MANAGING FARMS FOR ENVIRONMENTAL STEWARDSHIP AND PROFIT
Location: Pasture Systems & Watershed Management Research
Title: The role of carbon dioxide in ammonia emission from manure
Submitted to: Atmospheric Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 11, 2012
Publication Date: January 20, 2012
Citation: Hafner, S.D., Montes, F., Rotz, C.A. 2012. The role of carbon dioxide in ammonia emission from manure. Atmospheric Environment. Available: http://dx.doi.org/10.1016/j.atmosenv.2012.01.026.
Interpretive Summary: Ammonia emissions from agricultural systems contribute to poor air quality and pollution of terrestrial and aquatic ecosystems. In the US, animal agriculture is estimated to contribute more than half of the total anthropogenic ammonia emissions with manure being the primary source. In addition to environmental concerns, ammonia emission is a loss of valuable fixed nitrogen from agricultural systems. Computer models are often used to estimate ammonia emissions, and also to develop better understanding of the emission process, which can ultimately contribute to better control of emissions. In this work, we present a new model that provides a better understanding of the emission process. Carbon dioxide, which is also emitted from manure, is thought to influence ammonia emission through its effects on manure pH (carbon dioxide forms a weak acid when dissolved in water). However, the interactions between carbon dioxide emission, manure pH, and ammonia emission are not fully understood. By developing a model that included relevant physical and chemical processes, we were able to confirm the carbon dioxide influence, and to identify relationships that need further study. By contributing to a more complete understanding of the processes of ammonia emission from manure, our work will ultimately contribute to better tools for predicting and mitigating ammonia emission from manure.
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 carbon dioxide emission, manure pH, and ammonia emission are not completely understood. Others have recognized that carbon dioxide emission from manure increases the surface pH, and so increases the rate of ammonia emission, but this interaction has not been fully described or quantified. A model of simultaneous ammonia and carbon dioxide emission was developed that includes equilibrium acid/base reactions, kinetically-limited carbon dioxide hydration/dehydration reactions, and diffusive transport. Our model accurately predicts the increase in ammonia emission from defined solutions due to the presence of carbon dioxide in solution, while an equilibrium-only model does not. Model predictions show that when ammonia and carbon dioxide emission occur simultaneously, carbon dioxide emission generally increases ammonia emission rate by causing an elevation in the 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, the surface mass transfer coefficient, and other parameters. Kinetically-limited carbon dioxide hydration/dehydration reactions moderate this interaction, so equilibrium-based models tend to over-predict ammonia emission in the absence of significant carbonic anhydrase activity. Predicted emission from deep, mixed manure shows less dependence on carbon dioxide emission, although higher rates of carbon dioxide hydration/dehydration increase this effect. Interactions between carbon dioxide and ammonia emission influence the effect of model parameters on ammonia emission and lead to some unexpected responses. Future work should clarify the processes controlling carbon dioxide speciation and transport in manure, including carbon dioxide minerals, bubble transport, and carbon dioxide hydration/dehydration rates. Our model can inform the development of simpler models for estimating ammonia emission, and the design of experiments aimed at quantifying processes that influence ammonia emission from manure. The effects of carbon dioxide on ammonia emission deserve more attention, and both experimental and modeling approaches are needed to understand the interactions that control ammonia emission.