EFFECT OF WETTING AND DRYING OF SOIL ON SORPTION AND BIODEGRADATION OF ATRAZINE

Short term incubations (4 d) were conducted to assess the effect of a wetting/drying cycle on atrazine sorption, as well as biodegradation, as a function of various atrazine concentrations (ca. 5, 10, and 25 mu g g(-1) soil) and levels of added crop residues (0, 5, and 10% cornstalks by weight), using a technique that allowed independent analysis of soluble and sorbed atrazine. Soil solution atrazine concentrations decreased, and K(d)s increased with increasing crop residues. The sorptive capacity of cornstalks for atrazine was estimated to be 860 mu g g(-1) vs 28 mu g g(-1) for unamended soil. Drying and rewetting resulted in lower soil solution concentrations and decreased extraction efficiencies (13 to 22%) for sorbed atrazine; the effect was most pronounced with added cornstalks. High recoveries of C-14 from soils (combustion data) indicated that atrazine was not lost to volatilization. Rapid rates of biodegradation were observed in cornstalk-amended soils shortly after rewetting; degradation was not observed in unamended soil. A longer incubation (6 wk) was conducted with ca. 10 mu g g(-1) atrazine and 5% cornstalks to assess metabolites and kinetics of biodegradation. Atrazine disappearance was observed after ca. 2 wk with concomitant production of deethyl- and deisopropyl-atrazine at a ratio of ca. 2:1. Dealkylated-atrazine accumulated after ca. 3 wk; there was no evidence for hydroxy-atrazine production. These data suggest that biodegradation may play an important role in atrazine losses in the field despite wetting/drying cycles. In addition, there may be apparent losses of atrazine due to decreased extraction efficiencies as a consequence of wetting/drying cycles, resulting in underestimation of field residues.