The role of carbon dioxide in emission of ammonia from manure

Abstract

Ammonia emission from manure is a significant loss of fixed N from agricultural systems and contributes to air pollution and ecosystem degradation. Despite the development of numerous mathematical models for predicting ammonia emission, the interactions between CO2 emission, manure pH, and ammonia emission are not completely understood. Others have recognized that CO2 emission from manure can increase the surface pH, and so increase the rate of NH3 emission, but this interaction has not been completely described or quantified. In this work, we present a model of simultaneous NH3 and CO2 emission that includes equilibrium acid/base reactions, kinetically-limited CO2 hydration/dehydration reactions, and diffusive transport. Our model accurately predicted the increase in NH3 emission from simple solutions due to CO2 emission, while an equilibrium-only model did not. Model predictions showed that when NH3 and CO2 emission occur simultaneously, CO2 emission generally increases NH3 emission rate by causing an elevation in surface pH. For thin stagnant layers, this response occurs under a wide range of conditions, although the magnitude of the effect is dependent on manure composition, temperature, surface mass transfer coefficient, and other parameters. Kinetically-limited CO2 hydration/dehydration reactions moderate this interaction, so equilibrium-based models tend to over-predict NH3 emission in the absence of significant carbonic anhydrase activity. Predicted emission from deep, mixed manure showed less dependence on CO2 emission, although higher rates of CO2 hydration/dehydration increase this effect. Interactions between CO2 and NH3 emission influence the effect of model parameters on NH3 emission and result in some unexpected responses. Future work should clarify the processes controlling CO2 speciation and transport in manure, including CO2 minerals, bubble transport, and CO2 hydration/dehydration rates. Our model can inform the development of simpler models for estimating NH3 emission, and the design of experiments aimed at quantifying processes that influence NH3 emission from manure. The effects of CO2 on NH3 emission deserve more attention, and both experimental and modeling approaches are needed to understand the interactions that control NH3 emission.