Tidal inundation and plant growth/decay impact redox-sensitive metal geochemistry and fluxes in salt marsh porewater.

In dynamic coastal ecosystems, environmental factors can play important roles in the biogeochemical cycle of redox-sensitive metals. This work is focused on the impact of tidal inundation, plant growth and decay on the biogeochemical cycle of redox-sensitive metals (e.g., Fe, Mn, Mo, V and U) in salt marsh wetlands. Samples were collected from the salt marsh wetlands of the Yellow River Estuary under different tidal states and growth stages of plants (Phragmites australis). Compared to the concentration of redox-sensitive metals in the river water and seawater near the study area, Fe, Mn and U were enriched in salt marsh wetland, which might become a potential source of Fe, Mn and U in the coastal sea. Tidal inundation, plant growth and decay can affect redox-sensitive metals through changes in redox conditions; the plant can also affect them directly via root absorption or plant residue decomposition, especially for Mo. Calculations of diffusion flux between sediment porewater and tidal water show that these processes can increase diffusion by at least 16.7 % or decrease it by at least 65.7 %, even reversing the direction of diffusion, which can affect the accumulation of redox-sensitive metals in salt marsh wetlands. The results showed that tidal inundation and the decay of plant residue were not conducive to the accumulation of Fe and Mn but were beneficial to the accumulation of V and U in salt marsh wetlands. The plant growth showed the opposite pattern. The accumulation of Mo in salt marsh wetlands largely depends on ingestion by plants and the decay of plant residue. This research provides a scientific basis for the budget calculation of redox-sensitive metals in salt marsh wetlands.

Changes of active particulate uranium under the Water-Sediment Regulation Scheme in the lower Yellow River: Potential impact to the uranium flux into the global ocean.

With the annual Water-Sediment Regulation Scheme (WSRS) transporting large amounts of suspended particulate matter (SPM) to the sea within several days, the behavior of uranium in the Yellow River during the WSRS is crucial to better understand the uranium flux. In this study, the active forms (the exchangeable, carbonate bounded, Fe/Mn oxides bounded, organic matter bounded) and the residual form of particulate uranium were extracted by the sequential extraction method and their uranium contents were measured respectively. Results show that the content of total particulate uranium was 1.43-2.56 µg/g and the active forms accounts for 11-32 %. Particle size and redox environment are the two main factors controlling the active particulate uranium. The flux of active particulate uranium at Lijin was 4.7 tons during 2014 WSRS, which was about 50 % of the dissolved uranium flux for the same period. Thus, the terrestrial uranium flux is significantly modified by artificial regulation.