Dissolved organic matter (DOM) facilitates nutrient and metal transfers in near surface soil environments and, therefore, is an important contributor to ecosystem processes and pedogenesis. In well-drained soils in the wet tropics, where high rainfall and primary productivity lead to high DOM production, DOM is likely a major transport mechanism for C to deep mineral soil horizons where the potential for stabilization is greatest. Volcanic soils, particularly Andisols, have high C storage capacities due to the accumulation of highly reactive, non-crystalline minerals. Previous research along a soil age gradient in Hawai’i found that allophanic soils in the intermediate weathering stage contained the largest soil C stocks with longest residence times. Potential mechanisms for long-term soil C stabilization include accumulation of chemically recalcitrant C, unfavorable microenvironmental conditions for microbial decomposition, and sorption to minerals and/or metals. We measured DOM concentrations throughout sequentially deeper organic and mineral horizons in an intermediate aged soil (ca. 350k years) under wet (ca. 3000 mm map) native tropical forest. We used 13C-NMR to analyze chemistry of OM accumulating at depth and performed a detailed study of soil morphology and preferential flowpaths that likely contribute to DOM delivery to depth.
Results/Conclusions
Soil C storage appears to be linked to DOM movement along infiltration pathways from organic to mineral horizons. SOM accumulating at depth differs chemically from decomposing roots, is C-rich and N-depleted, suggesting little microbial processing, and shows strong contributions of organic acids, pointing to a likely DOM source. High concentrations of DOM, Fe and Al moving in soil solution in the organic horizons, and high rainfall punctuated by drying periods, which result in severe shrink and swell of soils, contribute to (1) fluctuating redox conditions, (2) crack formation along ped surfaces, and (3) cemented layer development. Concentrations of DOM and total dissolved Fe and Al in solution decrease significantly in the mineral horizons, which are dominated by minerals with high sorptive capacities. Macropore flow and downward transport of metal-complexed OM is facilitated by a large network of cracks and open channels that cover up to 25% of 1 m2 area. Cemented horizons separate zones of microbial activity; soils below have wider C:N and are depleted in 15N. A better understanding of DOM transport and stabilization is important for predicting how changes in hydrology, climate and productivity may affect the potential of soils to retain nutrients and sequester carbon.
