<論文・報告>稲わら施用による農地土壌からの窒素流出の抑制 - 安定同位体窒素を用いた施肥窒素の追跡 -

Abstract

化学肥料の過剰な施用に伴う農地からの窒素の流出は，環境を汚染する．本研究では，微生物の基質である稲わらを土壌に加えることにより，微生物を増加させ窒素を吸収させることで，その流出の低減を試みた．化学肥料施用区（化肥区），化学肥料＋稲わら施用区（稲わら区）のポットでトウモロコシを10週間栽培した．15Nで標識した化学肥料を用いその追跡を行った．栽培中の窒素溶脱量と微生物量，栽培終了時の植物窒素吸収量と土壌窒素量を測定した．トウモロコシは化肥区で稲わら区より大きく成長した．微生物量は栽培期間を通して稲わら区で多かった．窒素溶脱量は化肥区で稲わら区よりも多かった．化学肥料由来の窒素は，化肥区では土壌，トウモロコシ，溶脱水中に半分ほどが残存しており，残りは脱窒したと考えられた．一方，稲わら区では約9 割が土壌に残った．作物生育の向上という課題は残るが，稲わらの施用は化学肥料由来の窒素の溶脱と脱窒を抑制した． Excess nitrogen derived from chemical fertilizer in agricultural fields causes environmental pollution. We hypothesize that rice straw application reduces nitrogen efflux from soil by increasing microbial biomass nitrogen. Maize was cultivated for 10 weeks in pots containing either chemical fertilizer (CF) or chemical fertilizer and rice straw (CF+RS). We used 15N as a tracer in fertilizer. We measured nitrogen in the leaching solution and soil microbial biomass during the cultivation period, in addition to nitrogen in both the soil and plants at the end of cultivation. Maize height was greater in CF pots than in CF+RS pots. The microbial biomass was greater with CF+RS compared with CF alone throughout cultivation. The amount of nitrogen in the leaching solution from CF pots was greater than that from CF+RS pots. About half the nitrogen derived from chemical fertilizer remained in the soil, plant, and leaching solution under CF treatment while the other half of the nitrogen was released into the atmosphere via denitrification. In contrast, about 90% of the nitrogen derived from chemical fertilizer remained in the soil of CF+RS pots. Although plant growth should be improved, rice straw application decreased the amount of nitrogen efflux due to leaching and denitrification.