The behavior of the herbicide terbuthylazine (TA) was studied in a clay loam soil after the addition of different organic amendments (OAs). Addition of poultry compost (PC) and urban sewage sludge (USS) retarded degradation of TA with half-life values of 60.3 and 73.7 d, respectively. In contrast, addition of corn straw (CS) did not significantly alter the degradation of TA (half-life 55.5 d) compared with its degradation in nonamended soils (half-life 57.3 d). Sterilization of amended and nonamended soils resulted in a partial inhibition of TA degradation, indicating that biotic and abiotic processes are involved in TA degradation in soil. Degradation of TA led to the formation of desethyl-terbuthylazine, which was detected in low amounts (<8% of the initially applied TA) in all soils. Adsorption of TA was relatively low, with Kd values ranging from 2.31 L kg(-1) in the nonamended soil to 3.93 L kg(-1) in the soil amended with USS. In general, Kd values increased with increasing soil organic carbon content. The dissolved organic matter extracted from the OAs did not appear to interact with the pesticide or the soil surfaces, suggesting that it would not probably facilitate herbicide transport. Desorption studies indicated a slight hysteresis of TA desorption in the amended soils compared with TA desorption in the nonamended soil, which was entirely reversible. These findings might have practical implications for the environmental fate of TA in agricultural soils, where the studied OAs are commonly used.

Terbuthylazine (TA) is a herbicide that has been introduced for weed control in corn cultivations as a direct replacement for atrazine. Because incorporation of different organic amendments (OAs) is a common practice in this crop, this study investigated the effects of different OAs, including urban sewage sludge, poultry compost, and corn straw on the dissipation and metabolism of TA. A column study and a field dissipation study were used. In the column study, no residues of TA and desethyl-terbuthylazine (DETA) were detected in the leachate of amended and non-amended columns. The addition of OAs increased the persistence of TA and DETA in the upper soil layers (0-10 cm) but did not affect the mobility of TA and DETA in either experiment. Although the presence of OAs led to a significant increase in DETA production in the upper soil layers, the presence of DETA in lower depths did not significantly differ with the non-amended soil in either experiment. A gradual accumulation of DETA was evident in the soil layers amended with corn straw, whereas a rapid formation of DETA and a gradual decline thereafter was observed in the other treatments. Overall, the addition of OAs did not appear to significantly influence the mobility of TA and DETA, which did not move below the top 30 cm, thus indicating low risk for ground water contamination. In addition, the dissipation rate of TA in the field was faster than that in the column study.