77-22 Use of Absorption Isotherms to Determine the Ammonium Sorption Potential of Beef Cattle Feedyard Manure.



Monday, October 17, 2011
Henry Gonzalez Convention Center, Hall C, Street Level

Heidi Waldrip1, Richard Todd2 and N. Andy Cole2, (1)USDA-ARS Conservation and Production Research Lab, Bushland, TX
(2)USDA-ARS, Bushland, TX
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 (Kd), 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. 
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