HR: 0800h
AN: B11A-0058    [Abstracts]
TI: A Predictable Terrestrial Signature to Riverine Dissolved Organic Carbon?
AU: * Sanderman, J
EM: jsandman@nature.berkeley.edu
AF: Division of Ecosystem Sciences University of California, Berkeley, 137 Mulford Hall - MC3114, Berkeley, CA 94720, United States
AU: Amundson, R
EM: earthy@nature.berkeley.edu
AF: Division of Ecosystem Sciences University of California, Berkeley, 137 Mulford Hall - MC3114, Berkeley, CA 94720, United States
AU: Baldock, J A
EM: jeff.baldock@csiro.au
AF: CSIRO Land and Water Adelaide Laboratory, PMB 2, Glen Osmond, SA 5064, Australia
AB: In small mountainous watersheds, the majority of dissolved organic carbon (DOC) is derived from terrigenous sources; however, there is much debate over the age and recalcitrance of these organic materials. To determine controls on the age and recalcitrance of DOC found in stream waters, we measured DOC composition in stream and soil water samples, using isotopic (¹³C and ¹⁴C) and spectroscopic (UV and ¹³C NMR) analyses, in conjunction with soil hydrometric conditions in two first-order watersheds with contrasting vegetation in northern California.

In a low-gradient coastal prairie stream, we found low concentrations of old (Δ¹⁴C = -200 permil) DOC that most resembled stabilized soil organic matter found deep within the mineral soil during baseflow. In contrast, during storm events where saturation overland flow dominated runoff, we found high concentrations of young (Δ¹⁴C = +75 permil) DOC resembling fresher organic matter. These results contrast with observations from a high-gradient coniferous forest where there is a much narrower range in age and chemistry of stream DOC over time. In the forest, runoff generation is dominated by subsurface stormflow with little if any overland flow and there is a much narrower range of stream DOC concentration, age and chemistry DOC, all of which is comparable to that of older, stabilized soil organic matter.

At both of these locations DOC in soil water varies with increasing depth: young to old and labile to recalcitrant - due to rapid exchange with surficially-bound organic matter on soil solids. Given this range in soil DOC properties, it appears that the flowpath of water through soils determines the age and composition of DOC as water enters the stream network. During throughflow conditions, the soil acts as a filter for fresh plant-derived DOC, releasing only aged and highly altered DOC to the stream. Shallow flowpaths will largely bypass this filter, resulting in the export of high concentrations of young and labile DOC. In these ecosystems, riverine DOC is not simply terrestrial or aquatic in origin. Within the "terrestrial" signature, we have found nearly as large a range of ages and chemistries as found within the soil profile itself, and the resulting DOC composition is largely predictable with knowledge of runoff generating mechanisms.
DE: 0428 Carbon cycling (4806)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0486 Soils/pedology (1865)
DE: 1804 Catchment
SC: Biogeosciences [B]
MN: 2007 Fall Meeting