Modeling ammonia volatilization following the application of synthetic fertilizers to cultivated uplands with calcareous soils using an improved DNDC biogeochemistry model

Simulation of ammonia (NH₃) volatilization by process-oriented biogeochemical models, such as the widely used DeNitrification DeComposition (DNDC), is an imperative need to identify the best management strategies that can improve nitrogen use efficiency in crop production while alleviating environmental pollution. However, scarce validation has been impeding the applicability of the DNDC for this purpose. Using the micrometeorological or wind tunnel-based observations of NH₃ volatilization in 44 cases with at seven nationwide field sites in China, which were cultivated with summer maize and winter wheat in calcareous soils and applied with synthetic fertilizers, the DNDC was tested, modified, and evaluated in this study. The following major modifications were made in the model source codes. Primarily, pedo-transfer functions were introduced into the model to provide three soil hydraulic parameters that are required to simulate soil moisture. Then, the temperature effect on ammonium bicarbonate decomposition, which was originally missing, was parameterized. Finally, the effect of soil texture on ammonia volatilization from the liquid phase was re-parameterized while an adaption factor was set. Seven typical cases were involved in the model modifications and the other 37 independent cases were used for the modified model evaluation. Compared to the original model, the modified DNDC performed better. For instance, it showed a higher index of agreement of 0.77 versus 0.38, a higher modeling efficiency (Nash-Sutcliffe index) of 0.19 versus -0.52, and a greater determination coefficient (R²) of 0.35 (p < 0.001) versus no available value (i.e., R² ≤ 0) in the zero-intercept linear regression of the observed cumulative NH₃ volatilizations during individual measurement periods against the simulations. Future studies are needed to further improve the modified DNDC so as to better simulate the effects of rainfall/irrigation and deep placement of fertilizers on NH₃ volatilization from calcareous soils cultivated with upland crops.