CHLORIMURON ETHYL SORPTION AND DESORPTION-KINETICS IN SOILS AND HERBICIDE-DESICCATED COVER CROP RESIDUES

Interaction between a herbicide and plant residue on the soil surface in plant residue management systems such as no-tillage or cover crop i s of interest in terms of environmental fate of the herbicide. This st udy was designed to evaluate sorption and desorption of chlorimuron et hyl [ethyl -pyrimidinyl)amino]carbonyl]amino]sulfonyl]benzoic acid} in herbicide-desiccated rye (Secale cereale L.) and hairy vetch (Vicia v illosa Roth) residues and soils collected from a field. Sorption was d etermined by reacting 0.5 g of soil or plant residue with 6 mt of [C-1 4]chlorimuron ethyl solution at five concentrations (0.06-12.04 mu mol L(-1)) for 72 h. Freundlich K-f was highest in hairy vetch (6.33) fol lowed by rye (3.95) and soils from no cover crop or beneath plant resi due (0.81-1.03). The N values (0.88-0.95) for soils and plant residues indicated nearly linear sorption. Time course sorption data analyzed by an equilibrium/kinetic model indicated that sorption was initially; rapid (within 1 h), followed by a negligible increase in soil or a sl ow increase in rye and hairy vetch residue. Chlorimuron ethyl K-f incr eased with increased decomposition of plant residue. The K-f for rye a nd hairy vetch residues sampled at 11 weeks after soybean planting inc reased by 54 and 49%, respectively, compared with residues sampled at planting. CaCl2-desorbable chlorimuron ethyl in two consecutive 24-h c ycles ranged from 54 to 65% in soils and from 42 to 49% of that sorbed in plant residues. Two additional 24-h cycles with methanol/(NH4)(2)C O3 did not completely desorb the remaining chlorimuron ethyl. Under fi eld conditions, plant residues can apparently intercept and temporaril y retain chlorimuron ethyl.