The Nature of Organic Matter and Sorption of Phosalone in Soil

Abstract

The nature and quantity of soil organic matter play dominant roles controlling the fate of pesticide in the environment. We hypothesized that land use such as cultivation has profound impacts to the nature and quantity of soil organic carbon (SOC), and thus affects the mechanisms as well as magnitude of pesticide sorption in soil. Twenty‐one soils with a wide range of properties under various land uses were examined in this study. The specific objectives were to evaluate the nature of SOC using solid state nuclear magnetic resonance (NMR), and to determine sorption behavior of a nonionic pesticide, phosalone (S‐6‐chloro‐2,3‐dihydro‐2‐oxobenzoxazol‐3‐ylmethyl O,O‐diethyl phosphorodithioate) in these soils, and to elucidate the relationships between SOC fractions and sorption parameters in cultivated and uncultivated soils. As revealed by NMR spectra and principal component analysis (PCA) of tested variables, the quantity and composition of SOC differed considerably between cultivated and uncultivated soils. The decline of O‐alkyl carbon fraction of SOC is proportional to the increase in fractions of aryl carbon, O‐aryl carbon, and ketones/aldehydes carbon with the correlation coefficients of 0.78***, 0.66***, and 0.82***, respectively (n = 21). This change in the SOC composition and quantity led to changes in the sorption behavior of phosalone in soil. Uncultivated soils exhibited a linear increase of pesticide binding strength to SOC (organic carbon [OC] partition coefficients) with the increase of the reciprocal OC content (r = 0.95***, n = 9), while this relationship was not observed in cultivated soils. The obtained results suggested that mechanisms of pesticide sorption were different in cultivated and uncultivated soils.