Elevated CO2 and arbuscular mycorrhizal abundance interact to regulate soil C decomposition in the rhizosphere of a C3 (but not a C4) grass

There is increased recognition of the importance of the plant rhizosphere in mediating impacts of climate change on ecosystem-scale C cycling. Future elevated atmospheric CO2 conditions are likely to alter soil C storage with potential further impacts on atmospheric CO2. Effects of elevated CO2 on soil C storage may be direct via plant C inputs to soil. However, more indirect effects via root associated organisms may also play a role. Arbuscular mycorrhizal abundance is known to respond to elevated CO2 conditions. Moreover, although they have mostly been studied in the context of their function in plant nutrient acquisition, their role on soil C cycling is starting to become evident. We investigated the interactive effects of elevated CO2 and arbuscular mycorrhizal abundance on microbial decomposition of rhizosphere soil C. We grew two temperate native grasses (Pascopyrum smithii, a C3 and Bouteloua gracilis a C4) from seed in their native soil, under 13C labeled ambient and elevated CO2 atmospheres. This approach enabled us to assess incorporation of plant-derived and native soil organic matter C into microbes, dissolved organic C and respiration via isotopic partitioning. To manipulate mycorrhizal abundance, soils were steamed and later re-inoculated with fresh soil suspension that either had (-AM) or had not (+AM) been passed through a 15-μm mesh to remove mycorrhizal propagules. Microbial communities were assessed with phospholipid fatty acids (PLFA). Elevated CO2 increased the biomass of both species but only the C3 species was responsive to the AM treatments. Reduced abundance of mycorrhizae led to a decrease in P. smithii biomass and to changes in soil organic matter decomposition. The effect of elevated CO2 on decomposition of P. smitthi rhyzosphere soil C was dependent on mycorrhizal abundance so that while under -AM elevated CO2 did not impact soil C decomposition, under +AM elevated CO2 significantly decreased it. Consistent with this, PLFA profiles were distinct for +AM treatments under elevated CO2 suggesting that the response of C decomposition rates was associated with changes in the decomposer community structure. Our results demonstrate that symbiont-mediated responses to elevated CO2 are species specific. Our results suggest that mycorrhizal abundance can regulate the impact of atmospheric CO2 on soil organic matter decomposition and thus feedback into soil C processing.