Comparative Studies on the Formation of Bound Residues in Soil in Outdoor and Laboratory Experiments

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Abstract
To evaluate the formation of bound residues in soil, standardized degradation experiments were designed with soil-plant systems such as pot experiments in the greenhouse and lysimeter experiments under outdoor conditions using 14C-labelled metamitron, and methabenzthiazuron. During the experiments different losses of 14C occurred due to mineralization of the labelling positions. More than 80% of the remaining radioactivity applied in pot and lysimeter experiments was detected in the 0–10 cm soil layer. Thirteen weeks after application of 14C-metamitron, 20% of the applied 14C remained in the lysimeter soils after extraction with organic solvents or 0.01 M CaCl2 solution while 50% remained unextractable in the soils kept at 22°C and 50% of the maximum water holding capacity of the soil. In the case of methabenzthiazuron, 25% of the applied radioactivity could not be extracted from the lysimeter soils with organic solvents, whereas, 30–40% remained in the soils of the laboratory studies. The results from the soil in the pot experiments were intermediate between the outdoor and laboratory experimental values. The study indicates that small-scale laboratory experiments do not provide realistic data about the formation of bound residues under practical field conditions.