Influence of Catchment Topography On Carbon and Dissolved Metal Concentrations in the Shale Hills Critical Zone Observatory.

Dissolved organic carbon (DOC) is said to be present in all ecosystems and can be important in carbon sequestration as DOC transport through soils can influence the distribution and stabilization of soil carbon. Furthermore, DOC provides the link by which soil carbon is made available for plant and microbial uptake, and may be an important factor in the mobility of metals. In this study, conducted at the forested Shale Hills Critical Zone Observatory in central Pennsylvania, we evaluated (1) the vertical profiles of soil organic carbon (SOC) and DOC along two transects (swale and planar hillslope) at different landscape positions (ridgetop, midslope, valley floor) and (2) the effects of DOC on the mobility of dissolved metal (Al, Mn and Fe) concentrations along the transects and the associated headwater stream. Soil cores were analyzed for SOC and water samples (collected along the transects and from the stream) were analyzed for DOC, Al, Mn and Fe concentrations. Preliminary data showed that both SOC and DOC decreased with increasing soil depth, but had distinct vertical profiles at different landscape positions for both transects. Additionally, DOC showed a significant correlation with metal concentrations, indicating that DOC may control the mobility of Fe, Al and Mn. Moreover, elevated concentrations of carbon and metals at the valley floor suggest hydrologic transport along the hillslope and re-deposition at the valley floor. Stream DOC and metal concentrations were observed to increase with increasing discharge, indicating the influence of fast hydrologic flowpaths. These findings suggest that both topography and hydrology play an important role in carbon dynamics and that DOC plays an important role in the mobility of metals within the Shale Hills catchment. Future efforts include the mechanistic simulation of transport and distribution of SOC and DOC, and the assessment of different flowpaths within the catchment.