Applying the signature lipid biomarker (SLB) technique toward the assessment of below ground microbal processes in th rhizosphere has provided new insight into the effects of elevated CO₂ levels on microbial populations. This research focused on the quantitave extraction of ester-linked polar lipid fatty acids (PLFA) and steroids in order to ascertain both microbial and eukaryotic biomass levels, community structures and nutritional/physiological status, in situ. The SLB technique provides an alternative to the classical method of culturing recoverable isolates, which have been shown to represent less than 1% of the in situ population and which discriminates towards those organisms best adapted to the media used. Results to date have shown that, within a forest setting, microbes in regardless of CO₂ levels. In the artificial setting, increases in microbal biomass per gram of sample were observed when nitrogen was added as a supplement at both ambienty and elevated CO₂ levels. Analysis of the PLFA profiles indicated an increase in the relative percentages of those fatty acids indicative of bacterial eukaryotic communities. Analysis of bulk soils, from both natural and artificial settings, showed those soils to contain patterns of PLFA distinct from those observed in the Rhizosphere suggesting that important plant microbe interactions were in place. Multivarient analysis of the data further supported the likelihood of an interaction by showing the similarities between fatty acid profiles to be related to plant type and/or site location