Lilleskov, Erik (USDA-FS, NCRSForestry Sciences Laboratory, 410 MacInnes Dr., Houghton, MI, 49931; Phone: 906-482-6303 ext 18; Fax: 906-482-6355; Email: elilleskov@fs.fed.us)

The “Carbon Cascade” into Forest Soils:BioticControl Points, With a Focus on Mycorrhizal Fungi

E.A. Lilleskov *, E.A. Hobbie, A.L. Friend

Carbon (C) fixed by plants travels through complex biogeochemical pathways that determine whether it will be rapidly respired or stored in more long-lived pools. We review known and hypothetical biotic control points along the belowground portion of this “carbon cascade” that might be regulated by human activities, focusing in particular on the role of mycorrhizal fungi. Mycorrhizal fungi are important belowground sinks for net primary production. By manipulating tree species and genotypes, altering site fertility, and changing atmospheric chemistry, humans are almost certainly affecting mycorrhizal biomass, production and inputs to soil C in most of the world’s forests, yet our understanding of these effects are at present rudimentary. Numerous challenges exist in characterizing pools and fluxes of mycorrhizal C in the field. Current methods being used to overcome some of these challenges include in-growth cores, stable and radioisotope analyses, and minirhizotron observations. Estimates of mycorrhizal fungal biomass in soil range widely, with larger biomass estimates for ectomycorrhizal fungi than arbuscular mycorrhizal fungi. Estimates of the portion of net primary production allocated to mycorrhizal fungi range from 3 to 20%, and may scale with belowground carbon allocation. However, the importance of mycorrhizal fungal production to soil C formation is poorly understood. Glomalin (a glycoprotein produced by arbuscular mycorrhizal fungi) and chitin (a fungal cell wall constituent) are two mycorrhizal fungal products whose contribution to soil organic C is being explored, with a particular emphasis on the importance of glomalin. Many other compounds produced by fungi could be important contributors to soil C, including other glycoproteins, mannoproteins, extracellular enzymes, glucans, hydrophobins, and melanins. Further characterization of the production and fate of mycorrhizal fungal biomass is necessary before we can fully assess the role that different fungal associations have in the soil “carbon cascade.”