Arbuscular mycorrhiza fungi are obligate symbionts and must obtain carbon (C) from the host plant, whereas EM fungi primarily uses host plant C. Yet, some taxa are capable of supplementing host C with supplies from the soil (Durall et al. 1994; Smith and Read 1997). Studies have estimated that 10-85% of net photosynthate is allocated to mycorrhizal fungi depending on the ecosystem (Allen 1991). Mycorrhizal plants consistently transfer more C belowground than nonmycorrhizal plants. Under low light or high nutrient conditions, mycorrhizae may suppress plant growth and the fungal partner has sometimes been considered a drain on the host (Johnson et al. 1997). However, a number of studies have shown that photosynthetic rates are enhanced in mycorrhizal plants to compensate for the cost of mycorrhiza development, regardless of nutrient availability (Allen 1991).
Mycorrhizal fungi are an important link in the chain of C transfer from plant to soil. We now realize the important role of mycorrhizae in sequestration of C in soil (Bonkowski et al. 2000; Rillig et al. 2001; Treseder and Allen 2000) and the potential for mycorrhizae to influence C cycling rates. Estimates of AM fungal biomass can account for 1-17% of the root weight (Fujiyoshi et al. 2000) and EM can comprise 20-30% of the root volume (Harley and McCready 1952). Processes involved in the cycling of fungal C include production, survivorship, and decomposition rates of fungal tissue (Treseder and Allen 2000). The production and turnover of fine hyphal networks exceeds the turnover rates for both root and shoot material (Fitter et al. 2000). However, a substantial amount of the C allocated to mycorrhizal tissue could remain in the soil for a longer period. Chitin, a recalcitrant polysaccharide, can constitute 60% of fungal cell walls and may persist years to decades in the soil. The functioning hyphal mycelium also secretes an insoluble glycoprotein called glomalin (Wright and Upadhyaya 1996) that sloughs from hyphae during the life span of root colonization (Wright and Upadhyaya 1999; Wright et al. 1996). Glomalin is important in forming water-stable aggregates and in soil fertility; measurement of this protein concentration in soils will help with the comparison of soils of different compositions and/or tillage or disruption practices (Wright and Upadhyaya 1996). Because hyphal productivity varies among AM fungi, glomalin production also varies (17 mg/mg hyphae-63 mg/mg hyphae) (Wright et al. 1996). Glomalin levels can be present in soil as high as 1.5% of the soil dry weight (Wright and Upadhyaya 1996) and constitute 30-60% of the recalcitrant carbon in undisturbed soils (Treseder and Allen 2000). Carbon dating of glomalin indicates that turnover occurs at time scales of several years to decades, much longer than the turnover estimates for AM hyphae (Rillig et al. 2001).
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