MINERALIZATION OF FREE AND CELL-WALL-BOUND ISOPROTURON IN SOILS IN RELATION TO SOIL MICROBIAL PARAMETERS

Although microbial degradation into CO2 offers one of the principal me ans for removing organic pesticides from soils, the influence of quant itative and qualitative soil microbial properties on mineralization of pesticides is poorly understood. In a laboratory study, we examined t he relationship between soil microbial biomass and activity, estimated by heat output and adenine nucleotide fractions, and the mineralizati on rates of C-14-ring-labelled free and plant cell-wall-bound isoprotu ron, a phenylurea herbicide, in four soils originating from different cropping systems. (CO2)-C-14 production of free isoproturon ranged fro m 14 to 23% of the initial activity over 67 d and differed significant ly between soils. (CO2)-C-14 formation from cell-wall-bound isoproturo n was about one-third of that of free isoproturon with only small diff erences in the mineralization capacities of all four soils. For free i soproturon, mineralization correlated significantly with soil microbia l biomass (estimated by substrate-induced heat production) in three so ils, while in a soil from a former hop plantation no correlation was f ound. For cell-wall-bound isoproturon, no correlation between soil mic robial biomass or activity and (CO2)-C-14 production was found. High C u concentrations in one soil, resulting from fungicide treatments duri ng former hop cultivation, are probably the reason for an increased me tabolic status of the soil microorganisms characterized by the relativ e quotient of heat production (rqheat) and for the higher mineralizati on rate of free and cell-wall-bound isoproturon per unit biomass compa red to the other sails. Our results suggest that non-specific soil mic robial properties are not sensitive enough to describe the influence o f soil microbial activities on the mineralization of free and plant ce ll-wall-bound isoproturon.