Evidence for formation of glomalin, a recalcitrant pool of soil organic matter, in Mount St. Helens pyroclastic substrates

Background/Question/Methods

Formation of stable soil organic matter is typically the result of a relatively slow series of decomposition processes that can be constrained, in early successional sites, by the availability of inputs from plants and animals or by environmental conditions that limit microbial activities. Alternatively, compounds such as glomalin, a glycoprotein of arbuscular mycorrhizal fungi origin, may form relatively early during soil development, and improve aggregate stabilization, water infiltration, and carbon and nitrogen storage. We measured an indicator of glomalin, Bradford reactive soil protein (BRSP), in Mount St. Helens pyroclastic soil collected at three depth increments (0-5, 5-10, and 10-20 cm) from five different plant community types. Sites included low (LDH), intermediate (IDH), or high (HDH) densities of grasses and forbs with few shrubs, and dense shrub-dominated thickets of upland alder (SDU), and riparian Salix (SDR) with low herb cover.

Results/Conclusions

We found BRSP, correlated with total and soluble soil-C and N, at all sites in patterns influenced by both habitat type and depth. Highest average concentrations of BRSP and greatest inter-site variability were observed at the 0-5 cm depth; lowest for LDH (0.33 mg g^-1 soil), intermediate for IDH, HDH and SDU (1.01-1.21 mg g^-1 soil) and highest for SDR (3.27 mg g^-1 soil). Concentrations and variability of BRSP decreased with depth and became similar at all sites at the 10-20 cm depth (0.24 mg g^-1 soil). Conversely, the proportion of BRSP associated with freshly produced glomalin increased with depth; 1.5-8.6% at 0-5 cm, 6.4-21.1% at 5-10 cm, and 5.7-39.9% at 10-20 cm suggesting downward migration of glomalin after it is formed. These data show glomalin can be present early in soil succession and vary by plant community. Even small amounts of such microbially-produced substances may have important effects on soil processes and plant development before accumulation of humified of soil organic matter.