FATE OF FERTILIZER NITROGEN FOR WINTER WHEAT/SUMMER MAIZE ROTATION IN NORTH CHINA PLAIN UNDER OPTIMIZATION OF IRRIGATION AND FERTILIZATION

A 15 N-microplot experiment for determination of fertilizer nitrogen fate in winter wheat/summer maize rotation was conducted in North China Plain,where irrigation and nitrogen fertilizers are conventionally over used.In this experiment,the fate of fertilizer nitrogen between conventional irrigation-fertilization and an optimized irrigation-fertilization,which was based on the soil Nmin(NO_3--N+NH_4+-N)and soil moisture,was compared.It was shown that the optimized irrigation and nitrogen fertilizer could decrease agricultural input on the basis of maintenance of grain yield as conventional practices.It was found that only 22.7% and 25.7% of applied nitrogen under conventional irrigation-fertilization was recovered by above-ground parts of winter wheat and summer maize in the growing crop season,respectively,and 28.44% for the whole rotation perod.Loss was 52.9%,35.7% and 47.0% of applied nitrogen in winter wheat season,summer maize season and the rotation period,respectively.While optimization of nitrogen fertilization under conventional irrigation practices could greatly increase crop recovery and decrease the loss,the corresponding recoveries were 45.1%,42.9% and 46.1%,and the corresponding loss were 33.3%,7.1% and 34.5%,respectively.With comparison of irrigation optimization and conventional irrigation,there was no significant difference in fertilizer nitrogen recovery under optimized fertilization,but optimized irrigation could increase the soil fertilizer nitrogen residue and decrease the loss.Due to effect of irrigation on mineralization of soil nitrogen,irrigation should be considered when nitrogen optimization was introduced.It was also showed that,in general,leaching played a minor role in the loss of applied nitrogen for winter wheat/summer maize rotation in North China Plain,denitrification would be the primary mechanism of fertilizer nitrogen loss.