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

Abstract

High‐latitude permafrost regions store large stocks of soil organic carbon (OC), which are vulnerable to climate warming. Estimates of subsurface carbon stocks do not take into account floodplains as unique landscape units that mediate and influence the delivery of materials into river networks. We estimate floodplain soil OC stocks within the active layer (seasonally thawed layer) and to a maximum depth of 1 m from a large field data set in the Yukon Flats region of interior Alaska. We compare our estimated stocks to a previously published data set and find that the OC stock estimate using our field data could be as much as 68% higher than the published data set. Radiocarbon measurements indicate that sediment and associated OC can be stored for thousands of years before erosion and transport. Our results indicate the importance of floodplains as areas of underestimated carbon storage, particularly because climate change may modify geomorphic processes in permafrost regions.

Reflections on the history of research on large wood in rivers

“…Important gaps remain in our knowledge of the influence of large wood on organic carbon dynamics. Among these gaps are how floodplain carbon stocks differ within a river network and among river networks, although recent studies have examined patterns in spatial variability within at least some river networks (Lininger et al ., 2018, 2019; Scott and Wohl, 2018a, 2018b; Sutfin and Wohl, 2019). The tropics represent a significant unknown in this respect.…”

“…Studies on this topic along river corridors began after a few papers by European investigators noted that floodplain soils can store significant organic carbon stocks (Hoffmann et al ., 2009; Cierjacks et al ., 2010). Subsequent research indicates that river corridors – channels and floodplains – can contain disproportionately large organic carbon stocks in the form of downed, dead wood and floodplain soils (Wohl et al ., 2012, 2017; Sutfin et al ., 2016; Scott and Wohl, 2018b; Lininger et al ., 2019). The relative importance of downed wood versus soil organic carbon varies between sites.…”

Long‐Term Storage and Age‐Biased Export of Fluvial Organic Carbon: Field Evidence From West Iceland

“…The greater C sink associated with the growth of the valley sediment fill (i.e., topset deposition) may be underappreciated as much research on the long‐term implications of the terrestrial OC cycle has highlighted only the riverine export fluxes of OC (Galy et al, ; Hilton, ) despite existing evidence for large floodplain reservoirs of OC (Cierjacks et al, ; Hoffmann et al, ; Lininger et al, ; Sutfin et al, ; Sutfin & Wohl, ; Wohl et al, , ). Both lateral migration and vertical aggradation are common to low‐sloping river systems (Dunne & Aalto, ; Muto & Steel, ), which highlights the potential application of these dynamics to terrestrial systems elsewhere in the global OC cycle over a wide range of timescales.…”

“…However, interpreting the 14 C content of bulk POC in terms of a storage time can be complicated due to the inheritance of radiocarbon‐dead “petrogenic” OC in particles eroded from ancient sedimentary rocks in the catchment (Bouchez et al, ; Galy et al, ; Masiello & Druffel, ). So, despite important previous work on the POC content of floodplains (Cierjacks et al, ; Hoffmann et al, ; Lininger et al, ; Sutfin et al, , Sutfin & Wohl, ; Wohl et al, , ), these works cannot be used to test for long‐term storage and/or age‐selective entrainment because they did not account for petrogenic carbon. In one effort to circumvent this complication, several studies of terrestrial POC storage times (Galy & Eglinton, ; Martin et al, ; Tao et al, ) analyzed the 14 C content of select “biomarker” compounds.…”

“…Existing paradigms for the terrestrial OC cycle focus on how biological and geochemical factors influence the duration of OC storage by modulating the relative rates of OC production and consumption (Raich & Schlesinger, ; Schmidt et al, ). However, the observation that a majority of terrestrial OC is physically stored as particulate OC (POC) within sedimentary deposits (Cierjacks et al, ; Doetterl et al, ; Hoffmann et al, ; Lininger et al, ; Pachauri et al, ; Stallard, ; Sutfin et al, , ; Wohl et al, , ) implies that sediment storage and transport processes should also play an important modulating role. For instance, estimates of the duration of sediment storage in terrestrial systems (10 3 to 5 × 10 5 years; Blöthe & Korup, ; Granet et al, ; Li et al, ; Wittmann et al, ) are of similar order to the inferred timescales of terrestrial POC storage (greater than 10 3 years; Galy & Eglinton, ; Tao et al, ; Torn et al, ).…”