Use of Digested Slurry from Livestock Manure in Paddy Fields and its Environmental Effect in Southeast Asia

Use of digested slurry from livestock manure in paddy fields and the associated environmental effects were examined using a regional diagnosis and field experiments with the aim of establishing a sustainable biomass use system in Southeast Asia. First, analysis of biomass use in a suburban village in Southern Vietnam identified several options for improving the regional environment and generating energy, resulting in a preferred proposal to use currently untreated livestock manure for biogas production and use digested slurry as a fertilizer in paddy fields.Second, the effects of using digested slurry as fertilizer on rice production and environmental impacts were evaluated through field experiments. This study clarified that digested slurry can be substituted for chemical fertilizers, and nitrogen loading and fecal contamination of receiving waters can be avoided if no water is discharged from the treated plot for a sufficient duration. Third, the economic feasibility of using digested slurry was evaluated in field experiments, which found that it is economically feasible if the nitrogen concentration of the digested slurry can be raised to five times that of present conditions. Finally, the effects of extending this regional scenario to all of Southeast Asia on nitrogen loads discharged to water bodies and on greenhouse gas emissions was estimated and evaluated.東南アジアにおける持続可能な地域バイオマス利用システムの構築を目指して，地域診断と現地圃場試験にもとづき，東南アジアの水田における家畜ふん尿消化液の液肥利用とその環境影響について検討した。稲作と畜産がさかんなベトナム南部の都市近郊農村を対象に，バイオマス利用に関する現状を分析し，家畜ふん尿や消化液の垂れ流しによる水域への窒素負荷状況とこれらのバイオガス生産への利用意義および消化液の水田液肥としての利用可能性を示した。また，現地の水田において，消化液の肥効や消化液施用後の田面水中の窒素および衛生指標菌の動態を調査し，施肥に伴う下流水域汚染は，消化液施用後に表面流出を一定期間生じないようにすることで防げることを示した。さらに，消化液の液肥利用を行う場合の施肥コストを試算し，消化液の濃度を現在の5 倍にすることで，慣行栽培より施肥コストを下げることが可能であることを示した。今後の家畜ふん尿利用戦略に資するため，家畜ふん尿のバイオガスへの利用と消化液の水田への液肥利用を，東南アジア全体に導入した場合の水域への窒素負荷削減効果ならびに温室効果ガス削減効果について推定評価した