Impacts of fertilization optimization on N loss from paddy fields: Observations and DNDC modeling case study in Shanghai, China

Nitrogen (N) loss from paddy fields is an important source of agricultural nonpoint source pollution and leads to widely existing water contamination issues. The current fertilization rate in paddy fields of Shanghai exceeds the actual demand of rice and leads to substantial N loss. The objective of this study is to optimize the current fertilization scheme in paddy fields of Shanghai with the combination of field observations and modeling approach. In this study, N loss from a typical flooded paddy field in a Shanghai suburb was observed with an embedded lysimeter system from 2009 to 2013 rice seasons. The denitrification-decomposition (DNDC) model was adopted to assist with the integration and upgrading of the observed data. During the experimental period, the observed paddy field generated a seasonal average N loss of 14.9 kg N.ha( -1) through runoff and leaching, which accounted for 5.0 % of the total fertilizer N. Surface runoff events that occurred shortly after fertilization largely contributed to the total N loss from the experimental paddy field. According to the validation test against observational data, the DNDC model simulated N loss from the experimental paddy field well both in terms of runoff and leaching. Sensitivity analysis and scenario simulations conducted with the DNDC model demonstrated that the current fertilization rate of 300 kg N.ha( -1) exceeded the actual crop demand at the experimental site. An application rate of 240 kg N.ha(-1) with the optimized fertilization time was the best fertilization scheme for the experimental paddy field, which significantly reduced total N loss by 31.6 % while maintaining the optimal rice yields. The mitigating efficiencies varied from 23.8%-37.2 % under different precipitation and clay content in Shanghai suburb. A combination of field experiments with the DNDC model is an efficient approach to optimize field management practices and transfer the field results to other regions, thereby achieving a balance between agricultural production and the surrounding environment.