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 means for removing organic pesticides from soils, the influence of quantitative and qualitative soil microbial properties on mineralization of pesticides is poorly understood. In a laboratory study, we examined the relationship between soil microbial biomass and activity, estimated by heat output and adenine nucleotide fractions, and the mineralization rates of 14C-ring-labelled free and plant cell-wall-bound isoproturon, a phenylurea herbicide, in four soils originating from different cropping systems. 14CO2 production of free isoproturon ranged from 14 to 23% of the initial activity over 67 d and differed significantly between soils. 14CO2 formation from cell-wall-bound isoproturon was about one-third of that of free isoproturon with only small differences in the mineralization capacities of all four soils. For free isoproturon, mineralization correlated significantly with soil microbial biomass (estimated by substrate-induced heat production) in three soils, while in a soil from a former hop plantation no correlation was found. For cell-wall-bound isoproturon, no correlation between soil microbial biomass or activity and 14CO2 production was found. High Cu concentrations in one soil, resulting from fungicide treatments during former hop cultivation, are probably the reason for an increased metabolic status of the soil microorganisms characterized by the relative quotient of heat production (rqheat) and for the higher mineralization rate of free and cell-wall-bound isoproturon per unit biomass compared to the other soils. Our results suggest that non-specific soil microbial properties are not sensitive enough to descirbe the influence of soil microbial activities on the mineralization of free and plant cell-wall-bound isoproturon.