Effect of soil reduction on phosphorus sorption of an organic-rich silt loam

Phosphorus flux from agricultural landscapes to surface waters may cause eutrophication. In the northeastern United States, P transport largely depends on P sorption of soils in variable source areas or in land treatment systems. Soil redox fluctuation commonly occurs in these areas. Nevertheless, the effect of soil redox on P sorption has been variable in the literature. This study investigated P sorption of an organic-rich northeastern glaciated silt loam (Langford) under air-dried, field-wet, and reduced conditions using batch P sorption experiments. Additionally, the influence of farm wastewater on soil P sorption was studied. Major results indicated that soil reduction increased the maximum amount of P that can be sorbed ([S.sub.max]) and decreased the aqueous P concentration at which P sorption and desorption are equal ([EPC.sub.0]), both determined from a modified Langmuir isotherm model. The slightly reduced field-wet soils had no significant difference in [S.sub.max] due to limited soil reduction. Using the diluted wastewater as the sorption solution matrices instead of 0.01 mol [L.sup.-1] KCl solution, the soils generally exhibited greater [S.sub.max] and lower [EPC.sub.0] except for the [EPC.sub.0] of a reduced surface soil, implying more complex P sorption in the field. Identified P sorption mechanisms include phosphate precipitation, ligand exchange with organic matter, and adsorption onto Fe hydroxides. Transformation of Fe compounds during soil reduction is primarily responsible for the changes in soil P sorption. Abbreviations: DOC, dissolved organic carbon; ICP-AES, inductively coupled plasma-atomic emission spectrometry; OM, organic matter; SRP, soluble reactive phosphorus; TC, total carbon; TN, total nitrogen; VSA, variable source area; VTA, vegetative treatment area. doi:10.2136/sssaj2009.0123