Sorption of atrazine and metolachlor by earthworm surface castings and soil

Atrazine and metolachlor were more strongly retained on earthworm (Lumbricu s terrestris L.) castings than on soil, suggesting that earthworm castings at the surface or at depth can reduce herbicide movement in soil. Herbicide sorption by castings was related to the food source available to the earth worms. Both atrazine and metolachlor sorption increased with increasing org anic carbon (C) content in castings, and Freundlich constants (Kf values) g enerally decreased in the order: soybean-fed > corn-fed > not-fed-earthworm -castings. The amount of atrazine or metolachlor sorbed per unit organic ca rbon (K-oc values) was significantly greater for corn-castings compared wit h other castings, or soil, suggesting that the composition of organic matte r in castings is also an important factor in determining the retention of h erbicides in soils. Herbicide desorption was dependent on both the initial herbicide concentration, and the type of absorbent. At small equilibrium he rbicide concentrations, atrazine desorption was significantly greater from soil than from any of the three casting treatments. At large equilibrium he rbicide concentrations, however, the greater organic C content in castings had no significant effect on atrazine desorption, relative to soil. For met olachlor, regardless of the equilibrium herbicide concentration, desorption from soybean- and corn-castings treatments was always less than desorption from soil and not-fed earthworm castings treatments. The results of this s tudy indicate that, under field conditions, the extent of herbicide retenti on on earthworm castings will tend to be related to crop and crop residue m anagement practices.