Microbial respiration and diffusive transport of O2,16O2, and 18O16O in unsaturated soils:: A mesocosm experiment

Although the flux of molecular O-2 between the atmosphere and the subsurface is intrinsically linked to the net soil production of greenhouse gasses, few, studies have focused on the controls affecting the isotopic composition of O-2 in the subsurface. Here, we developed and tested a stable oxygen isotope tracer technique and gas transport modeling approach to evaluate O-2 cycling and fluxes from the subsurface that used an environmentally controlled soil mescosm, We measured the O-2 and delta(18)O(2) profiles in a model unsaturated soil zone and quantified the O-2 consumption rates and the O-2 isotope fractionation factors resulting from the combined processes of subsurface microbial (including bacteria, fungi, and protozoa) consumption of O-2 and diffusive influx of O-2 from the atmosphere. We found that at high respiration rates in the mesocosm. there appeared to be very little isotope fractionation Of O-2 by soil microorganisms. Although the mesocosm respiration rates are not typical of natural soils in northern temperate climes, they may be more representative of soils in warm and moist tropical environments. Our findings caution against the indiscriminate application of laboratory-determined oxygen isotope fractionation factors to field settings. The oxygen isotope tracer and modeling approach demonstrated here may be applied to gain a better understanding of biogenic gas production and O-2 cycling in subsurface systems and soils. Copyright (C) 2002 Elsevier Science Ltd.