The activity of soil microorganisms has important implications for the response of soil ecosystems to perturbations over both time and space.  Traditional proxies of soil microbial ecosystem function and composition are generally limited in the amount of spatio-temporal information they can provide.  This research verified soil volatile organic compounds (VOCs) as useful indicators of subsurface microbial community composition shift over space and time.  Microcosm studies containing South Texas soils were used to validate VOCs as microbial community indicators as a function of changing environmental factors (e.g. substrate availability, water content, and soil type).  The relationship between VOCs and microbial community composition and function were validated by traditional microbial community (e.g., fatty acid methyl esters, carbon substrate utilization profiles) and activity methods (e.g., respiration measured by CO₂ production).  Findings included the identification of over seventy-two VOC metabolites by GC/MS, including ketones, alcohols, aldehydes, terpenes, carboxylic acids, aromatics, hydrocarbons, and esters.  During factor analysis the VOC method clustered treatments levels separately and provided similar results to traditional measures of microbial community composition.  Regression analysis showed that the measured VOCs were significant (ρ < 0.05) predictors of microbial community structure. Analysis of variance (ANOVA) indicated significant differences (ρ < 0.05) between data clusters.  GIS maps showed spatial differences in LANWR soils, where trends with abiotic and biotic soil measurements were identified.  Temporal studies indicated that VOCs increased over time similarly to CO₂ emissions.  ANOVA showed that the spatial and temporal differences were significant (ρ < 0.05) for over twenty-eight of the measured VOCs.