Tuesday, June 5, 2007
438

Biogeochemistry of Dissolved Organic Carbon in Midwestern Agricultural Streams

Thomas J. Warrner1, Todd V. Royer1, Natalie A. Griffiths2, Michelle A. Evans-White3, Jennifer L. Tank2, Emma J. Rosi-Marshall4, and Matt R. Whiles5. (1) School of Public and Environmental Affairs, Indiana University, 1315 East Tenth Street, Bloomington, IN 47405, (2) Biological Sciences, University of Notre Dame, Department of Biological Sciences, Galvin Life Science Center, Notre Dame, IN 46556, (3) Biology, Kansas State University, Division of Biology, 104 Ackert, Manhattan, KS 66506, (4) Biology, Loyola University Chicago, Chicago, IL 60626, (5) Department of Zoology and Center for Ecology, Southern Illinois University, Carbondale, IL 62901-6501

The biogeochemistry of dissolved organic carbon (DOC) in agricultural streams remains largely unstudied.  In July, 2006, DOC concentrations in six Midwestern agricultural streams ranged from 1.9 to 6.5 mg/L.  Carbon export ranged from 2.7 to 106 kg/day.  DOC in field drains ranged from 1.3 to 5.9 mg/L, representing allochthonous inputs of 2 to 114 kg/day.  We hypothesized that the summer DOC pool in these agricultural streams was primarily allochthonous and of low bioavailability.  Specific ultraviolet absorbance (SUVA) at 254 nm indicates aromaticity of the dissolved compounds and was used as a measure of lability of the DOC pool at each site.  SUVA ranged from 0.0083 L/mg C/m to 0.0252 L/mg C/m, with the highest values generally associated with increased discharge.  Experiments were conducted to assess DOC bioavailability under ambient and nutrient-enriched conditions.  Bioavailability was determined as the percent decrease in DOC concentration after incubating unfiltered stream water for 48 hours.  Across sites, 6 to 14% of the ambient DOC was bioavailable.  Amendment with nitrogen and phosphorus increased bioavailability to 14 to 23% across sites, suggesting nutrient limitation of DOC cycling.  In general, the bioavailability of DOC in these agricultural streams was similar to that from less disturbed landscapes.