Distribution of terbuthylazine and atrazine residues in crop-cultivated soil: The effect of herbicide application rate on herbicide persistence

Field experiments were carried out to compare the effect of herbicide application rate on the distribution and persistence of atrazine (ATR), terbuthylazine (TBA), and their dealkylated degradation products in two conventionally cultivated cornfield soils: a silt loam soil and a silty clay loam soil, both typical for the crop-producing area of continental Croatia. Herbicides were applied pre-emergence at the registered (R) (1.0–1.5kgha−1), double-registered (2R), and triple-registered (3R) rates. Topsoil (0–30cm) and subsoil (30–60cm) samples were collected periodically during a crop rotation cycle of 17months. Soil microcosm experiments were performed under controlled conditions to investigate the influence of the herbicide application rate on their biodegradation in herbicide-treated cornfield topsoils. The herbicide half-lives (DT50) calculated by using monophasic and biphasic first-order kinetics models were compared. The herbicide sorption intensity in the studied soils was evaluated in terms of the Freundlich sorption isotherm. Under field conditions, TBA was retained within the first 60cm of both agricultural soils at least twice as long as ATR. While ATR residues persisted in soil only until corn harvest (5months), the residues of TBA and its N-deethylated degradation product were still measurable at the end of the crop rotation cycle (17months). The persistence of both herbicides was higher in soil with higher herbicide sorption intensity. ATR and TBA application rates higher than registered generally did not significantly influence the herbicide biodegradation rates or dissipation from the topsoil layers. The enhanced application rate slightly decreased the TBA persistence only in soil exhibiting a rather slow biodegradation. A temperature-leaching potential index for ATR and TBA applied at R rate showed the same leachability class for both herbicides. The leachability class of TBA could be higher than that of ATR in soil with moderate TBA persistence (DT50=30–100days). From an environmental perspective, TBA could pose a similar risk for groundwaters as ATR.