Shifting China’s rice fields from nitrogen source to sink by innovative agricultural management

Rice field has been traditionally considered as a nonpoint source of reactive nitrogen (N) for the environment, while it, with surrounding ditches and ponds, also contributes to receiving N inputs from atmosphere and waterbodies and intercepting N outputs from rice field. However, due to the paucity of multi-site, long-term observation and control experiment data, as well as robust process-based model for nitrogen budget, a comprehensive assessment of the N source (i.e., outputs > inputs) or sink (i.e., inputs > outputs) of rice field for the environment, from site to regional scale, is lacking. Here, a 2-year systematic observation and process-based simulations of N budget across China, covering typical annual rice cropping systems including single rice, double rice and rotation, were conducted to identify the roles of rice field in nitrogen cycling in China. The cost-benefit analysis for shifting China’s rice fields from nitrogen source to sink in different climatic scenarios (wet, normal and dry year) by innovative agricultural management, without compromising crop yield or soil fertility, were further evaluated. Rice fields, with surrounding ditches and ponds, perform as a nitrogen sink for atmosphere but a source for waterbody, which was confirmed by both the observational data and regional assessment by process-based model, regardless of rice types or climatic scenarios. With the adoption of sustainable N and water regulation measures, single rice field across China could be shifted to N sink (12.9-29.9 kg N ha^-1) while the N source of double rice would be reduced by 51%-66%. For middle rice field (rotation with other crops), N sink would be achieved in dry year, while 31%-55% would shift to sink in wet and normal year. However, with such great benefits, the costs only accounted for 41%-49% of the expenditure for waste water treatment. Furthermore, through sorting all the measures and adopting economic ones (lower cost with higher benefit) in priority, we found that rice fields across China have great space for source-to-sink regulation with zero cost, which means the benefits would exceed the regulation costs, and even achieve overall N sink in dry year. Together these findings help us to update scientific knowledge to the role of rice fields in ecosystems, as well as highlight the significance and possibility for achieving environmental-friendly rice field, by improving agricultural management technologies.