Tillage effects on persistence and distribution of trifluralin in soil

The aim of this study was to investigate the effects of the substitution of plowing with harrowing-twice in a 3-yr crop rotation-on persistence and di stribution of a persistent herbicide such as trifluralin (alpha,alpha,alpha -trinuoro-2,6 dinitro-N,N-dipropyl-p-toluidine) to rate these two tillage s ystems for their environmental impact (e.g., regarding potential ground wat er contamination with pesticides). Therefore, studies have been performed i n Germany at tno sites? a clayey silt in Reinshof and a loamy sand in Eickh orst. The herbicide was applied to oilseed rape (Brassica napus L., ar. nap us), followed in consecutive years by applications to wheat (Triticum aesti vum L.) or to wheat followed by ge (Secale cereale L.). The disappearance t ime of 50% of the initial concentration (DT-50 value) decreased from >300 d after preplant incorporation of trifluralin to <150 d after pre-emergence treatment, losses being greater due to volatilization resulting from the la rk of soil incorporation. No influence of different tillage systems on trif luralin persistence could be observed. Distribution of the herbicide in the top 30 cm of the soils, however, was significantly influenced by the soil cultivation systems. Reduced tillage (harrowing) in two consecutive years r esulted in trifluralin concentrations in the soil layers 10 to 20 cm and 20 to 30 cm under the limit of determination (0.005 mg kg(-1)). In contrast, plowing led to trifluralin residues of up to 0.019 mg kg(-1) in these soil layers. Although no trifluralin residues could be found in the 30- to 45-cm and 45 to 60-cm soil layers under either soil cultivation system, our resu lts imply an increased risk of ground water contamination after plowing.