Scoping the applicability of the KOC concept for pesticides across soil particle size and carbon content.

Soil organic carbon (OC) is generally considered the determining soil fraction for pesticide sorption in soils. It is common practice to normalize the sorption distribution coefficient (Kd) to OC (KOC) to compare sorption across different soils. This approach ignores the potential role of mineral soil components and may lead to sorption overestimation in carbon poor soil fractions. This study investigates how OC contributes to sorption of 11 pesticides across different soil types and particle-size fractions and evaluates to what degree KOC can account for variability in sorption. Sorption affinity expressed as Kd increased consistently as soil size fraction decreased. OC normalization led to overestimation of sorption affinity for the carbon-poor fractions and increased inter-soil variability for fine fractions. While OC normalization was generally suitable for hydrophobic compounds even in carbon-poor fractions, response of polar, ionizable, and less hydrophobic pesticides was more variable. In carbon-rich fractions, this variability was probably caused by the heterogeneous nature of soil organic matter and the variable charge states of both soil constituents and pesticides, while in carbon-poor fractions, it likely reflected a shift to mineral-driven sorption. Our findings highlight limitations of relying on KOC derived from carbon poor soils for pesticide environmental fate assessment, especially for polar and ionizable pesticides.