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Mycorrhizal fungi and soil carbon storage
Soil carbon storage is an important function of terrestrial ecosystems. Soil contains more carbon than plants and the atmosphere combined.〔Tarnocai et al. 2009. Soil organic carbon pools in the northern circumpolar permafrost region. Global Biogeochemical Cycles, 23(2) doi: 10.1029/2008GB003327〕 Understanding what maintains the soil carbon pool is important to understand the current distribution of carbon on Earth, and how it will respond to environmental change. While much research has been done on how plants, free-living microbial decomposers, and soil minerals affect this pool of carbon, it is recently coming to light that mycorrhizal fungi—symbiotic fungi that associate with roots of almost all living plants—may play an important role in maintaining this pool as well. Measurements of plant carbon allocation to mycorrhizal fungi have been estimated to be 5-20% of total plant carbon uptake,〔Pearson JN and Jakobsen I. 1993. The relative contribution of hyphae and roots to phosphorus uptake by arbuscular mycorrhizal plants, measured by dual labeling with 32P and 33P. New Phytologist, 124: 489-494.〕〔Hobbie JE and Hobbie EA. 2006. 15N in symbiotic fungi and plants estimates nitrogen and carbon flux rates in arctic tundra. Ecology, 87: 816-822〕 and in some ecosystems the biomass of mycorrhizal fungi can be comparable to the biomass of fine roots.〔Wallander H, Goransson H and Rosengren U. Production, standing biomass and natural abundance of 15N and 13C in ectomycorrhizal mycelia collected at different soil depths in two forest types. Oecologia, 139: 89-97.〕 Recent research has shown that mycorrhizal fungi hold 50 to 70 percent of the total carbon stored in leaf litter and soil on forested islands in Sweden.〔K.E. Clemmensen et al. 2013. Roots and associated fungi drive long-term carbon sequestration in boreal forest. Science, 339: 1615-1618.〕 Turnover of mycorrhizal biomass into the soil carbon pool is thought to be rapid〔Staddon et al. 2003. Rapid turnover of hyphae of mycorrhizal fungi determined by AMS microanalysis of 14C. Science, 300: 1138-1140.〕 and has been shown in some ecosystems to be the dominant pathway by which living carbon enters the soil carbon pool.〔Godbold DL et al. 2006. Mycorrhizal hyphal turnover as a dominant process for carbon input into soil organic matter. Plant and Soil, 281: 15-24.〕
Outlined below are the leading lines of evidence on how different aspects of mycorrhizal fungi may alter soil carbon decomposition and storage. Evidence is presented for arbuscular and ectomycorrhizal fungi separately as they are phylogenetically distinct and often function in very different ways.
==Recalcitrance of mycorrhizal tissues==
Based on the magnitude of mycorrhizal fungal inputs to the soil carbon pool, some have suggested that variation in the recalcitrance of mycorrhizal biomass may be important for predicting soil carbon storage, as it would affect the rate at which the contribution of mycorrhizal fungi to soil carbon is returned to the atmosphere.〔Langley JA and Hungate BA. 2003. Mycorrhizal controls on belowground litter quality. Ecology, 84: 2302-2312.〕 The compound glomalin, produced only by arbuscular mycorrhizal fungi, has been found to accumulate in some soils, and may be a substantial fraction of the soil carbon pool in these ecosystems.〔Rillig et al. 2001. Large contribution of arbuscular mycorrhizal fungi to soil carbon pools in tropical forest soils. Plant and Soil, 233: 167-177.〕 However, a recent set of experiments demonstrates the presence of arbuscular mycorrhizal fungi results in net losses of soil carbon,〔Cheng et al. 2012 Arbuscular mycorrhizal fungi increase organic carbon decomposition under elevated CO2. Science, 337: 1084-1087.〕 calling into question the role of glomalin produced by arbuscular mycorrhizal fungi leading to increased soil carbon storage.〔Verbruggen et al. 2013. Arbuscular mycorrhizal fungi – short-term liability but long-term benefits for soil carbon storage? New Phytologist, 197: 366-368.〕 Proteomic work has revealed that most of the proteins isolated in the glomalin extraction are not of mycorrhizal origin, and therefore the contribution of this molecule to soil C storage has likely been overestimated.〔Gillespie, Adam W., et al. "Glomalin-related soil protein contains non-mycorrhizal-related heat-stable proteins, lipids and humic materials." Soil Biology and Biochemistry 43.4 (2011): 766-777.〕
Using a similar line of argument, Langley and Hungate (2003)〔Langley JA and Hungate BA. 2003. Mycorrhizal controls on belowground litter quality. Ecology, 84: 2302-2312.〕 argued that the abundance of chitin in ectomycorrhizal tissues may reduce decomposition rates of these fungi, under the assumption that chitin is recalcitrant. This possibility was tested and refuted recently. Fernandez and Koide (2012) show that chitin does not decompose more slowly than other chemical compounds in ectomycorrhizal tissues, and that chitin concentrations positively correlated with mycorrhizal biomass decomposition rates, rather than negatively.〔Fernandez CW and Koide RT. 2012. The role of chitin in the decomposition of ectomycorrhizal fungal litter. Ecology, 93: 24-28.〕
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