Pesticide sorption by allophanic and non‐allophanic soils of New Zealand

Abstract We investigated the sorption of five pesticides (metsulfuron methyl, atrazine, 2,4‐D, phorate, and terbufos) in a range of allophanic and non‐allophanic soils of New Zealand, using surface and subsoil horizons from 10 soil series. Sorption of pesticides was measured by a batch equilibrium technique using 14 C‐labelled pesticides. The effect of soil properties on pesticide sorption was also examined. Sorption of pesticides was adequately described by the Freundlich equation with an R 2 value > 0.97. The value of the exponent in the fitted Freundlich equation for the pesticide sorption varied from 0.79 to 0.98. The pesticide sorption, as measured by the distribution coefficient (K d , sorption per unit concentration), was as follows: terbufos > phorate > 2,4‐D > atrazine > metsulfuron methyl (K d = 20.7, 18.1, 4.88, 3.74, and 0.54 L/kg, respectively). Sorption of pesticides was higher for allophanic than for non‐allophanic soils and in general decreased with depth. Multiple regression analysis between K d and soil properties indicated that the important properties which explained most of the variation in pesticide sorption include: organic carbon (C) and pH for metsulfuron methyl ( R 2 = 0.81***); organic C and oxalate‐extractable aluminium (Al) for atrazine ( R 2 = 0.86***), phorate ( R 2 = 0.93***), and terbufos ( R 2 = 0.95***); and organic C, pH, and oxalate‐extractable Al for 2,4‐D ( R 2 = 0.87***). The distribution coefficient of the pesticides, based on organic C (K oc = K d / organic C) content, varied less between the soils than did the K d values. The variation in K oc was much less for non‐ionic pesticides (phorate and terbufos) than for ionic pesticides (metsulfuron methyl, atrazine, and 2,4‐D). Removal of organic C decreased the K d values of all pesticides but it caused an increase in K oc values of ionic pesticides (metsulfuron methyl, atrazine, and 2,4‐D). Empirical equations based on the octanol‐water partition coefficient (Kow) of pesticides were used to predict the sorption of non‐ionic pesticides in New Zealand soils. Most of the equations over‐predicted the sorption of non‐ionic pesticides in these soils. This illustrates the limitation of using empirical equations for predicting K oc values.