Formation and long-term fate of non-extractable residues in outdoor lysimeter studies

Long-term outdoor lysimeter studies using C-14-labelled compounds allow the quantification of the 'non-extractable residue fraction'. More than 20 lys imeter studies under realistic environmental conditions showed that more th an 80% of residual carbon of the molecule is retained in the topsoil layer even after several years. Generally, 50-90% of this residual radiocarbon is regarded as 'soil bound residue'. Microbial biomass is present in large qu antities in topsoil and continuously influences chemical and biochemical al teration of pesticide molecules that may interact directly with the total s oil organic matter. Labelling techniques using radioactive isotopes like C- 14 have been used to characterize these residues in the humus matrix. Our s tudies have been directed to the investigation of extractability and/or bio availability of these residues in long-term investigations. (C) 2000 Elsevi er Science Ltd. All rights reserved.