MINERALIZATION OF [C-14] GLYPHOSATE AND ITS PLANT-ASSOCIATED RESIDUESIN ARABLE SOILS ORIGINATING FROM DIFFERENT FARMING SYSTEMS

The biomineralization of [C-14]glyphosate, both in the free state and as C-14-residues associated with soybean cell-wall material, was studi ed in soil samples from four different agricultural fanning systems. A fter 26 days, [C-14]carbon dioxide production from free glyphosate acc ounted for 34-51% of the applied radiocarbon, and 45-55% was recovered from plant-associated residues. For three soils, the cumulative [C-14 ]carbon dioxide production from free glyphosate was positively correla ted with soil microbial biomass, determined by substrate-induced heat output measurement and by total adenylate content. The fourth soil, or iginating from a former hop plantation, and containing high concentrat ions of copper from long-term fungicide applications, did not fit this correlation but showed a significantly higher [C-14]carbon dioxide pr oduction per unit of microbial biomass. Although the cumulative [C-14] carbon dioxide production from plant-associated C-14-residues after 26 days was as high as from the free compound, it was not correlated wit h the soil microbial biomass. This indicates that the biodegradation o f plant-associated herbicide residues, in contrast to that of the free compound, involves different degradation processes. These encompass e ither additional steps to degrade the plant matrix, presumably perform ed by different soil organisms, or fewer degradation steps since the p lant-associated herbicide residues are likely to consist mainly of eas ily degradable metabolites. Moreover, the bioavailability of plant-ass ociated pesticide residues seems to be dominated by the type and stren gth of their fixation in the plant matrix.