Phosalone Sorption and the Nature of Soil Organic Carbon in Cultivated and Uncultivated Soils.

The nature and quantity of soil organic matter play dominant roles controlling the fate of pesticides in the environment. We hypothesized that land use such as cultivation has profound impact on the nature and quantity of soil organic carbon (SOC), and thus affects the mechanisms as well as capacity of pesticide sorption in soils. The specific objectives were to evaluate the nature of SOC using solid state nuclear magnetic resonance (NMR) in 21 soils with a wide range of properties and under various land uses; to determine the sorption coefficients (K_d) of phosalone, a nonionic pesticide, to this group of soils; and to reveal the relationships between SOC composition and phosalone sorption in soils of different cultivation history. Based on the NMR spectra, SOC was represented by six fractions: alkyl C, O-alkyl C, aryl C, O-aryl C, carboxyl C, and ketones/aldehydes C. The quantity of SOC and relative contributions of each OC fraction to total OC detected differed considerably between cultivated and uncultivated soils.  Fractions of aryl C, O-aryl C, and ketones/aldehydes C were higher and those of O-alkyl C were lower in cultivated soils than the uncultivated ones. Of the six OC fractions evaluated, carboxyl C had the closest relationship with OC partitioning coefficients (K_OC) based on principal component analysis.  The different amount as well as the composition of SOC in cultivated and uncultivated soils led to detectable differences in phosalone sorption in soils. Although K_d values of phosalone were generally positively correlated with OC contents in both cultivated and uncultivated soils, K_OC values showed significant positive correlation with the reciprocal OC content (r=0.93^***) only in uncultivated soils, while little correlation was found in cultivated soils.