Ammonia (NH3) emissions from manure constitute a significant loss of fixed nitrogen (N) from agricultural systems and contribute to air pollution and ecosystem degradation. Accurate models of such NH3 emissions will improve our understanding of the factors that control the emissions and allow appropriate mitigation actions to be identified and quantified. Although the importance of manure pH on ammonia emission has been recognized for decades, the physical and chemical interactions that control pH are not fully understood. Here we present a novel mathematical model that includes the dynamic and crucial pH changes in the surface of stored slurry or slurry applied in the field. In the model, slurry pH is calculated by simultaneously determining: (1) speciation of the acid-base reactions, (2) diffusion of each buffer species, and (3) emission of NH3 and CO2. New features of the model include a reduced variable that combines time and location and an analytical approach to solving the resulting system of equations using Mathematica. To evaluate the model, we made measurements of pH at a resolution of 0.1 mm in the top 30 mm of an ammonium bicarbonate solution. These measurements show the creation of a large pH gradient (>1 pH unit in <30 mm after 20 h) and its change over time due to simultaneous NH3 and CO2 emission from aqueous solutions. The model was able to accurately predict the development of pH gradients over time, suggesting that our understanding of the factors controlling pH is correct. New developments presented in the model should be useful for future work on understanding and predicting NH3 emission from manure.
Nutrient Cycling in Agroecosystems, 2014, Vol 100, Issue 2, p. 189-204