Duckweed (Spirodela polyrhiza) as green manure for increasing yield and reducing nitrogen loss in rice production

Increasing rice production to feed the world's growing population while protecting the environment requires more optimal use of fertilizers. In China, the current high input, high output and high reliance on synthetic nitrogen (N) fertilizer in agriculture has resulted in high N losses, especially ammonia (NH3) emission. Urea combined with green manure (GM) might be a promising approach to improve N fertilizer management. However, few studies have evaluated duckweed in this manner. Duckweed does not require arable land for cultivation and thus avoids competition with food crops. Therefore, a field experiment was conducted for three years with five treatments (CK, no N-fertilizer; CT, conventional practice, urea alone at 300 kg N ha−1; CTD, urea combined with duckweed at 300 kg N ha−1; RN, urea alone at 225 kg N ha−1; and RND, urea combined with duckweed at 225 kg N ha−1) in an intensive rice cropping system in the Taihu Region of China. The results for two years showed that urea combined with duckweed cover reduced NH3 loss by 36-52% over CT. This reduction was attributed primarily to the formation of a physical barrier and the uptake of NH4+ by duckweed. The 15N recovery for 15N balance conducted for one year was 38% higher and the 15N loss was 16% lower for CTD than that of CT. Furthermore, urea combined with duckweed increased N accumulation in the aboveground plants by 14-25% over CT for the 3 years. As a result, urea combined with duckweed achieved higher rice yield by 9-10%, and higher net economic benefit by 10-11% over CT for the 3 years; however, using the conventional rate of 300 kg N ha−1 did not increase rice yield over using the reduced N rate of 225 kg N ha−1, with or without duckweed. Thus, duckweed as GM combined with chemical fertilizer application provided an approach for increasing the rice yield without increasing inputs of N fertilizer and thereby provided a financially attractive option for farmers to achieve environmental integrity and ensure food security in rice production.