The factors controlling microbial distribution and activity in the shallow subsurface

The relationships among sedimentological, geochemical, and microbiological parameters in the vadose zone of a barrier sediment from the eastern shore of Virginia were examined. Pairs of samples were taken 10 cm apart in the vertical direction and 2 cm apart in the horizontal direction along three transects with one sample from each depth being processed aerobically and the other being processed anaerobically. Little variation was observed in the sedimentological and microbiological parameters tested. The sediment of all samples was fine-to-coarse sand, and the grain sizes ranged from 0.19-1.16 mm. Sediment moisture was low for all samples, but increased near the top and bottom of each sampling transect. These were regions where bioavailable Fe(III) concentrations were high. Rates of H₂ uptake ranged from below detection limit to 0.064 μmol H₂ · day^-1 · g¹ of sediment with a median rate of 0.01 μmol H₂ · day^-1 · g^-1 of sediment. The variation in bacterial numbers was slightly more than an order of magnitude range over the entire sampling face. Phospholipid fatty acid analysis showed a diverse but fairly uniform microbial community from sample to sample. We found that the quartile of aerobically processed samples with the highest H₂ uptake rates had statistically higher moisture content and bioavailable iron content than did the rest of the samples. The quartile of aerobically processed samples with the lowest H₂ uptake rates had significantly more gravel, less moisture, and less bioavailable Fe(III) than did the rest of the samples. Similar trends were observed for anaerobically processed samples, but the differences were not significant. Our data indicate that the spatial variation in microbial parameters is low within strata with uniform grain sizes.