Export of dissolved organic carbon from agricultural streams in Illinois, USA

In the Midwestern USA, agricultural soils contain a large pool of organic carbon, yet little is known about carbon export in agriculturally-influenced streams. We studied DOC in three streams draining intensively farmed areas in Illinois, all draining Mollisols with large organic matter pools. Water quality samples were collected (weekly to daily) for 3-10 years and discharge was monitored continuously at each site. In-stream DOC concentrations ranged from 1-16 mg L-1, and high concentrations of DOC occurred both during floods and periods of low discharge. Among sites, average flow-weighted DOC concentrations varied from 3.1 to 3.9 mg L-1. DOC in the streams appeared to originate from two sources: allochthonous DOC from drainage of cropland in late winter through early summer, and autochthonous DOC from algal blooms in late summer through autumn. Inputs of allochthonous DOC were under hydrological control, with most of the allochthonous DOC entering streams during floods. Watershed export of DOC ranged from 3-23 kg ha(-1) yr(-1). The mass of C exported from watersheds as DOC was strongly related to water yield (r = 0.98), and by inference to precipitation. Bioassays indicated that on average 18% of the native DOC was bioavailable. Stream export of DOC from the agricultural watersheds was a small flux relative to the pool of soil organic C. However, increases in soil organic C could lead to greater inputs of DOC to streams and increased rates of microbial respiration.