Soil bound residues: Characterization, long-term fate, risk potential

Outdoor lysimeter studies using C-14-labelled compounds allow the quantific ation of the 'non-extractable residue fraction' in long-term investigations . The results from more than twenty lysimeter studies under realistic envir onmental conditions demonstrated that about 80-90% of residual radiocarbon of different molecules is retained in the topsoil layer even after several years. Generally, 50 to 90% of this residual radiocarbon is potentially reg arded to be a 'soil-bound residue'. Microbial biomass is present in large q uantities in topsoil and continuously influences chemical and biochemical a lteration of xenobiotics (e.g. pesticide molecules) that may interact direc tly with the total soil organic matter. The combined use of C-14, C-13 and C-12 labelling techniques offers the opportunity to characterize the nature Of these residues in the humus matrix. For instance NMR spectroscopy assis ts in detection 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 artificially produced soil. Further aspects of extractability and/or bioavailability of these residues in long-term investigations and their ri sk potential will be addressed.