160 kg N ha^-1 as a base application and 100 kg N ha^-1 as a topdressing. Results showed that CSS-treated rice plants exhibited a lower N status and leaf chlorophyll content during the vegetative growth stage; however, the split application of centrate topdressing improved plant N status, resulting in an increase in biomass and grain yield. Centrate and CSS tended to increase the mineral content of rice; nevertheless, a significant accumulation of As in grains raised concerns about food safety. Combining CSS and centrate has the potential to increase rice production, improve grain nutritional value, and decrease reliance on synthetic fertilizers. However, it is essential to optimize this fertilization, mitigate environmental risks, and ensure food safety by employing appropriate fertilization dosing and timing as well as appropriate field management strategies.