Compartmental process-based model for estimating ammonia emissions from stored liquid dairy manure

Abstract

This study developed a compartmental process-based model (PBM) as an alternative approach to estimate ammonia emission from liquid dairy manure during storage to improve the accuracy of quantification tools for nitrogen cycling at the farm level compared to the currently used non-compartmental PBM. The compartmental PBM developed partitions stored manure into several layers in the vertical domain to facilitate the spatial temperature and substrate concentration calculations. In contrast, the non-compartmental PBMs currently used consider the bulk stored manure as a material with homogenous properties. The models used similar equations and processes from the well-established principles of heat and mass transfer and pertinent known biogeochemical processes for the production and emission of ammonia. The models were calibrated, and their performance assessed using literature-based experimentally derived ammonia emission rates. Additionally, a scenario analysis was conducted to compare ammonia emission rate estimates by the models from stored liquid manure at a dairy farm during two (cold and warm) storage periods. Model outputs were sensitive to ambient air temperature, manure pH, wind speed, manure total ammoniacal nitrogen concentration, and the two-way interactions of ambient air temperature, pH, and wind speed. Ammonia emission rates by the models and literature-based experimentally derived ammonia emission rates were similar (p > 0.05). In general, the compartmental PBM predicted lower ammonia emission rates compared to the non-compartmental PBM. Also, the compartmental PBM had a relatively better performance compared to the non-compartmental PBM. Compared to the non-compartmental PBM, the compartmental model estimated lower ammonia emission rates, i.e., 28% and 34%, for the cold and warm periods, respectively.