Watershed Fire Regime Effects On Particulate Organic Carbon Composition in Oregon and California Coast Range Rivers

Fire causes major changes to organic carbon, converting biological organic materials to pyrogenic-derived organic carbon (Py-OC), including black carbon. Wildfire also dramatically affects hydrological and erosion processes within watersheds, potentially increasing the erosion and discharge of Py-OC as particulate organic carbon (POC). We hypothesize that the proportion of the POC being discharged as Py-OC will be affected by the watershed’s fire regime, increasing with annual proportion of the watershed burned. During the 2008 and 2009 water years, suspended sediment samples were collected from the Alsea, Umpqua, Eel, Salinas, and Arroyo Seco Rivers draining the Coast Ranges of Oregon and California. Events and discharges of various magnitudes were captured in this sample set. This sample set also included suspended sediment collected from the Arroyo Seco River after a 2008 wildfire burned through a large portion of its watershed. Fine (<63 μm) and coarse (>63 μm) particulate material was analyzed for OC and N. We used cupric oxide oxidation to determine the contribution of Py-OC and unburned organic matter to the POC load of these rivers. The area weighted mean fire return interval decreases from the Douglas fir dominated forests in the Alsea River watershed in the north to the chaparral dominated Arroyo Seco River watershed in the south (Alsea > Umpqua > Eel > Salinas > Arroyo Seco). This translated into an increase in the proportion of each watershed burned from north to south. With the increase in annual proportion of watershed burned we found that the Py-OC content of coarse and fine POC increased from north to south. These results suggest that fire plays an important role in delivering POC to long-term carbon sinks in the coastal and ocean environment.