DEGRADATION OF PESTICIDES IN SOIL - COMPARISON OF LABORATORY EXPERIMENTS IN A BIOMETER SYSTEM AND OUTDOOR LYSIMETER EXPERIMENTS

This study elucidates the environmental significance of standard laboratory experiments for pesticide degradability in soil. Simazine, chlortoluron and phthalic acid bis-(2-ethylhexyl) ester (plasticizer, formulation additive) were used as model substances. Standard metabolism experiments with C-14-labelled substances were performed in a biometer system with <2 mm-soil fractions (silty sand and silty loam, respectively) at constant soil moisture and 20-degrees-C. Another set of samples was incubated under simulated outdoor conditions (alternating temperature and soil moisture, <4 mm-soil fraction). Outdoor experiments were conducted with non-labelled substances in small lysimeters of 20 cm diameter. In five of six parallel biometer experiments, DT50 values (half-lives) under simulated outdoor conditions were up to 2.5 times longer than in standard experiments. This is probably due to the lower average temperature under simulated outdoor conditions as compared with standard conditions. In all experiments, non-extractable residues were lower under simulated outdoor than under standard conditions. Comparison of DT50 values from biometer tests under simulated outdoor conditions to those of fallow lysimeter experiments revealed shorter DT50 values outdoors. Only for chlortoluron, the DT50 outdoors was 1.5 times longer than under simulated outdoor conditions. DT50 values determined in standard experiments gave better estimates of the disappearance of the investigated substances in fallow lysimeters.