『Abstract
Defining the control that hydrology exerts on organic carbon
(OC) export at the watershed scale is important for understanding
how the source and quality 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-1 yr-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.』
1. Introduction
2. Study area
3. Materials and methods
3.1. Stream discharge estimation
3.2. Estimation of annual dissolved organic carbon load
3.3. Sample collection and size fractionation
3.4. Organic carbon analyses
3.5. Cupric oxide oxidation
3.6. Stable carbon isotope analysis
3.7. Statistical analyses
4. Results
4.1. Dissolved organic carbon
4.2. Stable carbon isotope data
4.3. Lignin oxidation products
5. Discussion
5.1. Annual export of organic carbon from Big Pine Creek
5.2. Organic matter sources
5.3. Hydrologic and seasonal roles in terrestrial contributions
to stream organic matter
6. Conclusions
Acknowledgments
References