MODEL ECOSYSTEMS FOR PREDICTING THE BEHAVIOUR OF PESTICIDES IN THE ENVIRONMENT

摘要

Prediction of the behaviour of pesticides in the environment aids efficient management of agrochemicals, and use of radioisotopes offers a simple and highly reliable tool for such studies. Various experimental set-ups that simulate ecosystems have been used successfully under laboratory and outdoor conditions. Use of a soil biometer flask, a continuous flow-through system and a rice-fish ecosystem to follow the fate of ~(14)C labelled HCH isomers, carbaryl, nitrofen, carbofuran and glyphosate is discussed. By combining the agronomic practices of flooding and/or use of organic matter amendments such as green manure it was shown that the persistence of HCH isomers, carbaryl and carbofuran in soil could be considerably reduced. Use of labelled pesticides in conjunction with a continuous flow-through system enabled determination of the total ~(14)C mass balance and budgeting of the pesticides in the environmental compartments of flood water, organic volatiles, CO_2, extractable and unextractable (bound) residues. Faster degradation in flooded and/or organic matter amended soils was unequivocally shown by demonstrating mineralization to ~(14)CO_2 and low extractable ~(14)C residues. Use of ~(14)C labelled pesticides helps to determine the extent of formation of bound residues, and to follow their uptake in plants and their bioavailability through microbial release. Bound ~(14)C residues were formed in soils after using all the above pesticides. A soil biometer flask is of help in following the mineralization of bound residues, as seen with lindane (gamma-HCH) and carbaryl. Another use for a soil biometer flask is to follow the fate of ~(14)C pesticides through ~(14)CO_2 evolved either in soil or by microbial cultures. The extent of glyphosate degradation in different soil types was followed, since this herbicide is known to undergo co-metabolic mineralization to CO_2. Studies using a rice-fish model ecosystem showed that ~(14)C-carbofuran and ~(14)C-nitrofen did not accumulate in rice or fish and indicated that discriminate use of these pesticides is not hazardous under semi-tropical conditions. The above model ecosystem was also useful in following the fate of ~(14)C-DDT in clams and sediments in the marine environment.