Bioavailability of triazine herbicides in a sandy soil profile

The bioavailability of atrazine was evaluated in a Danish soil profile (Dre ngsted) using a combination of soil sorption, transport and mineralisation methods as well as inoculation using Pseudomonas ADP. Sorption of atrazine decreased markedly with depth as indicated by K-d values of 5.2 l kg(-1) fo r the upper soil and 0.1 l kg(-1) for the subsoils. The transport of atrazi ne was evaluated using soil TLC plates and the resulting R-f values were 0. 1 for the upper soil and 0.9 for the subsoil. Only a relatively small amoun t of atrazine leached through undisturbed soil columns taken from the upper 60 cm. Inoculating with Pseudomonas strain ADP (1x10(6) CFU g(-1) dry weig ht soil) revealed that the degradation of 0.01 ppm atrazine was fully compl eted (80% mineralisation) within 10 days in the subsoil, while it reached l ess than 15% in the upper soil. Over a period of 500 days, a total minerali sation of 37% of added atrazine in the upper soil was found (2 mg kg(-1) in cubated at 20 degrees C). However, in the subsurface soil where 0.02 mg kg( -1) of atrazine was incubated at 10 degreesC, the degradation was slower, o nly reaching about 12%. Terbuthylazine mineralisation was found to be tempe rature-dependent and low (less than 5%) in the upper soil and very much low er in the subsoil. Desethylterbuthylazine was the most frequently found met abolite. Finally, Pseudomonas strain ADP inoculated into soils from differe nt depths increased the mineralisation of terbuthylazine dramatically. Mode lling using a "two-compartment model" indicated that desorption of terbuthy lazine is the limiting step for its mineralisation.