Use of Absorption Isotherms to Determine the Ammonium Sorption Potential of Beef Cattle Feedyard Manure.

Ammonia (NH₃) volatilization from beef feedyards results in loss of agronomically important N and presents risks for human health and the environment.  Empirical and process-based models have been developed to estimate NH₃ emissions from various livestock operations; however, their applicability to feedyards is unknown.  One tenet of most models is that NH₃ volatilization is partially dependent on the proportion of free NH₃ in solution, and based on dissociation of NH₄⁺ into NH₃ and H⁺.  Sorption of NH₄⁺ to solids decreases ammoniacal N in the soluble phase, thus lowering volatilization potential.  Feedyards in the Southern High Plains are subject to hot, arid conditions, and surface manure is fibrous and contains little moisture or soil; therefore, it is questionable if constants for dissociation (K_d), equilibrium, and kinetic rate determined for soils or liquid manure systems are valid for estimating feedyard NH₃ emissions. The objectives of this study were to conduct batch equilibration experiments to characterize sorption of NH₄⁺ by feedyard manure.  In kinetic sorption studies with 500 mg L^-1 N from (NH₄)₂SO₄ + 0.01 M CaCl₂ at a 1:10 ratio of manure to solution, equilibrium was reached within 24 h, and average NH₄⁺-N sorption was 2.9 mg g^-1 manure.  Desorption studies revealed that 53-76% of the sorbed NH₄⁺ was readily removed by 0.01 M CaCl₂, and manures differed in binding strength.  Increasing concentrations of NH₄⁺-N from 100 to 5000 mg L^-1 resulted in a roughly linear (C-type) increase in NH₄⁺-N retention with a maximum of 29 mg g^-1.  After 3 d of air-drying, up to 73% of sorbed NH₄⁺-N was volatilized.  These results indicate that while association with manure solids may temporarily reduce the potential for NH₄⁺ dissociation, sorption may not be an important long-term mechanism in controlling NH₃ volatilization from feedlot surfaces.