Geomorphic regulation of floodplain soil organic carbon concentration in watersheds of the Rocky and Cascade Mountains, USA

Abstract

Abstract. Mountain rivers have the potential to retain OC-rich soil and store large quantities of organic carbon (OC) in floodplain soils. We characterize valley bottom morphology, floodplain soil, and vegetation in two disparate mountain river basins: the Middle Fork Snoqualmie in the Cascade Mountains and the Big Sandy in the Wind River Range of the Rocky Mountains. We use this dataset to examine variability in OC concentration between these basins as well as within them at multiple spatial scales. We find that although there are some differences between basins, much of the variability in OC concentration is due to local factors, such as soil moisture and valley bottom geometry. From this, we conclude that local factors likely play a dominant role in regulating OC concentration in valley bottoms and that interbasin differences in climate or vegetation characteristics may not translate directly into differences in OC storage. We also use an analysis of OC concentration and soil texture by depth to infer that OC is input to floodplain soils mainly by decaying vegetation, not overbank deposition of fine, OC-bearing sediment. Geomorphology and hydrology play strong roles in determining the spatial distribution of soil OC in mountain river corridors.

Geomorphology and climate interact to control organic carbon stock and age in mountain river valley bottoms

“…This study was conducted alongside work presented in Scott & Wohl (2018a, 2018b, and hence shares many of the same methods with those two works.…”

“…At each sample site in all four basins, we measured the total channel and floodplain wood load in jams and individual pieces within a reach surrounding the site defined as either 100 m or 10 channel widths, whichever was shorter (see Scott & Wohl (2018b) for detailed wood load measurement methodology). We converted total wood volume per unit area to wood OC mass per unit area (stock) by assuming a density based on estimated decay classes assigned to wood pieces and jams in the field (Harmon et al, 2011) and the approximation that half of the wood mass is carbon (Lamlom & Savidge, 2003).…”

“…Organic carbon (OC) stored in soil and organic material in freshwater systems is substantial (Aufdenkampe et al, 2011) and varies in both spatial distribution of concentration (Battin et al, 2008;Scott & Wohl, 2018b;Sutfin et al, 2016;Wohl, Hall, et al, 2017) and residence time (Barnes et al, 2018;Marwick et al, 2015;Omengo et al, 2016). Carbon dynamics in these systems over short to moderate timescales (10 1 to 10 5 yr) can thus strongly regulate carbon emissions to and sequestration from the atmosphere (Berner, 1990;Stallard, 1998), regulating global climate.…”

“…Erosion regulates the fate of OC (Doetterl et al, 2016;Hilton, 2017;Wang et al, 2017) and whether that OC is stored long-term in sedimentary sinks (Blair & Aller, 2012) or respired to the atmosphere by microbes (Falloon et al, 2011;Jobbágy & Jackson, 2000). Modeling indicates that sedimentation dynamics should regulate the age of OC in floodplain soils (Torres et al, 2017), complementary to the idea that geomorphic processes regulate OC concentrations in those soils (Lininger et al, 2018;Scott & Wohl, 2018b;Swinnen et al, 2019;Wohl, Hall, et al, 2017) and wood loads in valley bottoms (Scott & Wohl, 2018b).…”

The Importance of Coarse Organic Matter and Depositional Environment to Carbon Burial Behind Dams in Mountainous Environments

“…Similarly, the F5 facies, with its fine-grained proximity to inflowing organic material and its leaf beds, probably has relatively high TOC, but analogous systems report widely varying carbon content. Floodplain sediments from partly confined younger forest and unconfined old-growth multithread headwater streams in Colorado have been reported to contain as much as 12% TOC (Wohl et al, 2012); however, samples from floodplain sediments on the Middle Fork Snoqualmie, Washington, report median values of approximately 2.5% or less (Scott & Wohl, 2018), while samples from frequently inundated, low-lying floodplains in Quebec report average TOC of 1.74 ± 0.17 wt% (Saint-Laurent et al, 2016) and overbank sediments in rivers from the United Kingdom range from 2.17 to 5.07% (Walling et al, 2006). In contrast, samples from Facies G4, G8, and G9 in former Lake Aldwell average between 0.17 and 0.25 wt% TOC.…”