Defining the control that hydrology exerts on organic carbon (OC) export at the watershed scale is important for understanding how the source and quantity of OC in streams and rivers is influenced by climate change or by landscape drainage. To this end, molecular (lignin phenol), stable carbon isotope, and dissolved organic carbon (DOC) data were collected over a range of flow conditions to examine the influence of hydrology on annual OC export from an 850 km2 Midwestern United States agricultural watershed located in west central Indiana. In years 2002 and 2003, modeled annual DOC loads were 19.5 and 14.1 kg ha-1yr-1, while 71% and 85%, respectively, of the total annual OC was exported in flow events occurring during less than 20% of that time. These results highlight the importance of short-duration, high-discharge events (common in smaller watersheds) in controlling annual OC export. Based on reported increases in annual stream discharge coupled with current estimates of DOC export, annual DOC loads in this watershed may have increased by up to 40% over the past 50 years. Molecular (lignin phenol) characterization of quantity and relative degradation state of terrestrial OC shows as much temporal variability of lignin parameters (in high molecular weight dissolved organic carbon) in this one watershed as that demonstrated in previously published studies of dissolved organic matter in the Mississippi and Amazon Rivers. These results suggest that hydrologic variability is at least as important in determining the nature and extent of OC export as geographic variability. Moreover, molecular and bulk stable carbon isotope data from high molecular weight dissolved organic carbon and colloidal organic carbon showed that increased stream flow from the study watershed was responsible for increased export of agriculturally derived OC. When considered in the context of results from other studies that show the importance of flood events and in-stream processing of terrestrial organic carbon, our results show how hydrologic variability in smaller watersheds can reflect landscape-scale carbon dynamics in ways that cannot necessarily be measured at the outlets of large rivers due to multiple source signals and attenuated hydrology.
Bibliographical noteFunding Information:
We thank Dr. Patrick Louchouarn of Texas A&M University for insightful discussions that improved the presentation and discussion of these data. We also thank three anonymous reviewers for their helpful comments that improved the quality of this manuscript. This research was supported in part by: the USDA—National Needs Fellowship Program, Geological Society of America Student Research Grant Program (Grant Nos. 7118-02 and 7365-03), the Consortium for Agricultural Sequestration of Greenhouse Gasses (USDA; Grant No. 2001-38700-11092), and the Purdue Research Foundation. The authors also thank Dr. Ron Regal (University of Minnesota—Duluth), the Purdue University Department of Earth and Atmospheric Sciences, and the following people for their support through the course of this research: Matt Ruark, Chris Edwards, David Gamblin, Nilupa Guaranata and Joy Mattox. This is Purdue Research Climate Change Center (PCCRC) publication number 0608 .
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