Mycorrhizal fungi have a potential role in soil carbon storage under elevated CO2 and nitrogen deposition

In this review, we discuss the potential for mycorrhizal fungi to act as a source or sink for carbon (C) under elevated CO2 and nitrogen deposition. M ycorrhizal tissue has been estimated to comprise a significant fraction of soil organic matter and below-ground biomass in a range of systems. The cur rent body of literature indicates that in many systems exposed to elevated CO2, mycorrhizal fungi might sequester increased amounts of C in living, de ad and residual hyphal biomass in the soil. Through this process, the fungi might serve as a negative feedback on the rise in atmospheric CO2 levels c aused by fossil fuel burning and deforestation. By contrast, a few prelimin ary studies suggest that N deposition might increase turnover rates of fung al tissue and negate CO2 effects on hyphal biomass. If these latter respons es are consistent among ecosystems, C storage in hyphae might decline in ha bitats surrounding agricultural and urban areas. When N additions occur wit hout CO2 enrichment, effects on mycorrhizal growth are inconsistent. We not e that analyses of hyphal decomposition under elevated CO2 and N additions are extremely sparse but are critical in our understanding of the impact of global change on the cycling of mycorrhizal C. Finally, shifts in the comm unity composition of arbuscular and ectomycorrhizal fungi with increasing C O2 or N availability are frequently documented. Since mycorrhizal groups va ry in growth rate and tissue quality, these changes in species assemblages could produce unforeseeable impacts on the productivity, survivorship, or d ecomposition of mycorrhizal biomass.