Sorption-Desorption Indices to Estimate Risk of Phosphorus Loss From Soils in An Iowa Watershed.

Livestock grazing in pastures has been identified as a possible factor contributing to sediment and phosphorus (P) delivery to Rathbun Lake and associated water bodies in southern Iowa. Seven representative soil materials were collected from the 143,000-ha watershed, and the risk of P loss was estimated by Mehlich-3 (P_M3) extractable P, P sorption indices (Langmuir S_max and k values), degree of P saturation (DPS) and P desorption indices (equilibrium phosphorus concentration (EPC) and phosphorus equilibrium buffering capacity (PEBC)). In general, there was no consistence among the indices in estimating risk of P loss. Most of selected soils had low risk of P loss as indicated by P_M3 and DPS. However, EPC values indicated that some soils could behave as sources by releasing P to the water if they become suspended sediments. The correlation of selected soil properties (e.g., pH, particle size, total C, total N, total P, Fe_M3, Al_M3, Ca_M3, Mg_M3, oxalate-Fe and Al (Fe_ox and Al_ox), dithionite-Fe and Al (Fe_d and Al_d)) and k, EPC, and PEBC was investigated to better understand P adsorption and desorption. We found that k and PEBC were not correlated with any selected soil properties, but EPC was positively correlated with both Fe_M3 (r² = 0.72, p < 0.05) and Fe_ox (r² = 0.62, p < 0.05). These results suggest that P desorption from these soils was controlled by Fe oxides.