References

Assessing Functions of Soil Microbes with Isotopic Measurements 397
D-alanine, and diaminopimelic acid (Pelz et al. 1998). Methods for the isotopic
analyses of DNA or RNA have also been developed (Coffin et al. 1990),
and could be applied at different levels of taxonomic resolution (MacGregor
et al. 2002). Such analyses have been used in studies of microbial processes
in sediments and aquatic systems, but have been used little in soils. RNA
could be particularly useful since it degrades rapidly in the environment
and reflects the component of microbial biomass that is actively synthesizing
proteins. To date, these techniques have not supplied great insight into
microbial processes because uncertainties in what controls isotopic fractionation
at the microbial level have hindered the interpretation of isotopic
patterns.
Several techniques using compound-specific measurements coupled to
tracer studies have considerable promise for understanding the microbial
impact on a variety of biogeochemical processes, such as methane
oxidation (Bull et al. 2000; Radajewski et al. 2000), degradation of aromatic
compounds (Johnsen et al. 2002), formation of fatty acids in soils
(Lichtfouse et al. 1995), or in examining relative activities of fungal versus
bacterial communities on 13 C-enriched substrates (Arao 1999). This general
approach is reviewed in Jones and Bradford (2001). Nuclear magnetic
resonance (NMR) studies also have great potential for new insights into
metabolic processes, are nondestructive, and can provide position-specific
information on labeling patterns within molecules that is extremely difficult
to obtain using isotope ratio mass spectrometry. Such NMR studies
have used 13 C-applied substrates in culture studies of soil microorganisms
(Gaines et al. 1996), or directly in soil (Lundberg et al. 2001). In the latter
study, 13 C-labeled glucose was traced for 28 days into solid-state components(NMR-invisiblecomponentsofmicrobialbiomass),respiredCO2,and
triacylglycerols, with the triacylglycerols probably located in oil droplets
within fungi.
4
Conclusions and Future Research
This review will, I hope, stimulate further applications of isotopic techniques
to the roles of microbes in belowground processes. Several challenges
remain. Our ability to interpret results from culture studies at
natural abundance must be improved through better knowledge of the
main metabolic pathways of microbes. Once that is accomplished, we can
then extrapolate from culture studies to field studies with greater confidence
than now possible. Ongoing work, much of it driven by the potential
for microbes in industrial production of specific compounds, has demonstrated
that the main metabolic fluxes within bacteria and fungi can be