Field leaching and degradation of atrazine in a gradationally textured alkaline soil

Abstract

The leaching and degradation of atrazine to 40 cm was monitored over a 1-year period, following a spray application in May 1991, at a field site on a highly alkaline sandy loam cropping soil with a soil pH ≥8·5. To account for gradational changes in soil texture and pH with depth, separate dose response curves of an oat bioassay for each 10-cm soil-sampling interval were used, to quantify the soil concentrations of the herbicide.

Throughout the trial the movement of atrazine was not observed to exceed beyond 40 cm with total rainfall of 386 mm. The only significant leaching of the herbicide was detected in late winter 1991, when approximately 30·5% of the applied amount leached from the 0–10 cm to 10–20 cm soil layer, with trace amounts detected at greater depths. This leaching occurred during a period of rainfall of 50 mm when soil water contents in the 0–10 cm to 10–20 cm soil layers were at an optimum, and it was deduced that the extent of the leaching, when evaluated with other studies, was influenced by the pH of the soil.

Atrazine recovery decreased exponentially with sampling time. The data fitted a first-order exponential function (R² = 0·99), with a half-life time for degradation of 62 days. The good fit of the data to this function also indicated that the rate of degradation was apparently independent of seasonal changes in water content and soil temperature. From this, it was inferred that any lowering to the rate of degradation, owing to decreasing soil water contents in spring–summer, was offset with compensating rises in soil temperature. Edaphic conditions in this spring–summer period approximate those in other studies where chemical hydrolysis was an important process of breakdown of atrazine. The degradation of the herbicide was almost complete by the end of the trial in late May 1992. An applied amount of 2·7% remained in the 10–20 cm soil layer, which corresponded to a residue level of 0·02 µg atrazine/g soil. This residue level is well below the recorded phytotoxic threshold of select cultivars of wheat, barley, and lucerne.