IMPACT OF ELEVATED CO2 ON THE METABOLIC DIVERSITY OF MICROBIAL COMMUNITIES IN N-LIMITED GRASS SWARDS

The impact of elevated atmospheric CO2 on qualitative and quantitative changes in rhizosphere carbon flow will have important consequences f or nutrient cycling and storage in soil, through the effect on the act ivity, biomass size and composition of soil microbial communities. We hypothesized that microbial communities from the rhizosphere of Dantho nia richardsonii, a native C3 Australian grass, growing at ambient and twice ambient CO2 and varying rates of low N application (20, 60, 180 kg N ha(-1)) will be different as a consequence of qualitative and qu antitative change in rhizosphere carbon flow. We used the Biolog(TM) s ystem to construct sole carbon source utilisation profiles of these co mmunities from the rhizosphere of D. richardsonii. Biolog(TM) GN and M T plates, the latter to which more ecologically relevant root exudate carbon sources were added, were used to characterise the communities. Microbial communities from the rhizosphere of D. richardsonii grown fo r four years at twice ambient CO2 had significantly greater utilisatio n of all carbon sources except those with a low C:N ratio (neutral and acidic amino acids, amides, N-heterocycles, long chain aliphatic acid s) than communities from plants grown at ambient CO2. This indicates a change in microbial community composition suggesting that under eleva ted CO2 compounds with a higher C:N ratio were exuded. Enumeration of microorganisms, using plate counts, indicated that there was a prefere ntial stimulation of fungal growth at elevated CO2 and confirmed that bacterial metabolic activity (C utilisation rates), not population siz e (counts), were stimulated by additional C flow at elevated CO2. Nitr ogen was an additional rate-limiting factor for microbial growth in so il and had a significant impact on the microbial response to elevated CO2. Microbial populations were higher in the rhizosphere of plants re ceiving the highest N application, but the communities receiving the l owest N application were most active. These results have important imp lications for carbon turnover and storage in soils where changes in so il microbial community structure and stimulation of the activity of mi croorganisms which prefer to grow on rhizodeposits may lead to a decre ase in the composition of organic matter and result in an accumulation of soil carbon.