Large contribution of arbuscular mycorrhizal fungi to soil carbon pools in tropical forest soils

The origins and composition of soil organic matter (SOM) are still largely uncertain. Arbuscular mycorrhizal fungi (AMF) are recognized as indirect contributors through their influence on soil aggregation, plant physiology, and plant community composition. Here we present evidence that AMF can also make large, direct contributions to SOM. Glomalin, a recently discovered glycoprotein produced by AMF hyphae, was detected in tropical soils in concentrations of over 60 mg cm^–3. Along a chronosequence of soils spanning ages from 300 to 4.1 Mio years, a pattern of glomalin concentrations is consistent with the hypothesis that this protein accumulates in soil. Carbon dating of glomalin indicated turnover at time scales of several years to decades, much longer than the turnover of AMF hyphae (which is assumed to be on the order of days to weeks). This suggests that contributions of mycorrhizae to soil carbon storage based on hyphal biomass in soil and roots may be an underestimate. The amount of C and N in glomalin represented a sizeable amount (ca. 4–5%) of total soil C and N in the oldest soils. Our results thus indicate that microbial (fungal) carbon that is not derived from above- or below-ground litter can make a significant contribution to soil carbon and nitrogen pools and can far exceed the contributions of soil microbial biomass (ranging from 0.08 to 0.2% of total C for the oldest soils).