Elevated CO2: Plant associated microorganisms and carbon sequestration

• Altered rhizodeposition under eCO2 conditions is bound to impact plant associated microbial communities.
• Altered plant–microbe interactions are likely to influence below ground C-cycling.
• High growth efficiency, better degree of protection and recalcitrant C are key requirements for microbial C-sequestration.
• Plant–microbe interactions can be exploited for higher soil C sequestration under eCO2.

Alterations in plant rhizodeposition under elevated CO2 (eCO2) are likely to influence below-ground plant–microbe interactions and soil C dynamics. There are studies on influence of elevated CO2 on soil microorganisms and below-ground microbial processes. However there is general lack of information on how altered plant–microbe interactions under eCO2 will influence belowground C-sequestration. In the present review we focus on the greenhouse gas CO2 with relevance to its effect on plant associated beneficial and pathogenic microorganisms in terrestrial ecosystems. Role of these microorganisms in belowground nutrient cycling and soil aggregation is discussed with reference to soil C-sequestration. This review demonstrates that eCO2 influence the richness, composition and structure of soil microbial community and the influence is more on active microbial communities and in the vicinity of roots. High C:N ratio under eCO2 favors fungi with wider C:N ratio and nutrient acquisition ability and biological nitrogen fixers. The ecosystems with fungal-dominated soil communities may have higher C retention than bacterial dominated soil communities. However, soil C-sequestration through plant growth, is strongly controlled by availability of nitrogen and nutrients required for biological nitrogen fixation. Nitrogenous and other chemical fertilizers show positive effect on C-sequestration but carry a carbon cost. Promotion of biological nitrogen fixers, and nutrient solubilizers and mobilizers may help in maintaining soil nutrient balance for higher C-sequestration. However more data need to be generated on the response of various plant beneficial as well as pathogenic microbial communities to eCO2. We suggest that plant associated communities and related processes to be researched in long term studies for alteration under eCO2 so as to assess their C-sequestration potential and identify management strategies for enhanced sequestration.