Estimating Losses of Efficacy Due to Pesticide Biodegradation in Soil: Model Simulations
1997
Shelton, Daniel R. | Doherty, Michael A.
A model was developed for describing rates of pesticide-substrate biodegradation, accounting for bioavailability and microbial growth. The model was used to simulate losses of efficacy for soil-applied pesticides. The model requires rate constants for rapid sorption-desorption to and from soil surfaces (k₁/k₋₁ = Kd₁); diffusion into and out of soil aggregates-organic matter particles (k₂/k₋₂ = Kd₂); microbial growth [yield (Y), maximum growth rate (µₘₐₓ), half-saturation growth constant (Kₛ), and initial biomass concentration (X₀)]; initial mass of substrate (s₀); and gravimetric water content (θg). Simulations of microbial growth and substrate depletion were conducted assuming no sorption (aqueous solution), sorption to soil surfaces only, and sorption in conjunction with diffusion. The time required to achieve a soil solution concentration of 1 µg mL⁻¹ was defined as a hypothetical loss of efficacy (LE₁). Certain relationships were consistently observed, regardless of sorption or diffusion: LE₁ was found to be related to Kₛ linearly, to X₀ logarithmically, to µₘₐₓ geometrically, and to initial pesticide-substrate concentration (S₀) nonlinearly. Sorption to soil surfaces resulted in decreased equilibrium soil solution concentration (Sₑ), depending on the magnitude of θg and Kd₁. Rates of biodegradation-growth were a function of Sₑ, as opposed to total (soluble + sorbed) concentration. Sorption coupled with diffusion decreased both Sₑ and time-dependent availability, resulting in slower rates of biodegradation. In general, larger values of S₀ resulted in faster rates of biodegradation, i.e., decreased the time required for a loss of efficacy.
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