Dissolved organic matter in upland soils: Fluxes, chemistry, and its importance in the terrestrial C cycle

The role of leaching in decomposition, transport, stabilization and loss of soil organic matter (SOM) is rarely represented in conceptual or numerical models of belowground C cycling. This study addressed the influence of the hydrologic cycle on the C cycle in two coastal California soils through the use of chemical, isotopic and hydrologic analyses. In a 100 year old redwood stand, rainfall mobilized 320 kgC ha^-1 yr^-1 as dissolved organic carbon (DOC) from the O horizon while only 74 kgC ha^-1 was leached below the A horizon and <10 kgC ha^-1 was exported to the stream. Nearly 90 kgC ha^-1 yr^-1 was mobilized from the A1 horizon in a coastal prairie soil dropping to 10 kgC ha^-1 below 100 cm. Batch absorption and bioavailability experiments indicate that this large drop in DOC fluxes with depth is primarily due to absorption reactions in these fine textured soils. We also found a progressive shift in the age and recalcitrance of the DOC as water percolates through the soil profile which we attribute to the continuous exchange with native SOM in both of these soils. Evidence for significant exchange with solid phase OM also comes from tracking δ¹³C changes in soil solution during the laboratory sorption experiment. Finally, we couple these DOC flux measurements with soil CO₂ production data into a numerical simulation model of SOM dynamics. While we find that DOC leaching is not a significant overall loss mechanism for soil C, greater than 20% of the C stored at depths greater than 40 cm in the forest soil is due to DOC transport and retention within the mineral soil. This contrasts with the prairie soil where we find only about 2% of the stored C below 20 cm was attributable to DOC movement.