Nitrogen Efficiencies of 9 Green Manures Labeled with 15N in Paddy Rice Cultivation.

Rice is the staple and main crop in Japan. For developing a sustainable and environmentally conscious rice production system, green manures attract attentions as fallow season crops in south area of Japan. In this study, the relationships between N mineralization rates from 9 different green manures and N uptake by paddy rice were investigated by both incubation test and field test. The green manures used were 4 legumes (white clover, Chinese clover, hairy vetch and crimson clover), 3 Gramineae (Oat, rye and Italian ryegrass), Brassica hirta and Phacelia tanacetifolia and cultivated with ¹⁵N labeled fertilizer. In incubation test at 30C, the 4 legumes showed higher N mineralization rates than the other green manures. Especially hairy vetch represented the highest rate. All of the 3 Gramineae have low N mineralization rates. Never the less, all the residues decomposed so quickly. By 30 days incubation, the mineralization rate of N in the legumes and the non-legumes ranged 68-81% and 57-67% of total mineralization, respectively. In field experiment, all the green manures increased the rice yield except for Italian ryegrass. Application of hairy vetch and white clover gave the highest yield. In the all residues, the yields highly related to the number of panicles m^-2 and the amount of N mineralized during 0-30 or 31-60 d incubation. But there is little relationship of the mineralized N to rice grain number, percentage of ripened grains and 1000-grain weight. From these data, the leguminous green manures, especially hairy vetch and white clover, are suitable green manures for following paddy rice cultivation in the south area of Japan.