Development of a field bioassay for the side effects of pesticides on decomposition

A method has been developed for assessing the side effects of fungicides on decomposition using litterbags. Twenty dried leaf disks of Chinese cabbage, Brassica oleracea, were placed in litterbags (20 x 20 cm) made of nylon fabric (25 microns mesh). The litterbags were laid in a treated plot and 5, 10, 20, and > 200 m from the treated plot and covered with 1 cm of standard soil. After 1 week, the dry weight of the leaf disks was determined. In an iterative procedure the method was optimized and significant differences of 5% in decomposition rate between litterbags were found using the final method presented here. In and around agriculturally managed plots, negative effects on decomposition of two fungicides could be traced. Effects on decomposition were found after application of captan (in fruit, 1.5 kg a.i./ha) and maneb (in potatoes, 3 kg a.i./ha) up to 10 m from the treated plot, at moderate wind speeds (3-5 m/s). In one case the concentration of captan in the soil covering the litterbags was measured. A negative correlation was established between captan concentration and decomposition. These experiments were repeated in an experimental plot under more controlled conditions with respect to the amount and time of application. The results of the field experiments could not be reproduced on the experimental plot. Additional experiments with soil fungi in agar again demonstrated a significant effect of exposure to captan, however. Given the results, the method using litterbags requires further development before a standard field trial can be designed. Given the knowledge gained with respect to the key factors involved, a bioassay using soil fungi seems to have high potential, however.