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Huang Dong-mai, Gao Jia-hua Zhu Pei-li (Institute of Soil and Fertilizer, Jiangsu Academy of Agricultural Science)  
The application of chemical fertilizer combined with farm manure is popularly practised in chinese farming. In order to clarify the characteristics of nitrogen supply in such a fertilization system, pot experiments were conducted using ~(15)N labelled crotalaria and ammonium sulphate as organic and inorganic nitrogen sources respectively. Some preliminary results are summarized as follows: 1. Rice plant utilized much more soil nitrogen than fertilizer nitrogen. In all treatments only about one third of the nitrogen assimilated by the rice plant came from the fertilizers. The ratio of soil nitrogen to fertilizer nitrogen taken up by rice plant was closely related to the kinds of soil. In general, the more fertile the soil, the more soil nitrogen was assimilated by rice plant. 2. As ammonium sulphate was applied with crotalaria both at the same nitrogen level, rice absorbed relatively more (NH_4)_2SO_4-N in the first 30 days after transplanting. Thereafter, it absorbed approximately equal amount of nitrogen come from ammonium sulphate and crotalaria, the available nitrogen supplying capacity of crotalaria seemed to be potentially similar to that of ammonium sulphate from maximum tillering stage till maturity. In addition, the availability of ammonium sulphate nitrogen revealed more long lasting in the presence of crotalaria. 3. Good correlations were found between the A-value and the crop yields, as well as soil nitrogen uptake by rice plant. 4. Comparative studies on different soils by application of different nitrogen sources showed that the priming effect was only taken place in the case of the application of chemical nitrogen fertilizer, but this effect under green manure applied remains to be studied. 5. Experimental results showed that in combined fertilization, the inorganic nitrogen fertilizer promoted the mineralization of organic nitrogen in green manure, and the amount of immobilized inorganic nitrogen was considerably increased by the existence of organic nitrogen. These mutual effects seemed to be an essential characteristic of such a fertilization system. As a result, the availability of organic fertilizer nitrogen was increased, more of the inorganic nitrogen applied was immobilized and kept in soil. 6. Different fertilization changed the status of the nitrogen balance sheet. Basal application of crotalaria showed that the amount of residual erotalaria nitrogen retained in soil was greater than the amount of crotalaria nitrogen taken up by rice plant. On the contrary, in the ammonium sulphate treatment, the amount of ammonium sulphate nitro- gen uptake by rice plant was greater than the immobilized portion. The nitrogen balance sheet of combined application showed an intermediate feature between those of crotalaria and ammonium sulphate applied alone. Besides, it was indicated that the loss of chemical nitrogen could be reduced when it was dressed with organic fertilizers.
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