Diffusion Limitation for Atrazine Biodegradation in Soil

Effects of sub-millimeter scale heterogeneity in chemical and microbial distributions on atrazine degradation were examined using Pseudomonas sp. strain ADP introduced into soil at a population mimicking atrazine-adapted soils (~2000 cells/g), and employing a range of soil water pressures (−100, −300, −500 kPa). Heterogeneous cell distribution was employed in all treatments whereas uniformity of distribution was a variable for atrazine introduction. Two methods of initially distributing atrazine in soil were examined. Proximally-applied atrazine (PAA) was intended to yield elevated atrazine concentrations in the vicinity of the degraders. Dispersed atrazine (DA) was introduced to distribute the chemical uniformly as compared to the distribution of degraders. Both rate and extent of degradation were greater than PAA, regardless of water content, presumably due to proximity of atrazine to degraders. Biodegradation decreased with decreasing water content for both application methods, attributed to decreases in atrazine’s effective diffusion. Mineralization of nearly 100% of DA in soils receiving a heterogeneous inoculum with a greater cell density (~10 cells/g) indicates that biodegradation was limited by the distance atrazine had to diffuse. Results support the hypothesis that enhanced populations of atrazine degraders, as reported elsewhere for atrazine-adapted soils, though heterogeneously distributed, may overcome bioavailability limitations.