Direct measurements of the activity of soil microorganisms have been a goal of soil biologists for a long time (Newman and Norman, 1943). This results from the basic thermodynamic fact that as organisms undergo metabolic activity, they emit heat from the enthalpy of reactions occurring in net catabolism (Battley, 1987). With the continuing trend toward miniaturization of circuitry and much better, more sensitive thermocouples, it is now possible to obtain direct measures of metabolic activities of organisms in small samples of soils with only a few milligrams of biomass (Sparling, 1981; Battley, 1987; Alef, 1995). Flow-microcalorimeters are now available that allow for the simultaneous measurements of CO2 and N2O production in soils (Albers et al., 1995).
Another approach to direct measurement of microbial metabolic activity in a more fine-grained fashion is to measure microbial metabolic processes and identify the microorganisms responsible for particular biochemical reactions under field conditions. Radajewski et al. (2000) pioneered stable isotope probing of community-extracted DNA as a laboratory-based means of identifying microbial populations involved in 13C-substrate metabolism. Padmanabhan et al. (2003) combined the approach of Radajewski et al. (2000) with an assay of a range of labile and recalcitrant organic compounds, linking the 13C field release assay of respired 13CO2 to DNAextraction analyses of the active microbial populations. Transient peaks of 13CO2 released in excess of background were found in glucose- and phenol-treated soil within 8 hours of application. Across the 30-hour time span of the experiment, neither naphthalene nor caffeine additions stimulated 13CO2 release above background. A total of 29 full sequences revealed that active populations included relatives of Arthrobacter, Pseudomonas, Acinetobacter, Massilia, Flavobac-terium, and Pedobacter spp. for glucose; Pseudomonas, Pantoea, Acinetobacter, Enterobacter, Stenotrophomonas, and Alcaligenes spp. for phenol; Pseudomonas, Acinetobacter and Variovorax spp. for naphthalene; and Acinetobacter, Enterobacter, Stenotrophomonas, and Pantoea spp. for caffeine. All these genera belong to bacterial divisions or subdivisions that were recovered from soils in more than 25% of the studies surveyed in the review. This approach is a useful first step in taking powerful analytical tools to the field. However, Padmanabhan et al. (2003) note that the amendment-based approach used in this study would not be likely to identify less responsive, slow-growing (K-selected) members of the soil microbial community.
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