Long-term effects of organic amendments on the rice yields for double rice cropping systems in subtropical China

Organic amendment (OA) has positive effects on rice yield in literature, but few studies have reported positive effects on rice yield trend in a region, which need to be distinguished for the development of sustainable agriculture in terms of management of organic resources available in agroecosystems. The objectives of the present study were to examine the long-term effects of OA on rice yield, yield trends and soil fertility change under the same double rice cropping system at three experimental sites established in 1980s in subtropical China. Organic amendments at the sites covered different strategies in use of locally available green manure, rice straw and farmyard manure to partially substitute inorganic fertilizers at different doses of recommended N, P and K in combination at Jinxian, Jiangxi Province, to partially substitute inorganic N at rates of 30%, 50% and 70% of the recommended rate at Nanchang, Jiangxi Province, and to fully substitute single element of inorganic N, P and K at Qiyang, Hunan Province. The actual rice yield showed no significant time trends at Jinxian and Nanchang and negative trends in the treatments with low N application rates at Qiyang. Analysis of the relative yield trends showed significant positive effects of OA on the yield trends for either first or second rice crop in the treatments except for those without sufficient nutrient supply. The positive effects can be attributed mainly to the increased soil organic carbon and soil nutrient capacity due to the long-term application of OA. The results suggest that the organic amendments strategies should be encouraged as partial substitution of inorganic fertilizers for the consideration long-term yield trends and soil fertility.