Soil-bound residues

Long-term outdoor lysimeter studies using C-14-labelled compounds allow the quantification of the 'soil-bound residue fraction'. Referring to more tha n twenty lysimeter studies under realistic environmental conditions, it can be concluded that more than 80 % of residual carbon of the molecule is ret ained in the top soil layer even after several years. Usually 50 to 90 % of this residual radiocarbon can be considered to be a 'soil bound residue' o nce the chemical structure has been clarified. The high amount of microbial biomass in the topsoil continuously influences the chemical and biochemica l metabolism of pesticide molecules interacting directly with the total soi l organic matter turnover. The combined use of e.g. C-14, C-13 and C-12 lab elling techniques offers the opportunity to characterize the nature of thes e residues in the humus matrix. For instance NMR spectroscopy assists in de tection of molecules bonding in the humus matrix by both enrichment of the test compound with C-13 and depletion of C-13 in the organic matter of an a rtificially produced soil. Aspects of bioaccessibility and/or bioavailabili ty as well as the environmental relevance of these residues will be address ed. Important conclusions of a workshop on bound residues organized on beha lf of the German Research Foundation in 1996 will also be presented.