Sediment Carbon Concentration and Transport From Small Pastured Watersheds.

      With the current emphasis on the role of carbon in the environment, agricultural systems and their impacts on the carbon cycle are important parts of the overall issue.  Pasture systems and carbon that is transported attached to sediment has been addressed at the North Appalachian Experimental Watershed near Coshocton, Ohio.  In a pasture system where a beef cow-calf herd was rotated weekly among 4 paddocks during the grazing season, one paddock was used for winter feeding.  Surface runoff was collected throughout the year from 4 watersheds, one WS per paddock.  The WS in the winter paddock was the only WS to yield sediment samples, and those were collected during the dormant season.  With continuous winter occupancy, the percent vegetative cover would often fall below 50% in the late winter/early spring, which increased the potential for soil loss.  During the 10-year period of sample collection, there were 403 runoff events and 115 events with sediment loss ≥10 kg ha^-1.  The largest 6 events carried nearly 50% of the total sediment and sediment-attached C lost during this period.  Annual losses of sediment and sediment-C varied considerably but averaged 2,642 and 140 kg ha^-1, respectively.  There was no correlation between the amount of sediment transported during a single event and the C concentration on that sediment.  The pasture sediments have a C enrichment ratio of 1.4-1.5 compared with the 0-2.5 cm soil layer.  Pasture sediment-C concentrations were more than 2 times the C concentrations on sediments from nearby row crop watersheds.  Pasture sediment C losses can be reduced by using management practices that reduce sediment losses.