Effects of carbon dioxide emission, kinetically-limited reactions, and diffusive transport on ammonia emission from manure

Volatilization of ammonia 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. 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 in order to assess the importance of CO₂ volatilization, 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 CO ₂ 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 was 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 that cause higher convective mass transfer from the surface. Additional work is needed to assess the importance of the processes studied here for different conditions.