The quality of soil organic matter, accessed by C solid state nuclear magnetic resonance, is just as important as its content concerning pesticide sorption.

The global increase of food production has been achieved mainly through the intensive use of inputs such as pesticides. Once released to the soil, sorption (which could be represented by Freundlich solid-water distribution coefficients - K) and degradation are two governing processes that determine the distribution and persistence of pesticides in the environment. In spite of the huge dataset, the only apparent generalisation is the high correlation between K and soil organic matter (SOM) content. However, in this work no correlation was observed between K and organic C content (OC) and so the obtained K (K normalised by OC) spread out in a wide range: 1100 to 11,400 mL g for abamectin; and 30-150 mL g for atrazine, both ranges corroborate with data from literature. These high variabilities indicate that other soil components or SOM quality strongly interfere in the pesticide sorption in addition to SOM content. Seeking to estimate the influence of SOM quality in the abamectin and atrazine K values, the humic acids, a fraction of the SOM, was analysed by C nuclear magnetic resonance spectroscopy (C NMR) and Principal Component (PC) Regression. The first PC of C NMR spectra presented negative loadings for aliphatic compounds and positive loadings for aryl C, typical of partially oxidised pyrogenic C. Their scores showed strong correlation with the abamectin K values (R = 0.91, p < 5 10) and weaker with atrazine K (R = 0.63, p < 0.0001), in addition to a smaller standardised slope: 1.01 for abamectin and 0.76 for atrazine. These results could be explained by the higher hydrophobicity of abamectin, being thus more prone to interact with the polycondensed aryl groups from the pyrogenic C. It is also important to highlight that humic acids are useful proxies for understanding the paramount interaction of SOM with pesticides.