Long-Term Fate and Transport of Organic C in Forested Inceptisol and Ultisol Pedons Under Natural Conditions and Fertilization.

Experiments were undertaken in an attempt to improve scientific understanding of long-term (5 y) soil C cycling in terrestrial ecosystems in a forested ecosystem under natural and fertilized conditions. Pedons measuring 2 m x 2 m x 3 m depth were instrumented in contrasting soil types (Inceptisol, Ultisol) with coarse and fine soil solution samplers and the mobility of dissolved organic C (DOC), pH, NO3- and other anions was observed during storms. Despite similar inputs, higher concentrations of DOC were consistently observed in the Inceptisol versus the Ultisol. Two possible explanations are offered, one geochemical and the other hydrological. Geochemically, the Inceptisol has higher concentrations of stabilized organic C than the Ultisol and therefore has a lesser capacity to sorb additional DOC. Hydrologically, the structure of the Inceptisol is more fractured than that of the Ultisol, which may promote vertical displacement of DOC in the former. Both soils exhibited evidence of lateral flow at 1 m depth, but the Ultisol also exhibited a 20 y old residue of dissolved Br- below 1 m depth. Manipulations of the Ultisol involving the application of urea fertilizer successfully displaced DOC from the litter layer into the deeper BE horizon, suggesting both hydrological and geochemical transport mechanisms were important. The depth-dependent carbon isotope signature of the solid-phase was analyzed to quantify the extent of organic C mobilization.