THE AGE AND LABILITY OF ORGANIC CARBON IN HEADWATER CATCHMENTS WITH VARYING LAND USE

Recent studies have shown that the age of dissolved organic carbon (DOC) and particulate organic carbon (POC) being transported by large river systems on the east coast of the United States can be several thousand years old and recalcitrant in nature. Carbon age in large river systems may be obscured by factors such as carbon production, large-scale land use, and sewage treatment plants. We are studying three headwater catchments with varying land uses to better understand organic carbon age, lability, and the processes that control the export of this material to larger river systems. We hypothesize that land use plays a critical role in controlling the watershed hydrology and organic carbon dynamics of these catchments. For the past year, we have captured two storm events and sampled soil horizons along a catena within an old growth forest, once logged forest, and agricultural area. These watersheds are located in close proximity to each other, are underlain by the same bedrock type, and are less than 30 km² in size. The streams were sampled in late summer and spring to assess seasonal changes in DOC composition and quality. Soil horizons were restricted to the upper 30 cm and were sampled during the summer. In most locations we combined the O/A-horizons and sampled the B-horizon separately. One site contained an E-horizon. DOC and POC samples from baseflow and peak DOC concentration in both storms were radiocarbon dated. DOC and POC leached from specific soil horizons were also radiocarbon dated. We assessed DOC biolability throughout the course of the storm events and for each soil horizon. Initial results from the first storm show a rapidly cycling (< 50 yrs) of DOC from all of the watersheds. POC ages were quite variable depending on land use type and were older than 800 yrs BP. The bulk of organic matter is exported from watersheds during storm events. We hope to clarify the age and lability of the organic matter within our study catchments, with the intent of showing how this may influence larger river systems.