The rhizosphere of plants harbors a microbial community which is more responsive to ecosystem changes such as those induced by global changes. Environmental changes effect the growth and metabolism of plants and hence root exudations of aromatic compounds which potentially influence the microbial rhizosphere population. In this study we investigated the microbial diversity of aromatic compound degraders in the rhizosphere of the grassland plants Avena and Geranium on a 6-year old FACE (Free Air CO₂ Enrichment) experiment near Stanford, California, in which global climate changes were simulated by treatments of added nitrogen and carbon dioxide. The SIP (Stable Isotope Probing) technique was applied to detect active aromatic compound degrading bacteria since these products were expected to be different. We added ¹³C labelled substrates of benzoic acid or biphenyl to the rhizosphere and after incubation, extracted the bacterial DNA and RNA which incorporated the ¹³C labelled carbon. T-RFLP analysis of ¹³C labelled DNA revealed differences in the rhizosphere population of non-treated plants compared to those treated with elevated CO₂ or NO₃ concentrations at the grassland field area. New primers were developed using HMMER, to detect functional genes involved in aromatic compound degradation. This implementation of the profile hidden Markov model (profile HMM) for biological sequence analysis was used to search databases for homologues in sequence families. Our primers targeted the functional gene sequence of the small subunit of biphenyl and benzoic acid dioxygenases (bphA and benA) involved in the first step of oxidizing the aromatic ring structure. DNA Microchip arrays are being used to characterize the functional gene diversity of the rhizobacterial population of those grassland plants that have responded simulated global climate change conditions.