Influence of hydrodynamics on dissolved and particulate organic carbon release from sediments in vegetated flows

Hydrodynamic processes are crucial for carbon storage in wetlands. Intense hydrodynamic activity can cause erosion, thereby mobilizing buried carbon and reintroducing it into the environment. To estimate the hydrodynamic impacts on the resuspension of organic carbon in vegetated sediments, we exposed patches of Scirpus with different shoot densities to gradually increasing unidirectional currents characterized by plant Reynolds numbers ranging from 901 to 2160 in a hydraulic flume with a constant water column height of 0.3 m. The study found that low turbulent kinetic energy (TKE) favored dissolved organic carbon (DOC) accumulation (C1-F1, C1-F2, C1-F3, C2-F1, and C3-F1), whereas relatively high TKE in high-density vegetation (such as C3-F3) triggered organic carbon desorption due to elevated particulate concentrations, resulting in the highest DOC concentrations. Additionally, particulate organic carbon (POC) concentrations decreased with increasing hydrodynamic intensity in cases with higher average suspended sediment concentrations. This was presumably due to the enhanced hydrodynamic conditions and elevated dissolved oxygen levels, which directly promoted the consumption of organic carbon and influenced the adsorption/desorption processes of suspended particles. During sediment resuspension, suspended sediment concentration (C_s) was the dominant controlling factor for DOC in both bare bed and low-density Scirpus cases, while total dissolved solids (TDS) also played a significant role. In high-density Scirpus cases, DO exhibited the strongest statistical association among the measured water quality parameters, with C_s and TDS closely following in terms of permutation importance. In the cases with the highest average suspended sediment concentrations, namely low-density and high-density Scirpus under high TKE (C3-F2, C2-F3, and C3-F3), C_s remained the primary controlling factor for POC. The kinetic fitting results indicated that near-bottom POC adsorption under high-density Scirpus and high TKE (C3-F2) was well described by the pseudo-first-order and pseudo-second-order models. In contrast, POC dynamics in the low- and high-density Scirpus cases (C2-F3, C3-F3) could not be accurately captured by these models. This discrepancy is likely because POC desorption was not governed solely by adsorption-desorption equilibrium but was also influenced by a net release process involving the detachment of suspended particulate matter (SPM) biofilms, decomposition of organic matter, or particle fragmentation. Overall, these results provide valuable insights into the interactions among vegetation density, hydrodynamic conditions, and organic carbon dynamics. The findings suggest that increased hydrodynamic disturbance may enhance the short-term mobilization of sedimentary organic carbon at the sediment-water interface under controlled flume conditions.