Sediment-bound and dissolved carbon concentration and transport from a small pastured watershed

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. Organic carbon lost to streams and rivers can promote bacterial production and microbial respiration of CO2 to the atmosphere. Although pastures and grasslands are major land uses in the humid U.S., row crop agriculture has received most of the carbon research focus. The objective of this study at the North Appalachian Experimental Watershed near Coshocton, Ohio, was to assess organic carbon transported from a pasture system, particularly on a runoff event basis. A beef cow–calf herd rotationally grazed a paddock during the growing season and was fed hay in this paddock during the dormant season (November–April). Surface runoff and sediment loss was measured and sampled throughout the year from the small watershed in the paddock. Most of the sediment samples were collected during the dormant season. With continuous winter occupancy, the percent vegetative cover was often ≤50% in the late winter/early spring, which increased the potential for soil loss. During the 10-year period of sample collection, there were 376 runoff events and 115 events with sediment loss ≥10 kg ha−1 from the watershed in the winter feeding area. 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 2642 and 140 kg ha−1, respectively. There was no significant correlation between the amount of sediment transported during individual events and the C concentration on the associated sediment. The pasture sediments have a C enrichment ratio of 1.2–1.5 compared with the 0–2.5 cm soil layer. Pasture sediment-C concentrations were >2x the C concentrations on sediments from nearby row crop watersheds.

Research highlights
► Soil organic C of top soil layers greater with pastures than row crops.
► In “sacrifice areas” vegetative cover decreases, runoff, sediment, C loss increase.
► Dissolved C (DOC) losses quantitatively similar to sediment-bound C losses.
► Sediment, sediment-C, and DOC concentrations not correlated with runoff event amount.
► Sediment C and DOC transports strongly correlated with sediment transport.