The dynamics of soil microorganisms have important implications for the response of subsurface soil ecosystems to perturbations. Traditional indicators such as functional, community, activity, and carbon and nutrient pathway methods have been used to characterize soil microbial processes and ecological function; however, many of these indicators lack the ability to measure changes over large (e.g., landscape) spatiotemporal scales in soil environments. This research introduces the analysis of soil volatile organic compound (VOC) metabolites as nondestructive indicators of subsurface microbial activity and community composition as a function of varying environmental factors. Results of method validation using laboratory microcosms are presented, where VOC metabolites as characterized by gas chromatography and mass spectrometry (GC–MS) were related to CO2 evolution as a measure of microbial activity and to community‐level physiological profiles (CLPPs) and fatty acid methyl ester (FAME) community structure techniques. Results included the identification of 72 VOC metabolites produced from the soils, where significant (ρ < 0.05) differences in the estimated amounts and types of compounds produced were observed between treatments. Temporal measurements indicated similarity between VOC production and CO2 evolution, where increased amounts over time were detected. Principal component analysis (PCA) and hierarchical cluster analysis showed that the VOC results clustered similarly to FAME and CLPP results. Our results suggest there is promise for the use of naturally produced VOCs as potential indicators of soil microbial ecosystems over large spatiotemporal dynamics and environmental perturbations.

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Bottom-water hypoxia effects on sediment-water interface nitrogen (N) transformations in Corpus Christi Bay (TX, USA) were examined using continuous-flow intact sediment core incubations. Sediment cores were collected from three sites in August 2002 (summer hypoxia) and April 2003 (normoxia). Oxygen (O 2 ) and hydrogen sulfide (H 2 S) depth profiles were generated with microelectrodes. Membrane inlet mass spectrometry was used to measure sediment O 2 demand and net N 2 flux and combined with isotope pairing to determine potential denitrification and N fixation. Potential dissimilatory nitrate reduction to ammonium (DNRA) was measured using highperformance liquid chromatography. Sediment O 2 penetration depths ranged from 5 to 10 mm. H 2 S ranged from being present in overlying water and throughout the sediment column in August to not detectable in overlying water or sediment in April. Sediment O 2 demand was higher during bottom-water normoxia conditions versus hypoxia. Sediments were a significant source of NH þ 4 to overlying water during hypoxia but not during normoxia. Net N 2 fixation was observed at one station in August and all stations in April. Denitrification rates were significantly higher during hypoxia at two of three sites. Potential DNRA was observed during both oxic states, but rates were significantly higher during hypoxia, which may reflect sulfide enhancement and absence of cation exchange with 14 NH þ 4 . DNRA may contribute to formation and maintenance of bottom-water hypoxic events in this system. These results show that N transformation pathways and rates change when bottom-water O 2 concentrations drop to hypoxic levels. Since south Texas is a semiarid region with few episodic runoff events, these results indicate that Corpus Christi Bay sediments are a N source most of the year, and denitrification may drive N limitation between episodic runoff events.

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