A MODEL DESCRIBING PESTICIDE BIOAVAILABILITY AND BIODEGRADATION IN SOIL

A model is proposed for describing pesticide-substrate bioavailability and rates of biodegradation in soil. The model accounts for sorption to soil surfaces, diffusion into the internal matrix of soil organic m atter or aggregates, and microbial growth. Rates of sorption and diffu sion are approximated by first order kinetics while microbial growth i s approximated by Monod kinetics. Model verification was performed usi ng 2,4-D (2,dichlorophenoxyacetic acid) degradation data from high and low organic matter soils inoculated with pure cultures of 2,4-D degra ding bacteria. Estimates of sorption, diffusion, and Monod constants w ere obtained sequentially by fitting the defining differential equatio ns to the data using nonlinear regression techniques. Independent esti mates of initial biomass (X-0) and growth yield (Y) were required, alt hough X-0 could he approximated from the number of colony-forming unit s assuming a bacterial weight of 0.1 pg cell(-1). The model could acco unt for the partitioning of 2,4-D between soluble and sorbed phases an d provided estimates of the Monod constants, mu(max) and K-S, which we re generally consistent with values previously determined in pure cult ure. In conjunction with pest toxicology data, the model may be useful in predicting the time between application and loss of efficacy (i.e. , window of efficacy) for biodegradable pesticides.