Increasing yield and N use efficiency with organic fertilizer in Chinese intensive rice cropping systems

In China, the current high reliance on chemical nitrogen (N) in agriculture has resulted in low N use efficiency and high N loss. Reducing N fertilizer input is necessary for environmental protection, but it has not been attractive to Chinese farmers because of insufficient increase in net income. Reduced use of N fertilizer might be attractive if reduced N combined with organic fertilizer could increase rice yield. Therefore, a three-year field trial and a 15N micro-plot experiment were conducted to study the effect of organic fertilizer combined with chemical N on rice yield, NH3 loss and the fate of 15N in an intensive rice cropping system in Taihu Lake region. Six treatments were used: control (0 N, zero N application), organic N alone (OrgN, 41 kg N ha−1), local farmers' N practice (FN, 300 kg N ha−1), reduced N (RN, 225 kg N ha−1), organic fertilizer combined with FN (FN + OrgN) and organic fertilizer combined with RN (RN + OrgN). Compared to chemical N, organic fertilizer markedly increased soil organic matter content, promoted grain N accumulation, and improved rice production. Organic fertilizer treatments showed 11%-13% higher yield and 4%-5% higher net economic benefit over FN for three years. Organic fertilizer produced high NH4+ in the floodwater and increased NH3 loss. Although FN + OrgN had significantly higher 15N-NH3 loss than FN, it increased soil residual 15N and decreased 15N loss by 29%. There was no significant difference of yield and NEB between FN + OrgN and RN + OrgN, while RN + OrgN was superior to FN + OrgN because of the lower NH3 loss and higher N recovery efficiency (NRE). Therefore, organic fertilizer is an economically attractive practice to increase NRE and rice yield without increasing chemical N input.