Nitrogen supply via internal nutrient cycling of residues and weeds in lowland rice farming.

摘要

Development of alternative soil fertility management is opted to mitigate the side effects associated with the excessive use of synthetic fertilizer. Soil fertility management practice through strengthening internal nutrient cycling (agroecological management: AEM) was examined in lowland rice (Oryza sativa) farming in Tochigi Prefecture, Japan. The principal study fields were established AEM (EAEM), where AEM was established in 1999 and transitional AEM (TAEM), where AEM has been practiced since 2009 and the field is still in transient stage. A field with conventional nutrient management (CNM) was studied for reference purposes. In EAEM field, potential amount of soil N supply was estimated at 41.9 g N m-2 in spring 2011. Components in AEM maintained labile N pool were basic mineralizable N (21.0 g N m-2) and annually changing mineralizable N (20.9 g N m-2). Annually changing mineralizable N include soil drying effect (3.6 g N m-2), internal inputs (13.2 g N m-2) such as rice straw, rice bran, spring and winter weed as well as external inputs (4.1 g N m-2) such as biological N₂ fixation, precipitation and guano. Estimated NH₄-N released from soil during the cropping season was 28.5 g N m-2 in 2011. As the N uptake of rice was 11.5 g N m-2 in 2011, EAEM supplied sufficient N to satisfy N demand by the rice plants. Relatively large amount of NH₄-N released in EAEM implies that the recycling of on-site N sources would supply substantial amount of N to rice plants. In TAEM (two and three year practice), limited amount of NH₄-N (17.8 g N m-2) available during the cropping season compared with EAEM emphasized the importance of long-term practice to ensure effects of AEM through increase of basic soil mineralizable N. The research result suggests that AEM might be a sustainable and appropriate soil fertility management option for lowland rice farming if measures such as adjusting application rate of rice bran are employed to control soil N supply.