Modeling ammonia volatilization from urea application to agricultural soils in the DayCent model

Abstract

Nitrogen (N) loss through ammonia $({\mathrm{NH}}_{3})$ volatilization in agricultural soils is a significant source of atmospheric ${\mathrm{NH}}_{3}$, contributing to low N use efficiency in crops, risk to human health, environmental pollution, and is an indirect source of nitrous oxide $({\mathrm{N}}_{2}\mathrm{O})$ emissions. Our objective was to develop an ammonia volatilization method within the DayCent ecosystem model that incorporates key 4R N management practices (right type, right rate, right placement, and right timing) that influence ${\mathrm{NH}}_{3}$ volatilization associated with application of urea-based nitrogen fertilizers to agricultural soils. The ${\mathrm{NH}}_{3}$ volatilization method was developed with Bayesian calibration using sampling importance resampling methods and Bayes factors to select the level of complexity in the model that best represents ${\mathrm{NH}}_{3}$ volatilization given the observed data. The final model included urea hydrolysis and the influence of urease inhibitors; short-term soil pH changes following fertilization; fertilizer incorporation into the soil (mechanically and through irrigation/precipitation); and specification of the fertilizer placement method (i.e. broadcast vs. banding and surface vs incorporated). DayCent predicts ${\mathrm{NH}}_{3}$ volatilization with a root-mean-squared error of 158 (95% interval ranging from 133 to 192), bias of 7 (95% interval ranging from − 106 to 102) g NH₃-N ha⁻¹ day⁻¹, and with a Bayesian R² value of 0.39 (95% interval ranging from 0.17 to 0.62). Furthermore, the model incorporates key management options influencing ${\mathrm{NH}}_{3}$ volatilization related to placement method and fertilizer type with and without urease inhibitors that can be used to evaluate management and policy options for reducing losses of NH₃ from urea fertilization.