Effects of intact rhizosphere microbial communities on the mineralization of surfactants in surface soils

Abstract

This work addressed the impact that the rhizosphere has on the biodegradation of foreign organic compounds. This was examined with live plants grown in a sewage sludge amended agricultural soil and a woodlot soil. Microcosms were designed that contained soil, roots, and microbial biomass in a compartment that excluded most of the plant stem and all of the leaf biomass. Control microcosms contained soils without plants and had no plant-induced rhizosphere microbial communities. Microbial mineralization of 14C-labelled organic compounds (anionic, cationic, and nonionic surfactants) was measured by quantifying cumulative production of 14CO2 over a 40- to 55-day period. Regardless of the treatment, the surfactants were mineralized without lag periods. The mineralization results were fitted to mineralization models by nonlinear regression techniques. A mixed-order (3/2 order) mineralization model best fit the data. This model is similar to a first-order rate model but contains additional parameters that describe growth events or the mineralization of 14C that has been incorporated or bound by the microorganisms. Of the compounds tested, sodium stearate was mineralized to the greatest extent (65% conversion to 14CO2) in the sludge-amended agricultural soil. A cationic surfactant, dodecyltrimethylammonium chloride, was mineralized to the least extent (8%) by the microbes associated with corn roots in the sludge-amended soil. Linear alcohol ethoxylate was mineralized at the highest initial rate (1.5 day−1) in the presence of a soybean-induced rhizosphere in the woodlot soil. Overall, the rhizosphere treatments significantly increased the initial rates of mineralization by a factor of 1.1–1.9. However, there was no significant difference between the rhizosphere treatments and the soil-alone controls in the asymptotic yields of 14CO2. The results suggest that rhizosphere microbial communities enhanced the biodegradation of surfactants in soils in terms of their effects on the initial rates of mineralization. However, because the total amounts of 14CO2 production were the same as in the no-plant controls, the extents of biodegradation of xenobiotic compounds may be limited by other factors, such as chemical and (or) physical interactions between the soil and the chemicals. Key words: rhizosphere, biodegradation, surfactants, soils, kinetics.