Concentrations, Loads, and Yields of Organic Carbon in Streams of Agricultural Watersheds

Carbon is cycled to and from large reservoirs in the atmosphere, on land, and in the ocean. Movement of organic carbon from the terrestrial reservoir to the ocean plays an important role in the global cycling of carbon. The transition from natural to agricultural vegetation can change the storage and movement of organic carbon in and from a watershed. Samples were collected from 13 streams located in hydrologically and agriculturally diverse watersheds, to better understand the variability in the concentrations and loads of dissolved organic carbon (DOC) and particulate organic carbon (POC) in the streams, and the variability in watershed yields. The overall annual median concentrations of DOG and POC were 4.9 (range: 2.1-6.8) and 1.1 (range: 0.4-3.8) mg C L-1, respectively. The mean DOG watershed yield (+/- SE) was 25 +/- 6.8 kg C ha(-1) yr(-1). The yields of DOC from these agricultural watersheds were not substantially different than the DOG yield from naturally vegetated watersheds in equivalent biomes, but were at the low end of the range for most biomes. Total organic carbon (DOC + POC) annually exported from the agricultural watersheds was found to average 0.03% of the organic carbon that is contained in the labile plant matter and top 1 m of soil in the watershed. Since the total organic carbon exported from agricultural watersheds is a relatively small portion of the sequestered carbon within the watershed, there is the great potential to store additional carbon in plants and soils of the watershed, offsetting some anthropogenic CO2 emissions.