Clear effects of soil organic matter chemistry, as determined by NMR spectroscopy, on the sorption of diuron

Organic matter has long been recognized as the main sorbent phase in soils for hydrophobic organic compounds (HOCs). In recent times, there has been an increasing realization that not only the amount, but also the chemical composition, of organic matter can influence the sorption properties of a soil. Here, we show that the organic carbon-normalized sorption coefficient (KOC) for diuron is 27–81% higher in 10 A11 horizons than in 10 matching A12 horizons for soils collected from a small (2 ha) field. KOC was generally greater for the deeper (B) horizons, although these values may be inflated by sorption of diuron to clays. Organic matter chemistry of the A11 and A12 horizons was determined using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. KOC was positively correlated with aryl C (r2 = 0.59, significance level 0.001) and negatively correlated with O-alkyl C (r2 = 0.84, significance level <0.001). This is only the second report of correlations between whole soil KOC and NMR-derived measures of organic matter chemistry. We suggest that this success may be a consequence of limiting this study to a very small area (a single field). There is growing evidence that interactions between organic matter and clay minerals strongly affect KOC. However, because the soil mineralogy varies little across the field, the influence of these interactions is greatly diminished, allowing the effect of organic matter chemistry on KOC to be seen clearly. This study in some way reconciles studies that show strong correlations between KOC and the chemistry of purified organic materials and the general lack of such correlations for whole soils.