[Current status of nitrogen and phosphorus losses and related factors in Chinese paddy fields: A review]

Rice is one of the main crops in China. Therefore, it is important to understand the current status and influencing factors of nitrogen and phosphorus losses from paddy fields in China, which would facilitate assessing the potential of chemical fertilizer reduction in different rice cultivating regions. We summarized the current knowledge on nitrogen and phosphorus losses from surface runoff in major rice cultivating areas in China, as well as their influencing factors, such as rainfall, planting pattern, cultivation techniques, fertilization management, water management strategies, etc. The total nitrogen (TN) and total phosphorus (TP) losses from runoff in six major rice cropping areas ranged from 5.09 to 21.32 and 0.70 to 3.22 kg·hm^-2, respectively. The highest losses of TN and TP were the South China double rice cropping area. The TN runoff losses were the lowest in the North China single rice cropping area, while the lowest TP runoff losses occurred in plateau single and double rice cropping area of the Southwest China. The peaks of TN and TP in surface water of paddy fields were generally higher than those of the runoff water based on farmers' conventional fertilization in different rice cropping areas. The peak period of nitrogen and phosphorus losses was in a week after rice fertilization. There could be a potential of 20% reduction of nitrogen and phospho-rus for farmers' conventional fertilization compared with the optimized fertilization. Among all the factors, rainfall and fertilization management were the main ones affecting the runoff losses of nitrogen and phosphorus in paddy fields. Fertilization management and water management strategies were the mostly controllable, including reduction of fertilizers, application of new fertilizers, replacement of chemical fertilizers by organic fertilizers, water-saving irrigation, etc. Overall, the risk of nitrogen and phosphorus losses in paddy fields was more prominent in the Southern China than in any other areas of China. Rice cultivation should be carried out in a more resource-efficient way to reduce nutrient loss. Future research should focus on non-point source pollution monitoring of paddy fields, nitrogen and phosphorus losses risk assessment, nitrogen and phosphorus losses characteristics and mechanisms, and new technologies for reducing chemical fertilization inputs and environmental risks.