Raising atmospheric Carbon dioxide: Effect on structure of soil microbial communities and functions related to terrestrial N cycle

CO2 in atmosphere is approaching an alarming 400 ppm mark, which is a big concern for present day environment. There is a lack of understanding on how potential it is in altering microbial structure and functions in soil and in particular those that are involved in N-cycle. Terrestrial C and N cycle are closely linked so we believe that there should be a direct role of raised CO2 on microbial functions involved in soil N cycle. To fill this gap soil microbial communities and functional genes involved in N cycling in soil were studied . To carry out this study special fields were designed where high levels of CO2 (550 ppm ) were maintained in an experiment called Free Air Carbon dioxide Enrichment abbreviated as FACE. Soil samples were collected from rhizosphere and bulk of wheat crops . To compare FACE experiment site soil, a control field having same variety of wheat but with normal atmospheric CO2 was maintained. Three replicates of each, rhizosphere and bulk from FACE site and three replicates from rhizosphere and bulk from control field were sampled to proceed with further studies. DNA was extracted and PCR was performed to prepare amplicons for bacterial and archaeal 16 S rDNA. These amplicons have been sent for sequencing. Sequencing results will give an estimate of microbial populations present in different conditions. Shifts in proteolytic communities producing alkaline proteases were studied using DGGE. Similarly neutral metalloproteases gene (npr)and other genes involved in N transformations including nifH (nitrogen fixation), amoA (Ammonia oxidation), nitrate reduction (nirS and nirK), will be studied using DGGE. All these genes are considered for this study as the enzymes coded by them not only take part in terrestrial N cycle but also are major components of soil health.Apart from shifts in communities, gene abundance will be studied using quantitative PCR. Elevated levels of CO2 seems to have an effect on shifts in alkaline protease genes as observed in DGGE results. Furthermore it has been observed plant rhizosphere has a pronounced effect on proteolytic community structure under both conditions i.e. in soil from FACE site and in control . We are now working to see changes in gene diversity and abundance for other genes for N transformation i.e. npr, nifH, amoA, nirK and nirS. Preliminary results suggest that the proteolytic communities in soil are adapting themselves in response to rising CO2 in soil. We are working on whether it also has effect on other functions related to N cycle. Interpretation of sequencing results will help us to understand the dynamics in soil microbial populations in response to carbon dioxide.