Elevated atmospheric CO2(eCO2) is known to alter soil C decomposition but the mechanisms involved are yet to be understood. Recent findings suggest that the impact of eCO2 on decomposition varies for different soil carbon(C) pools. At the same time, eCO2 can have impacts on soil communities, also a critical factor controlling decomposition. Furthermore, there is evidence that the role of the soil communities on C-processing is dependent on C substrate chemistry. However, it is still unknown whether the role of soil communities will be affected by future eCO2 conditions, if so, whether this is dependent on the type of soil C pool.
We designed a factorial experiment manipulating CO2 and soil communities and measured C loss of two substrates in a Eucalyptus FACE experiment in Australia. We used a novel nested mesh-bag system composed of an outer mesh-bag with three pores sizes (1µm-excludes roots, most fungi and fauna; 25µm-excludes roots and some micro-fauna; 1000µm-intact community) filled with soil, containing an inner mesh-bag with decomposition substrates (root litter and soil). This design allowed us to simultaneously assess C dynamics of two different pools, controlling the effect of decomposing communities surrounding the substrates, one of the main setbacks of traditional mesh-bag approaches.
Results/Conclusions
Our nested mesh bags proved to be successful in creating different communities surrounding the decomposition substrates. eCO2 conditions significantly affected the composition of the overall microbial community, assessed by phospholipid fatty acid profiles (PLFAs) and significantly increased the concentration of microbial groups such as Actinobacteria, Gram positive bacteria and saprotrophic fungi. Ectomycorrhizal fungal frequency diminished with mesh size while saprotrophic fungi remained constant throughout mesh sizes.
The decomposition of soil, assessed as C loss with a novel isotopic approach was higher when intact soil communities were present and under eCO2 conditions. Likewise, the remaining mass of root litter was lower for intact communities but it was not affected by eCO2 conditions. C degrading enzymatic activities were not affected by eCO2 but were higher for intact communities and when soil present as a decomposition substrate. Finally, microbial biomass C and N increased with mesh size for both types of substrates, but were generally higher for root litter. Our results demonstrate that in this Eucalyptus woodland, the role of the soil communities is dependent on CO2 conditions and less important for the root litter pool.