DESORPTION AND BIOAVAILABILITY OF AGED SIMAZINE RESIDUES IN SOIL FROM A CONTINUOUS CORN FIELD

The sorption/desorption behavior of field weathered (aged) simazine (2-chloro-4,6-bis(ethylamino)-s-triazine) residues from a 20-year continuous corn (Zea mays L.) field was compared to that of C-14-simazine recently added to the same soil. The apparent sorption coefficients of the aged residues determined from 24-h and 48-h desorption experiments were approximately 15 times higher than the sorption coefficients of added simazine. Aged simazine residues were also shown to be biologically unavailable to sugarbeet (Beta vulgaris L.) and to microbial degraders, whereas recently added simazine showed herbicidal damage to sugarbeet and was substantially (ca. 48%) degraded during a 34-d incubation in soil. Apparently, the process of pesticide aging results in simazine residues that are less bioavailable, as shown by their resistance to plant uptake and microbial degradation, and that desorb slowly into soil solution. The concentration of simazine in water from field soil-samples (C) was monitored throughout a growing season, and compared to the equilibrium aqueous phase concentration (C(e)) predicted from the laboratory determined sorption coefficient (K(oc)) of added C-14-simazine. The fractional equilibrium values (C/C(e)) were approximately 1 at the time of simazine application in May but fell to 0.05 during July and August. These results show that the concentration of simazine in water contacting field soil is likely to be substantially lower than that predicted from laboratory determined sorption coefficients, except immediately following application.