MICROBIAL ABUNDANCE AND ACTIVITIES IN RELATION TO WATER POTENTIAL IN THE VADOSE ZONES OF ARID AND SEMIARID SITES

Numbers and activities of microorganisms were measured in the vadose z ones of three arid and semiarid areas of the western United States, an d the influence of water availability was determined. These low-moistu re environments have vadose zones that are commonly hundreds of meters thick. The specific sampling locations chosen were on or near U.S. De partment of Energy facilities: the Nevada Test Site (NTS), the Idaho N ational Engineering Laboratory (INEL), and the Hanford Site (HS) in so uthcentral Washington State. Most of the sampling locations were uncon taminated, but geologically representative of nearby locations with st orage and/or leakage of waste compounds in the vadose zone. Lithologie s of samples included volcanic tuff, basalt, glaciofluvial and fluvial sediments, and paleosols (buried soils). Samples were collected asept ically, either by drilling bore-holes (INEL and HS), or by excavation within tunnels (NTS) and outcrop faces (paleosols near the HS). Total numbers of microorganisms were counted using direct microscopy, and nu mbers of culturable microorganisms were determined using plate-count m ethods. Desiccation-tolerant microorganisms were quantified by plate c ounts performed after 24 h desiccation of the samples. Mineralization of C-14-labeled glucose and acetate was quantified in samples at their ambient moisture contents, in dried samples, and in moistened samples , to test the hypothesis that water limits microbial activities in vad ose zones. Total numbers of microorganisms ranged from log 4.5 to 7.1 cells g-1 dry wt. Culturable counts ranged from log <2 to 6.7 CFU g-1 dry wt, with the highest densities occurring in paleosol (buried soil) samples. Culturable cells appeared to be desiccation-tolerant in near ly all samples that had detectable viable heterotrophs. Water limited mineralization in some, but not all samples, suggesting that an inorga nic nutrient or other factor may limit microbial activities in some va dose zone environments.