3Power J E. Nitrate contamination of ground water in north American. Agron Ecosys Environ , 1989,26: 165-187.
4Guillard K. Nitrogen utilization of selected cropping system in the U S A northeast: soil profile nitrate distribution and accumulation. Agron J, 1995,87: 199-207.
5Westerman R L. , Boman R K, Raun W R, et al. Ammonium and nitrate nitrogen in soil profile of long-term winter wheat fertilization experiments. Agron ,J , 1994,86: 94-99.
6Kessavalou A, Doran J W, Powers W L, et al. Bromide and nitrogen-15 tracers of nitrate leaching under irrigated corn in central Nebraska. J Environ Qual, 1996,25: 1008-1014.
7immons D R, Dylla A L. Nitrogen leaching as influenced by nitrogen management and supplemental irrigation level. J Environ Qual, 1981,10:421-426.
8Ritter W F. Nitrate leaching under irrigation in the United States A review. J Environ Sci Health A ,1989,24:349-378.