Studies on the Control of Fate of Water, Heat, and Nitrogen in Agricultural Soils

从环境问题的角度阐明农业土壤中水、热、氮的去向和控制具有重要意义。模拟了土壤中水、热、氮的矿化、硝化和反硝化过程，揭示了通过人工控制氮肥、水肥用量和土壤温度来控制氮素运移和转化的可能性。具体内容如下：(1)利用两种典型的日本旱地土壤(沙土和红壤)，明确了土壤中产生硝酸盐的硝化过程的特性。红壤的硝化速率远大于沙土。土壤水分和土壤温度对红壤硝化作用的影响很大。(2)沙地土壤入渗过程中，施氮量和施氮期对土壤剖面中各种阳离子和阴离子的影响较大。包括硝化作用在内的土壤氮素和水分去向的模拟技术可以反映沙土和红壤的入渗试验结果。(3)长期的水氮迁移模拟表明，分肥分水施入可以降低氮素向地下水淋失的风险。温室栽培条件下土壤温度的升高会激活氮素的硝化作用，增加氮素淋失的风险。(4)加州玉米-谷物轮作灌溉条件下，牛粪液态氮的去向可以用数值方法模拟，包括矿化、硝化和反硝化作用。(5)在梯田中，地下水位的时间变化在坡面上部比坡面下部大得多。坡下土壤处于还原状态，水文地球化学和分子生物学研究表明，坡面的存在对减少硝态氮通过反硝化作用向下游排放具有重要意义。

It is important to clarify the fate and the control of water, heat, and nitrogen in agricultural soils in the sense of environmental issues. We modeled the fete of water, heat, and nitrogen including mineralization, nitrification, and denitrification in soils and showed the possibilities to control the nitrogen movement and transformation by the artificial control of nitrogen and water applications and soil temperature. Specific knowledge is as follows;(1) Properties of nitrification process that produces nitrate in soils were become clear using two typical Japanese upland soils (sand and andosol). The rate of nitrification of andosol was much larger than one of sand. Nitrification of andosol highly depended on soil moisture and soil temperature.(2) The amount and timing of nitrogen applications effected on soil profiles of various cations and anions during infiltration process in the case of sand. Modeling technique of nitrogen and water fates in soil including nitrification could represent the results of infiltration experiments for sand and andosol.(3) Long-term water and nitrogen transfer simulations showed that split fertilizer and water applications could reduce the risk of nitrogen leaching into groundwater. Increase of soil temperature under green house cultivation would activate nitrification and give rise to the risk of nitrogen leaching.(4) Fate of liquid dairy manure nitrogen in an irrigated double crop corn-grain rotation in California could be modeled numerically including mineralization, nitrification, and denitrification.(5) In terrace paddy fields, temporal changes in groundwater level were much larger at the part of upper side of the slope than at the part of under side of the slope. Soils at under side of the slope were always under deoxidized condition and hydrogeochemical and molecular biological approaches showed that the existence of the slope was important to reduce the nitrate discharge to downstream by denitrification.