ARS | Publication request: Effects of carbon dioxide emission, kinetically-limited reactions, and diffusive transport on ammonia emission from manure

Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: May 14, 2010
Publication Date: June 20, 2010
Citation: Hafner, S.D., Montes, F., Rotz, C.A. 2010. Effects of carbon dioxide emission, kinetically-limited reactions, and diffusive transport on ammonia emission from manure. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE). Paper No. 1008895.

Technical Abstract: Volatilization of ammonia (NH3) from animal manure causes significant loss of fixed N from livestock operations. Ammonia emission from manure is the culmination of biological, chemical, and physical processes, all of which are well-understood. In this work, we present a speciation and transport model for simultaneous CO2 and NH3 emission from manure. Our model was implemented using the geochemical software, PHREEQC, to assess the importance of CO2 emission, equilibrium speciation, kinetically-limited reactions, and aqueous diffusion on NH3 emission dynamics from thin layers (1-10 mm) of dairy cattle manure. Preliminary predictions show that emission of CO2 leads to a rapid increase in manure pH, which causes a substantial increase in NH3 flux. Kinetic limitations on conversion of carbonic acid to dissolved CO2 were predicted to significantly influence NH3 emission by limiting the effect of CO2 volatilization on solution pH. Therefore, these reactions should be considered in the development of chemical models of NH3 emission. For the limited number of conditions studied in this work, diffusion of solution species through the aqueous phase had only small effects on NH3 emission from 1 and 5 mm layers. However, aqueous-phase diffusion can be limiting for thicker layers or for conditions with higher convective mass transfer from the surface. Additional work is needed to assess the importance of the processes studied over different conditions.