Reply on RC2

Abstract

Accurate simulation of ammonia (NH3) volatilization from fertilized croplands is crucial to enhancing fertilizer-use efficiency and alleviating environmental pollution. In this study, a process-oriented model, CNMM-DNDC (Catchment Nutrient Management Model – DeNitrification-DeComposition), was evaluated and modified using NH3 volatilization observations from 44 and 19 fertilizer application events in cultivated upland areas and paddy rice fields in China, respectively. The original CNMM-DNDC model not only performed poorly in simulating NH3 volatilization from upland areas but also failed to simulate NH3 volatilization from paddy rice fields. In the modified CNMM-DNDC model, the major modifications for simulating NH3 volatilization from uplands were primarily derived from a peer-reviewed and published study. NH3 volatilization from uplands was jointly regulated by the factors of wind speed, soil depth, clay fraction, soil temperature, soil moisture, vegetation canopy, and rainfall-induced canopy wetting. Moreover, three principle modifications were made to simulate NH3 volatilization from paddy rice fields. First, the simulation of the floodwater layer and its pH were added. Second, the effect of algal growth on the diurnal fluctuation of floodwater pH was introduced. Finally, the Jayaweera-Mikkelsen model was introduced to simulate NH3 volatilization. The modified model showed remarkable performances in simulating the cumulative NH3 volatilization of the calibrated and validated cases, with drastically significant zero-intercept linear regression of slopes of 0.94 (R2 = 0.76, n = 40) and 0.98 (R2 = 0.71, n = 23), respectively. However, the volatilized NH3 simulated by the modified model still exhibited some deviation from the observations when deep/mixed-placement, irrigation/precipitation, and prosperous/depressed algal biomass accompanied the fertilizer application events. Future studies still need to solve these problems to further improve the performance of the modified model. Nevertheless, the modified model could provide an available method for developing NH3 emission inventories and reducing environmental pollutions.