Floodplain carbon stocks in soil and large wood across diverse environments: controls and comparisons

Abstract:

Floodplains are increasingly recognized as important sites of organic carbon (OC) storage. However, the amount and residence time of OC stored in river corridors (channels and floodplains) and the geomorphic influences on the spatial distribution of OC are not well understood. Floodplain soil and downed, dead large wood (LW) are two of the largest OC stocks within floodplains. I present case studies on floodplain OC stocks in soil and LW to elucidate controls on carbon stocks using sites in the boreal region and from the Colorado Front Range. I draw on additional studies to compare the magnitude of and controls on stocks in other environments. Hydrogeomorphic processes and vegetation dynamics influence OC stocks. Hydrogeomorphic processes set the physical template for vegetation dynamics, but also affect the capacity for soil OC retention by influencing grain size, moisture conditions, disturbance regime, and the residence time of floodplain sediment. River corridor vegetation provides a source of LW and contributes OC to floodplain soil. Across large spatial scales, floodplain soil OC stocks are influenced by climate, but there is large variability within climate zones. Within a drainage basin, variation exists in soil OC stocks depending on the timescale of alluvial storage, river characteristics, grain size, and disturbance regime. Floodplain LW OC stocks are likely highest in regions with high primary productivity and slow decay rates, resulting in a large OC stock on the floodplain. Productive forests provide a source for LW, while slow decay rates allow for the build-up of LW on the floodplain surface. Within a drainage basin, floodplain forest stand density also influences the amount of LW stored on the floodplain; dense floodplain forests impede transport of LW onto the floodplain, while floodplains lacking sufficient trees limit trapping locations where LW accumulations can form. Disturbance plays an important role in the amount of LW stored on floodplains, since large floods can significantly increase LW loads and thus OC storage. Human modifications of river corridors have significantly altered the amount of OC in soil and LW, limiting our understanding of unmodified conditions. To conclude, I highlight gaps in our understanding of floodplain OC stocks and provide suggestions for further study.