Dissolved Organic Matter as a Mechanism for Carbon Stabilization at Depth in Wet Tropical Forest Volcanic Soils.

Dissolved organic matter (DOM) plays an important role in many biological and chemical processes in soils. In well-drained soils under wet climates, DOM is potentially a primary pathway for the transport of C from the zones of highest microbial activity to deeper mineral horizons, where the potential for long-term stabilization increases. We are studying DOC production, transformation, and loss pathways in volcanic soils dominated by highly reactive, non-crystalline minerals (allophane). We have installed tension and zero tension lysimeters throughout sequentially deeper organic and mineral horizons in an intermediate aged soil (ca. 350k years) under wet (ca. 3000 mm mean annual rainfall) native tropical forest in Hawai’i. Previous research at similar sites has measured soil organic matter (SOM) with very long mean residence times in the deeper mineral horizons. Potential mechanisms for long-term stabilization include an accumulation of chemically recalcitrant C, microenvironmental conditions which result in incomplete decomposition, and strong sorption of soluble and otherwise labile C to mineral and/or metals, making them inaccessible to decomposers. While C concentrations decreased exponentially with depth, they remained high (5-10 %) even down to 1 m. SOM accumulating at depth showed high C to N ratios (40-50), matching that of DOM in the surface organic horizons. Low pH did not seem to explain this accumulation of C-rich, N-depleted SOM, as soils became less acidic with depth. Soil C:N were positively correlated with alumina, oxalate-extractable Al, dithionite citrate-extractable Al, and less strongly with pyrophosphate-extractable Al, while DOC concentrations also increased with total dissolved Al in soil solution. Metals appeared to be implicated in the mobilization of C in solution and its stabilization in mineral horizons. To better understand the source of C stabilized at depth and the mechanisms responsible for its persistence, we are measuring changes in DOC hydrophobicity and bioavailability with depth.