Dynamics Of Mineral N Residual In Dryland Soil And Its Affecting Factors

Application of N fertilizer is an important measure in increasing crop yield. However, more and more input of N fertilizer has unavoidably led to a great deal of mineral N residual in cropland soils due to the low recovery of fertilizer N by crops, which subsequently has caused serious pollution of ground and underground water, and atmosphere, as well as the other ecological environment. In this thesis, factors affecting determination of soil mineral N were studied using different kinds of soils, and the optimum determination method was designed. Then, field experiments were carried out, using winter wheat and summer maize as test crops, to study the dynamics of residual N in dryland soils and its affecting factors during the crops growing season over years. The main conclusions are as follows:1. The determination of soil mineral N was affected by the method of soil sample processing, diameter of sieve, ratio of extract solution to soil, vibration time and storage method of the soil extract to different degrees. The optimum method to determine the soil mineral N is using fresh soil sample, adopting 3 mm sieve, 10:1 extract solution to soil ratio, and 60 min of vibration time, and determining the mineral N in soil extract right after extraction by Continuous Flow Analyzer.2. A great deal of residual mineral N was found during the growing season of winter wheat, and the amount of residual mineral N was increased after seeding with the maximum value being reached at jointing stage and the most of the residual N in soil layers bellow 100 cm. After the stage of jointing, the amount of soil ammonium-N increased rapidly, and even became higher than nitrate-N in soil. The accumulation peak of mineral N in soil profile was found to move down to deep layers with the extension of growth stages, especially the soil ammonium-N. At harvest of winter wheat, the accumulation peak of ammonium-N had already moved down to the 180-200 cm soil layer, with the amount of 29.6 kg N hm-2. 3. The amount of nitrate-N residue in 0-200 cm soil layer was obviously decreased by growing winter wheat, but the amount of ammonium-N and its distribution in soil profile was not significantly affected until harvest of winter wheat. The amount of soil ammonium-N was observed to positively change with the change of temperature of soil and air. No certain relationship was found between changes of the amount of soil ammonium-N or nitrate-N and the soil moisture content, and soil nitrate-N was hardly leached down to deep soil layer by the lower amount of precipitation during the late growing season of winter wheat.4. The amount of mineral N residual in 0-200 cm soil layer was as high as 861.8kg hm-2 during the growth season of summer maize, and the amount of soil nitrate-N is obviously higher than that of ammonium-N at the same stage. With the prolongation of growing season, mineral N in soil profile showed obvious trend of moving down to deep soil layers, especially soil nitrate-N. The accumulation peak of soil nitrate-N had moved down to soil layers bellow 200 cm under soil surface.5. The amount of residual nitrate-N was obviously decreased by growing summer maize, while the amount of ammonium-N and its distribution in soil profile was not affected. Application of N fertilizer significantly increased the amount of residual nitrate-N, and affected its distribution in the soil profile. The amount of soil ammonium-N was observed to positively change with the change of temperature of soil and air, and nitrate-N negatively changed with the temperature. However, the change of the amounts of soil ammonium-N and nitrate-N was consistent with that of soil moisture, although the former was changed slower in time than the former in different degrees. In northern China, soil nitrate-N was able to be leached down to soil layers bellow 200 cm under soil surface by the heavy rainfall during the summer season.