Dissolved organic carbon in streams from artificially drained and intensively farmed watersheds in Indiana, USA

Dissolved organic carbon (DOC) in streams draining hydrologically modified and intensively farmed watersheds has not been well examined, despite the importance of these watersheds to water quality issues and the potential of agricultural soils to sequester carbon. We investigated the dynamics of DOC for 14 months during 2006 and 2007 in 6 headwater streams in a heavily agricultural and tile-drained landscape in the midwestern US. We also monitored total dissolved nitrogen (TDN) in the streams and tile drains. The concentrations of DOC in the streams and tile drains ranged from approximately 1-6 mg L^-1, while concentrations of TDN, the composition of which averaged >94% nitrate, ranged from <1 to >10 mg L^-1. Tile drains transported both DOC and TDN to the streams, but tile inputs of dissolved N were diluted by stream water, whereas DOC concentrations were generally greater in the streams than in tile drains. Filamentous algae were dense during summer base flow periods, but did not appear to contribute to the bulk DOC pool in the streams, based on diel monitoring. Short-term laboratory assays indicated that DOC in the streams was of low bioavailability, although DOC from tile drains in summer had bioavailability of 27%. We suggest that these nutrient-rich agricultural streams are well-suited for examining how increased inputs of DOC, a potential result of carbon sequestration in agricultural soils, could influence ecosystem processes.