Effect of long term organic amendments on adsorption–desorption of thiram onto a luvisol soil derived from loess

The objective of this work was to assess the influence of soil organic amendments on the sorption properties of the fungicide thiram. The organic amendments studied were organic household compost (COM), sewage sludge from municipal water treatment facilities (SLU) and farmyard manure (FYM), which were compared to mineral fertilizer application (MIN). Sorption–desorption experiments were performed using the batch method and the results indicated that the adsorption isotherms were non-linear and were found to conform to the Brunauer–Emmett–Teller (BET) model, suggesting multilayer adsorption and adsorbate–adsorbate interactions after the saturation of the surface layer. In general, distribution coefficient values, KD, are dependent on, but not proportional to, the initial concentration of thiram. For a fixed thiram initial concentration, a significant correlation (r2 > 0.851; p < 0.001) between KD values and the soil organic carbon content (OC) was observed. The highest value of KD was observed for the soil amended with compost, which is the one with the highest organic carbon content. KD values were divided by the soil organic carbon contents in order to obtain organic carbon partition coefficients KOC. Comparing KOC means from 3 (initial concentrations) × 4 (soil organic matter compositions) × 3 (replicates) factorial ANOVA allow us to conclude that there is a significant but not proportional influence of the initial concentration of thiram on those values, but changes in the soil organic matter composition, associated to different soil amendments, have no significant influence on adsorption of thiram. To evaluate the reversibility of thiram adsorption, two consecutive desorption cycles were performed with CaCl2 0.01 mol L−1. The desorption KD values were consistently higher (approximately twice) than those for adsorption at the same equilibrium concentrations for all soil samples supporting the existence of hysteresis in the adsorption–desorption behavior of thiram. Despite the fact that the adsorption KD values were proportionally increased with increasing total organic carbon content, this was not the case for the desorption KD values.