The effect of landuse on soil organic carbon chemistry and sorption of pesticides and metabolites

Earlier studies had shown significant differences in sorption of nine pesticides in soils collected from two landuses (native vegetation and market gardens), which could not be explained on the basis of organic carbon content alone. Consequently it was hypothesised that the differences in sorption behaviour between the two landuses may be due to variation in the chemistry of the organic carbon. In this study the relationship between sorption behaviour of the nine chemicals and soil organic carbon chemistry, as determined by solid-state 13C NMR spectroscopy, was investigated. No significant differences were found between the two landuses in the distribution of the four main spectral regions of the 13C NMR spectra of soil OC, except for the carbonyl fraction (165–220 ppm), which may reflect the low OC content of the soils from both landuses. For all chemicals, except prometryne, the most significant (P < 0.01 or P < 0.001) relationship between Kd values and types of OC was found with the aromatic (110–165 ppm) or the alkyl (0–45 ppm) fraction. A comparison was made of the variability of Kd values normalized over OC (i.e. Koc), alkyl, aromatic and alkyl + aromatic fractions. Expressing Kd values for all chemicals, except azinphos methyl, in soils under native vegetation as Kalkyl or Karomatic greatly decreased the variability compared with the Koc value. However in the cultivated soils only the sorption coefficients for DEA, DIA and fenamiphos showed a decrease in variability when expressed as Kalkyl or Karomatic. This reflected the stronger relationship between sorption coefficients and the alkyl and aromatic fraction of soil OC in soils from native vegetation compared with those determined from the market garden soils. The different relationships between sorption coefficients and types of OC of the two landuses also suggests that the type of aromatic and alkyl carbon under the two landuses is different and NMR characterisation of the OC was not sufficient to distinguish these differences.