Improvement Of Nitrogen Fertilizer Use Efficiency By Water-saving Cultivation In Winter Wheat-summer Maize Production System In Northern China

Application of nitrogen fertilizer is one of the most important measures for increasing crop yield in agriculture. However, misuse of nitrogen fertilizer (particularly overuse) in traditional winter wheat-summer maize production system in northern china was frequent, which caused low nitrogen fertilizer use efficiency and related environmental problems (for example NO3"-N leaching to groundwater). In order to improve agricultural resources (water and nitrogen fertilizer) use efficiency and crop yield simultaneously, field and undisturbed soil column experiments were carried out at Wuqiao Experiment Station of China Agricultural University in Hebei Province from 2002 to 2004. The objects of this study are: (1) investigating dynamics, spatial distribution and accumulation of soil NO₃^--N in the summer maize-winter wheat production system, (2)examining plant nitrogen accumulation, distribution and use efficiency under different nitrogen fertilizer applied rates and irrigations in waiter wheat season., and (3)testing the ability of winter wheat to capture and re-use subsoil ¹5N-labeled fertilizer nitrogen under water-saving cultivation in the condition of field or soil column. The results are as follows.1. There was leaching of nitrogen fertilizer applied in summer maize season influenced by rainwater, and a part of nitrogen fertilizer has been leached out of maize root zone and accumulated in subsoil (120²00cm ) in the form of residual NO3-N because of these factors such as shallow roots, much rainfall and great ability of soil nitrogen provision . Residual soil NO3-N in 2m soil profile increased with more nitrogen applied, and the treatments of nitrogen applied (N90,N180,N270,N360) increased the residual soil NO3-N content in the 120²00cm soil profile by 50.4 150.4kg NO3-N/hm2 compared to non-nitrogen applied treatment (NO) at harvest stage. The soil column results indicated that fertilizer ¹5N distinctively move to deep soil layer (below soil depth 100cm) and some ¹5N fertilizer nitrogen leaching out of summer maize roots zone under the condition of ¹5N-labeled Urea applied. The proportion was amount to 18.28% of 1.0².0 m soil profile residual ¹5N in whole 2.0m soil profile. It was suggested that in summer maize season some nitrogen was leached out of root zone, and could not be absorbed, which not only reduced nitrogen fertilizer use efficiency, but also might cause ground water pollution.2. The residual ¹5N-labeIed fertilizer nitrogen of 2m soil profile from summer maize season could be captured and re-used by winter wheat under water-saving cultivation due to the significant difference of roots spatial distribution between winter wheat and summer maize. There are distinctively difference of rooting depth or roots strap spatial distribution between summer maize and winter wheat whether in the field or in the column conditions. The most root depth was about 120cm for summer maize and exceeded 2.0m for winter wheat at anthesis stage. Compared with traditional cultivation (w4 treatment) in winter wheat, water-saving cultivation (wO or w2 treatment) significantly increased the percentage of root dry weight at deep subsoil, which could explain why ¹5N-labeled nitrogen placed in deep subsoil could be captured under w2 treatment or wO treatment condition in the field condition. In the undisturbed soil column, wheat could also capture ¹5N-labeled nitrogen in deep soil. Recovery rates of ¹5N-labeled nitrogen in 100¹10cm,120¹30cm,and 140 150cm soil layers were 16.26%,7.33% and 4.38% respectively. Winter wheat growth in soil column could capture and re-use the residual fertilizer ¹5N in whole 2.0m soil profile from front stubble summer maize under the treatment of one-time irrigation in spring. The range of residual fertilizer ¹5N recovery rate (%) by winter wheat absorbed is 22.98% 4.41% and more lower with depth increase. Compared withconventional irrigation treatment (W4N 157.5 ) , water-saving and fertilizer-saving cultivation (W2N157.5) Significantly decreased subsoil NO3-N content in field winter...