Biodegradation of 3-chlorobenzoate as affected by rhizodeposition and selected carbon substrates

The plant rhizosphere has been hypothesized to generally increase the population numbers and activity of soil microorganisms that degrade various xenobiotic contaminants. However, the relative importance of increased C availability in the rhizosphere to microorganisms that directly metabolize xenobiotics as a C source for growth of that degrade xenobiotics through growth-linked metabolism and cometabolism is not well understood. We hypothesized that when a xenobiotic can be used directly as a growth substrate, the rhizosphere would have relatively little influence on the catabolizer population since the substrate provides an independent nutritional niche. However, increased C levels in the rhizosphere might enhance the numbers of microorganisms that degrade the xenobiotic through cometabolism or metabolism that is linked to growth on other substrates. To further examine this hypothesis, experiments were conducted with soil containing indigneous microorganisms capable of degrading the model compound 3-chlorobenzoate (3-CB). The results showed degradation of 3-CB in 1:1 soil/water slurries from ryegrass rhizosphere soil that had not been previously exposed to 3-CB had a faster initial rate of 3-CB degradation than nonrhizosphere soil. This rhizosphere effect could be simulated in nonrhizosphere soil using glucose, mannitol, or benzoate. A long-term experiment further showed that the population size of microorganisms that degraded 3-CB in the absence of supple-mental C (catabolizers), and those that degraded 3-CB in the presence of supplemental C, for example, via growth-linked metabolism or cometabolism, were enriched by 40- and 250-fold, respectively, in rhizosphere soil, In contrast, planted and nonplanted microcosms that were repeatedly exposed to 3-CB over 24 wk were similarly enriched for 3-CB degraders, which had a MPN of 4 x 10(8) g(-1) soil, Our results suggest that under conditions where there is not constant exposure of the microbial population to 3-CB as a C source, C provided by rhizodeposition may primarily enhance the population numbers of microorganisms that degrade 3-CB by cometabolic or growth-linked metabolism, and thereby promote rapid depletion of 3-CB from soil during short term or low level exposure to the xenobiotic.