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

The origins and composition of soil organic matter (SOM) are still largely uncertain. Arbuscular mycorrhizal fungi (AMF) are recognized as indirect co ntributors through their influence on soil aggregation, plant physiology, a nd plant community composition. Here we present evidence that AMF can also make large, direct contributions to SOM. Glomalin, a recently discovered gl ycoprotein produced by AMF hyphae, was detected in tropical soils in concen trations of over 60 mg cm(-3). Along a chronosequence of soils spanning age s from 300 to 4.1 Mio years, a pattern of glomalin concentrations is consis tent with the hypothesis that this protein accumulates in soil. Carbon dati ng of glomalin indicated turnover at time scales of several years to decade s, much longer than the turnover of AMF hyphae (which is assumed to be on t he 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 am ount (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 biom ass (ranging from 0.08 to 0.2% of total C for the oldest soils).