Nitrogen Migration In Soil-Wheat System And The Ecological Basis For Efficient Nitrogen Utilization

In Middle and Lower Yangtze River Basin in China, wheat and rice are intensively rotated. In wheat production, excessive nitrogen fertilizer together with inappropriate application methods led to low N use efficiency and high N losses through runoff, leaching, denitrification, and volatilization, resulting in a series of environment problems such as the water pollutions and greenhouse gas emissions. On the other hand, increased fertilizer input had increased production costs leading to lower net returns for farmers. Efficient N utilization should be realized in agriculture for environmental and economic reasons. For the efficient N utilization, it should be firstly clear on nitrogen dynamics and fates of fertilizer-N after application. So, field experiments and15N micro-plots experiments in four sequential growth seasons during2006-2010were conducted to study the effects of nitrogen applications and seeding rates on nitrogen balance in soil-wheat system, flow of fertilizer N, allocation and remobilization of nitrogen, root growth and development, grain yield and nitrogen utilization efficiency in winter wheat(Triticum aestivum L.). Here are the main results:1. This study made clear the regulation of soil inorganic nitrogen migration and apparent soil nitrogen budget. Nitrogen fertilizer application increased nitrate nitrogen (NO3--N) in0-60cm soil layer and ammonium nitrogen (NH4+-N)0-40cm soil layer; Soil inorganic nitrogen content increased with increasing of basal nitrogen rate before jointing and NO3--N would leach out60cm soil layer at jointing stage; soil inorganic nitrogen content increased with increasing of topdressing rate after jointing stage. At the same N level, seeding rate had little influence on soil inorganic nitrogen content, but soil inorganic nitrogen content decreased with increasing of seeding rate. Apparent nitrogen budget had evident stage, nitrogen surplus before jointing was the major nitrogen surplus. Through the whole growth stage, apparent nitrogen surplus amount increased with increasing of nitrogen rate and basal ratio, decreased first and then increased with increasing of seeding rate. At the same nitrogen rate, an appropriate topdressing rate and seeding rate decreased NO3--N leaching, maintained the balance in soil-wheat system.2. Flow and balance of nitrogen(N) fertilizer during different growth stages were clarified. The ratio of recovery and soil residual decreased and loss increased significantly with nitrogen rate increasing; it had advantage to improve nitrogen fertilizer recovery and soil residual when topdressing ratio increasing appropriately. Under the same N level, nitrogen recovery increased, and soil residual and loss decreased with increasing of seeding rate appropriately. On average, the recovery and residual of topdressing nitrogen was higher significantly than basal nitrogen, but loss was opposite. The uptake and loss of basal nitrogen were the highest before jointing, and the uptake and loss of topdressing N were the highest in the duration from jointing to anthesis stage, and the stage uptake and loss amount increased with basal or topdressing rate. Loss of basal N before jointing was the major loss of fertilizer N. Both basal and topdressing N moved down to60-100cm soil layer, and the residual amount of basal or topdressing nitrogen increased with increasing of basal or topdressing rate, but mostly remained in the0-20cm soil layer. The residual amount of total fertilizer N in0-100cm soil layer increased with increasing nitrogen rate and topdressing ratio, decreased with increasing of seeding rate. On average, residual of topdressing N was higher than basal N significantly. At the same nitrogen rate, an appropriate topdressing rate and seeding rate improved nitrogen uptake, reduce nitrogen loss.3. This study made clear the characteristic of nitrogen transfer and nitrogen metabolism. The results showed that accumulation rate of nitrogen from different sources reached the highest from jointing to anthesis. Distribution proportion of total nitrogen in grain decreased with increasing of nitrogen rate and topdressing ratio, increased with increasing of seeding rate, but distribution of nitrogen from different sources in different organs showed inconsistent. Remobilization of fertilizer N and soil N increased with increasing of nitrogen rate and seeding rate, increased first and then decreased with increasing of topdressing ratio. Remobilization efficiency and its contribution to grain nitrogen decreased with nitrogen rate and topdressing increasing, increased with seeding rate increasing. Remobilization efficiency of topdressing N was higher than basal N, fertilizer N was higher than soil N in all treatments. Post-anthesis nitrogen accumulation increased with increasing of nitrogen rate and topdressing ratio, but decreased with increasing of seeding rate, soil N was the major nitrogen accumulation after anthesis. Analysis showed the further research was to improve fertilizer N accumulation after anthesis and transfer efficiency of soil N. 4. This study expounded the regulation of root growth and its relationship with yield and nitrogen utilization in winter wheat. Root weight ratio increased in0-20cm soil layer and decreased below20cm soil layer with nitrogen rate and topdressing ratio, but there was an opposite tendency with increasing of seeding rate. Total root length, root volume and root surface area increased with increasing of nitrogen rate, and an appropriate topdressing ratio and seeding rate, but the tendency was different at different soil layer. The correlation analysis showed that there were significant positive correlation between root weight, root total length, root volume, root surface area and grain yield, nitrogen accumulation amount. Root growth decreased with the soil depth from the highest values in0-20cm soil layer, root in0-20cm soil layer had the highest contribution to nitrogen accumulation amount and grain yield. Root only proliferated to20cm soil layer at overwintering stage, which resulted in nitrate nitrogen leaching out root zone and great loss of basal N because root could not uptake or capture more nitrogen at earlier growth stage. The roots grew and proliferated rapidly in the duration from jointing to anthesis stage (appeared peak value at anthesis a stage), especially in upper soil layer. As a result, topdressing N at jointing would be absorbed rapidly by plant or intercepted by the major root growth in upper soil layer. So, topdressing N had higher recovery and residual but lower loss than basal N.In conclusion, it had advantage to improve root growth, reduce loss of NO3--N leaching, increase nitrogen uptake and allocation, remobilization to grain, decrease loss of fertilizer N, maintain nitrogen balance in soil-wheat system, and then enhance yield and, nitrogen utilization efficiency when nitrogen rate was reduced to225kg hm-2and topdressing ratio was increased to50%and seeding rate were increased to225×104hm-2appropriately.