Water content mediated microaerophilic toluene biodegradation in arid vadose zone materials

We investigated the conditions promoting toluene biodegradation for gasoline-contaminated nearsurface (0.6 m depth) and subsurface (4.7 to 5.0 m depth) vadose zone soils sampled from an and environment. At both depths, water addition was required for toluene biodegradation to occur. In near-surface samples, no inorganic nutrient addition was necessary and (i) biodegradation was fastest at 0.0 MPa, (ii) biodegradation rates decreased with decreasing water potential down to -1.0 MPa, and (iii) biodegradation was undetectable at -1.5 MPa. For subsurface material, toluene depletion was stimulated either by slurrying with a nutrient solution or by adjusting the moisture content to 20% (0.0 MPa) with nutrient solution and lowering the oxygen concentration (to effectively 1 mg L-1 in the aqueous phase). Thus, in the subsurface material, toluene depletion was microaerobic and nutrient-limited, occurring only under low oxygen and with inorganic nutrient addition. Our studies implicate microaerophily as an important characteristic of the toluene-degrading communities in these dry soils, with soil water as a primary controller of oxygen availability.