Sorption and desorption of ammonium from swine lagoon waste in two Kansas soils

High NH₄+ concentrations in the soil beneath waste lagoons of concentrated animal operations (CAO) pose an environmental threat via nitrification followed by potential NO₃− leaching to groundwater. Ammonium sorption and desorption play a key role affecting NO₃− leaching because it can regulate available NH₄+ for nitrification. This study was conducted to evaluate sorption-desorption behavior of NH₄ + in Kennebec silt loam and Haynie very fine sandy loam soils exposed to a complex matrix such as liquid swine waste and a simple matrix representing an aqueous solution of (NH₄)₂SO₄. The NH ₄+ sorption isotherms fitted to the Langmuir model showed significantly greater sorption of NH₄+, from swine waste than from (NH₄)₂SO₄ solution, in soils. The Kennebec soil sorbed significantly more NH₄+ as compared to the Haynie soil. These findings were further supported by the results of kinetics studies which demonstrated the time-dependent sorption of swine waste-derived NH₄+. The amount of NH ₄+ sorbed in soil was positively correlated with the dissolved organic carbon (DOC) concentration of liquid swine waste. The higher sorption of NH₄+ in soil exposed to the complex matrix may have been due to the high levels of DOC in liquid swine waste. When varying DOC concentrations were tested at pH 4.0, 6.0, and 8.0, high solution pH tended to enhance NH₄+ sorption especially at higher DOC levels. Increased concentration of K+ reduced the amount of NH₄+ sorbed in soil, probably due to competition with NH₄+ ions for the binding sites. Significantly higher NH₄+ sorption in the Kennebec soil compared with the Haynie soil may be attributed to higher cation exchange capacity, clay content, and organic matter content of the Kennebec soil, Desorption of NH₄+ was studied by conducting kinetics and sequential extraction experiments. Desorption results indicated that the Haynie soil retained NH₄+ more strongly than the Kennebec soil inspite of its low sorption capacity indicating possible NH₄ + fixation in the Haynie soil. Significantly less NH₄ + was desorbed from soils exposed to liquid swine waste as compared to (NH₄)₂SO₄ solution suggesting a possible complexation of NH₄+ ions with organic molecules in swine waste. The DOC in swine waste was identified as a key factor influencing sorption-desorption of NH₄+ in soil. The use of (NH₄)₂SO₄ solutions could underestimate the sorption and overestimate the desorption of NH₄+ in soils exposed to swine waste effluents, Both sorption capacity and holding or retention capability of soil and unique characteristics of waste should be considered when studying the fate of animal waste-derived NH ₄+ in soil.