| Attention has been paid more and more to the grain yield increase and food security than ever before, and the application of N fertilizers has played an important role in this aspect. Nitrogen supply affects photosynthesis, yield construction and grain filling process and characteristics as well as the uptake of N, P and K, and determines to a large extent the growth and development, yield and quality of wheat.The study was conducted from October 2006 to June 2008 at the Irrigation Station, Northwest A & F University, using wheat as test crop with four N rates (without application of N, Adding 80 kg, 160 kg and 240 kg N per ha, respectively, with four replications).. During the period of experiments, measurements were carried out at different crop stages of aboveground biomass, N, P and K contents and their uptake amounts as well as nitrite N in 0-3 m profile to investigate the effects of N rates on the yield increase, biomass production, harvest index, N efficiency, N, P and K uptake and nitrate N accumulation in soil profile. The main results obtained are as follows:1 Under the condition of low organic matter, and low N supply capacity of a soil, N fertilization had a significant function for the increase of crop yield, the magnitude of crop yield increase depending on nitrate N cumulative in soil profile and crop growing factors. In 2006-2007, the soil accumulated relatively high amount of nitrate N, the effect of N fertilizer on yield increase was low. However, in 2007-2008, the nitrate N in 1 m soil layer was almost exhausted, and the precipitation was abundant. As a result, application of 80 kg N per ha greatly increased wheat yield; per kg N increased wheat seeds by 33 kg, and the N recovery rate reached almost to 100%. Even the N rates amounted to 160 and 240 kg, the recovery was still as high as 77% and 61%, respectively. In this case, wheat uptake N from N fertilizer was much higher than that from soil, showing that the N uptake from soil was changed with growing condition, but was not always higher than that from fertilizer..2 Application of N raised plant water contents at early stages while decreased their contents at late stages. The contribution rate of the accumulated water (defined as the ratio of grain yield at harvest to the maximum water amount accumulated in plants) was 0.52,0.60,0.65 and .62 for N rate of 80, 160 and 240 kg per ha, respectively.3 Application of N fertilizer significantly increased N and K contents (concentrations) in plants and N, P and K uptake amounts while had no influence on P concentration or decreased its concentration. For the aboveground plants of wheat, N, P and K uptake amounts were increased with plant growth, and the peak amount occurred in grain filling stage for N and P whereas that for K in heading stage. The peak time occurring at early stage was a typical characteristic of K.4 Nitrogen fertilization played an important role for the accumulation of N, P and K in grain at late stage. The N and P accumulation in wheat grain was mainly from the transfer of N and P in leaves and stems while that for K was totally from the transfer and therefore the uptake rate of K during the grain filling stage was negative values. Nitrogen and P taken up by wheat was mainly in the grain, and therefore the harvest index was 69-76% for N and 79-83% for P. In contrast, K was mainly in leaves and stems and thereby the harvest index was 23-31%.6 After wheat harvest, the amount of the nitrate N accumulated in 0-3 m soil layer was surprisingly high. In 2007-2008, the nitrate N quantity cumulative in 0-3 m soil layer was significantly related with N rates applied. With application of 80, 160 and 240 kg N per ha, the total amount of nitrate N cumulative in 0-3 m layer of soil was 39, 104 and 278 kg N per ha, respectively. The residual nitrate N may cause some environmental issues if it is not rationally managed and utilized.
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