Modeling ammonia volatilization following urea and controlled-release urea application to paddy fields

Ammonia volatilization, a main pathway of nitrogen (N) loss in paddy fields, can easily lead to environmental problems, such as haze and water eutrophication. Field measurement of ammonia volatilization is time-consuming and expensive. Soil-crop system models can simulate the effect of different fertilizer management practices on NH3 volatilization and compensate for the limitations of field measurement. Therefore, the WHCNS (soil Water Heat Carbon Nitrogen Simulator) model was used to evaluate the feasibility of simulating NH3 volatilization in paddy fields under different N fertilizer types and application methods. The field experiment, including four treatments of farmer's practice (FP), basal application of urea in one dose (SBN), split application of urea in three doses (SPN), and basal application of controlled-release urea in one dose (CRU), was conducted from 2013 to 2014. All treatments were conducted with a fertilizer rate of 165 kg N ha(-1), except for FP with a rate of 210 kg N ha(-1). The measured dry matter weight, grain yield, N uptake, and NH3 flux were used to test the WHCNS model. The model evaluation indices of Nash-Sutcliffe efficiency (NSE) and index of agreement (d) were close to 1 for the measured variables, except for NSE of yield (0.16). The normalized root mean square error (NRMSE) of NH3 flux was slightly over 30%, but lower than 30% for the other items. Results showed that the model can successfully simulate rice yield and NH3 flux of urea and controlled-release urea under different application methods. The simulated annual average cumulative NH3 losses of FP, SBN, SPN, and CRU accounted for 18.9%, 17.8%, 20.6%, and 10.1% of their N inputs, respectively. Compared with the FP treatment, the ratios of NH3 losses of the SBN and CRU treatments were decreased by 1.2%-8.8%. Compared with the SBN treatment (the same rate of 165 kg N ha(-1)), SPN reduced the peak of NH3 flux but increased the cumulated NH3 losses and CRU significantly reduced the NH3 flux and cumulative NH3 losses. The application rate of controlled-release urea could be further optimized by refined model calculations to reduce NH3 volatilization and total N loss while maintaining yield.