Global and Regional Pools of Arbuscular Mycorrhizal Fungi, a Common Soil Microbe

Belowground carbon pools in ecosystems have historically been difficult to quantify. Particularly challenging to measure are arbuscular mycorrhizal fungi, a functional group that forms mutualistic symbioses with plants and augments nutrient uptake. These fungi also produce glomalin, a recalcitrant protein that is abundant enough in the soil to constitute a potentially significant carbon sink. We estimated standing stocks of arbuscular mycorrhizal hyphae using "bottom-up" and "top-down" approaches based, respectively, on published values of root colonization by arbuscular mycorrhizal fungi, and a model incorporating net primary productivity derived from remote sensing data. Global biomass of arbuscular mycorrhizal hyphae was approximated as 43 Tg in bottom-up estimates and 140 Tg in top-down calculations. Thus, hyphal biomass was equivalent to only 0.3 to 1% of standing root biomass. However, the estimated total length of arbuscular mycorrhizal hyphae (18 x 10¹⁸ to 59 x 10¹⁸ m) was more than 20 times the length of their associated live fine roots. Temperate grasslands, savannas, and tropical rainforests contained the largest pools of arbuscular mycorrhizal hyphae. These budgets could assist researchers in predicting larger-scale responses of arbuscular mycorrhizal fungi--and ecosystems in general--to global change.