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Zhu Peili Huang Dongmai(Institute of Soil and Fertilizer, Jiangus Academy of Agricultural Sciences)  
The transformation of nitrogen was studied in a well-drained paddy soil by adding~(15)N labelled crotalaria, ~(15)N labelled wheat straw, (~(15)NH_4)_2SO_4 and (~(15)NH_4)_2SO_4 together with cellulos so as to adjust the C/N ratio of applied fertilizer from zero to 50 at the same N level. Rice was planted under three soil water regimes, i.e. submerged condition, submerged and percolated condition (1.5cm/day), and dryland condition kept with 60% W. H. C. The effect of soil water regimes on rice yeild seemed to be more predominant than that of C/N of manure applied, submergence was an essential factor for increasing the yield potential of rice. Nagative correlation was found between the carbon content of manure and N uptake from manure, a critical level of C/N of mapure applied to a well drained paddy soil was examined, which indicated that if the C/N of applied manure was greater than 20,the rice yield would be decreased, and the reverse was also true. With addition of carbon sources to soil, it was found that the slope of the equation of net immobilization and net release of nitrogen under submerged condition was close to that of dryland condition. It was supposed that the impact of water regime on rate of N-transformation might be slight in present soil, however, the amount of immobilized fertilizer N detected under dryland condition was higher than that in submerged condition. The loss of nitrogen in soil through percolation amounted to 10—25% of the total N loss in present experiment, results of ~(15)N isotopic determination showed that about 80% of nitrogen in percolatde water was native soil nitrogen. The amount of residual nitrogen derived from (NH_4)_2SO_4 applied was approximately compensated by the release of soil nitrogen due to priming effect. Therefore, it is considered that the nitrogen level of soil is hardly to be increased only by the application of nitrogenous chemical fertilizers. The critical value of C/N of organic manure, which is defined as the value at which the degree of nitrogen immobilization equals to that of nitrogen release in soil, has been determined and discussed in view of practical purposes.
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