Geomorphic Controls on Floodplain Soil Organic Carbon in the Yukon Flats, Interior Alaska, From Reach to River Basin Scales

Abstract

Floodplains accumulate and store organic carbon (OC) and release OC to rivers, but studies of floodplain soil OC come from small rivers or small spatial extents on larger rivers in temperate latitudes. Warming climate is causing substantial change in geomorphic process and OC fluxes in high latitude rivers. We investigate geomorphic controls on floodplain soil OC concentrations in active‐layer mineral sediment in the Yukon Flats, interior Alaska. We characterize OC along the Yukon River and four tributaries in relation to geomorphic controls at the river basin, segment, and reach scales. Average OC concentration within floodplain soil is 2.8% (median = 2.2%). Statistical analyses indicate that OC varies among river basins, among planform types along a river depending on the geomorphic unit, and among geomorphic units. OC decreases with sample depth, suggesting that most OC accumulates via autochthonous inputs from floodplain vegetation. Floodplain and river characteristics, such as grain size, soil moisture, planform, migration rate, and riverine DOC concentrations, likely influence differences among rivers. Grain size, soil moisture, and age of surface likely influence differences among geomorphic units. Mean OC concentrations vary more among geomorphic units (wetlands = 5.1% versus bars = 2.0%) than among study rivers (Dall River = 3.8% versus Teedrinjik River = 2.3%), suggesting that reach‐scale geomorphic processes more strongly control the spatial distribution of OC than basin‐scale processes. Investigating differences at the basin and reach scale is necessary to accurately assess the amount and distribution of floodplain soil OC, as well as the geomorphic controls on OC.

Significant Floodplain Soil Organic Carbon Storage Along a Large High‐Latitude River and its Tributaries

“…The total carbon stock increases by approximately 40-360% across individual study river floodplains when estimating stocks using field data (Figure 3). Samples taken horizontally into cut banks ranged from 1 to 4.2 m below the top of the bank, and the OC concentrations of these samples were similar to the OC concentrations of mineral sediment within the top 1 m (median = 2.1% OC from cut banks compared to median = 2.2% OC in mineral sediment within top 1 m; Lininger et al, 2018). In contrast, previous comparisons with NCSCD have found that the NCSCD may overestimate stocks in some areas (Zubrzycki et al, 2013).…”

“…We sampled floodplain sediments over two field seasons (Summer 2014 and 2015) along five rivers within the YF with varying drainage areas (Text S1 in the supporting information). For further description of sampling methodology and fieldwork, see Lininger et al (2018) and supporting information Text S1. We sampled mineral sediment in increments of approximately 18 cm with an auger or a soil corer, stopping when we reached 1 m in depth or point of refusal.…”

“…We also took core samples into the face of cut banks at a few locations to assess the range of OC concentrations below 1 m in depth. For further description of sampling methodology and fieldwork, see Lininger et al (2018) and supporting information Text S1.…”

The persistence of beaver‐induced geomorphic heterogeneity and organic carbon stock in river corridors

“…With respect to carbon inputs, recent work on other floodplains in the Rocky Mountains and elsewhere suggests that autochthonous inputs dominate (Scott and Wohl, in review; Lininger et al, ). The lack of buried, high OC concentrations at depth in the soil cores from the beaver meadows suggests that autochthonous, rather than allochthonous, inputs also dominate these floodplains.…”

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

“…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).…”