Pesticide adsorption and degradation in fine earth and rock fragments of two soils of different origin

The soil skeleton (larger than 2 mm fraction) can provide a source of organic carbon which may represent an unexplored possibility of adsorbing and degrading pesticides. The present paper reports a laboratory experiment on the adsorption of metobromuron and terbuthylazine on the skeleton and fine earth of two Italian soils derived from two parent rocks, a calcareous marl and a sandstone. KF values of 1.30 and 2.22 for metobromuron and 1.24 and 2.21 for terbuthylazine were found in the fine earth of sandstone and calcareous marl, respectively. Surprisingly, the soil skeleton, intended as the 2–10 mm fraction, showed a good adsorption capacity of about 40% and 20% with respect to the fine earth fraction for sandstone and calcareous marl soils, respectively. The derived Koc values for the skeleton turned out to be much higher than those of the fine earth indicating an adsorption activity of organic carbon in the skeleton higher than that of the fine earth. Pesticide degradation followed first order kinetics in all samples and half-life values in the skeleton were in the same range as those in the fine earth demonstrating a biodegradation activity of this substrate. A degradation trial in sterilized skeleton of the two soils supports the previous statement, since degradation was almost absent. The experiment performed indicated a possible contribution of the soil coarse fraction to pesticide adsorption and degradation, helping in their disappearance from the environment.