In this study, 15N-labeled organic materials were used to evaluate the nitrogen (N) dynamics associated with the application of organic materials to paddy fields. The decomposition of three types of organic materials, namely wheat straw, rice straw, and cattle manure compost, was evaluated by using the glass fiber-filter paper bag method. The N retention rates obtained by evaluation of N content were different from those obtained by evaluation of 15N abundance. The fates of N in seven types of 15Nlabeled organic materials, namely cattle manure compost, poultry manure compost, swine feces, rice straw compost, rice bran, rice straw, and wheat straw, were studied during a crop season in a warm region in Japan. The nitrogen use efficiency for swine feces, poultry manure compost, and rice bran relative to that for chemical fertilizers was as high as approximately 70%, whereas the nitrogen use efficiency for cattle manure compost relative to that for chemical fertilizers was as low as 16-19%. The N uptake from 15N-labeled swine manure compost or rice bran simultaneously applied with cattle manure compost was slower than those from swine manure compost or rice bran alone. Furthermore, I investigated the fate of N derived from 15N-labeled cattle manure compost over a period of three years in a region with a cool climate. Well-composted cattle manure compost was found to be a stable N source for rice plants for at least three years after application. A large amount of N derived from the cattle manure compost remained in the soil even three years after application.
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