Geomorphically mediated carbon dynamics of floodplain soils and Implications for net effect of carbon erosion

Timothy A. Quine, Elizabeth L. Cressey, Jennifer A. J. Dungait, Sarah de Baets, Jeroen Meersmans, Matthew W. Jones, Andrew P. Nicholas

Research output: Contribution to journalArticlepeer-review

Abstract

The fate of organic carbon deposited in floodplain sediments is an important control on the magnitude and direction of the carbon flux from anthropogenically accelerated erosion and channelization of the riverine network. Globally, rates of deposition and the mean residence time (MRT) of carbon within different geomorphic settings remains poorly constrained. We sampled the soil profile to 0.8 m depth from two geomorphic zones: active channel belt (ACB) and lowland floodplain, under long-term pasture adjacent to the river Culm in SW England, UK. We evaluated sedimentation rates and carbon storage using fallout radionuclide 137Cs, particle size and total carbon analyses. Variation in decomposition was assessed via empirical and numerical techniques, using soil aggregate size and density fractionation combined with natural abundance C analysis. Carbon decomposition was simulated using the RothC model and catchment implications were explored using a floodplain evolution model. Sedimentation and carbon accumulation rates were 5-6 times greater in the ACB than the floodplain. Carbon decomposition rates also varied with geomorphic setting. Soil carbon in floodplain cores had more rapid decomposition rates indicated by greater 13C-enrichment and subsoils dominated by mineral-associated soil organic carbon. The carbon in ACB cores was less processed and 13C-depleted by comparison, with more light fraction and macroaggregate-carbon throughout the cores. Decomposition rates in the ACB were estimated to be 4-fold less than the floodplain, as indicated by the RothC model. Including the ACB in floodplain carbon MRT calculations in the floodplain evolution model increased overall MRT by 10%. The major differences in the balance of sedimentation and decomposition rates between active and inactive floodplains suggest that the relative extent of these contrasting floodplain zones is critical to the overall carbon balance of floodplains. Restoration projects could enhance soil carbon storage by maximising active floodplain areas by increasing river channel complexity
Original languageEnglish
JournalHydrological Processes
Early online date2 Aug 2022
DOIs
Publication statusE-pub ahead of print - 2 Aug 2022

Keywords

  • floodplain
  • sedimentation
  • stable isotopes
  • carbon dynamics
  • source/sink
  • mean residence time
  • erosion
  • carbon storage

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