Carbon mineralization in tidal freshwater marsh soils at the intersection of low-level saltwater intrusion and ferric iron loading

Low-level saltwater intrusion and ferric iron (Fe(III)) loading are predicted for many mineral tidal freshwater ecosystems under sea-level rise. To assess their effects on the carbon mineralization processes, in-situ experiments were conducted with four treatments: Fe(III) amendment, saltwater addition, saltwater addition plus Fe(III) amendment, and a control. Soil and porewater samples (0–30 cm) were collected to assess geochemical properties and soil incubation on Days 460 and 580 after the start of the experiment. Saltwater addition increased the salinity from freshwater (0.1 mg·g⁻¹) to oligohaline (<2 mg·g⁻¹) and elevated the concentrations of porewater Cl⁻, SO₄²⁻, and NH₄⁺. Fe(III) amendment increased the concentrations of soil Fe(III), Fe(II), and total reduced sulfur (TRS). Both Fe(III) amendment and saltwater addition enhanced the belowground biomass of marsh plants, while neither affected the total organic carbon (TOC) and total nitrogen (TN) pools or their ratios. Both Fe(III) amendment and saltwater addition improved the overall mineralization rates (i.e., CO₂ plus CH₄ production) of the soil at 0–10 cm depths. Methanogenesis decreased under both Fe(III) amendment and saltwater addition. Strong negative relationships were observed between methanogenesis rates and the concentrations of porewater Cl⁻ and SO₄²⁻, and soil Fe(III). Relative to the individual Fe(III) and saltwater treatments, the combined treatment did not further promote overall mineralization rates or further impede methanogenesis potential. Methanogenesis and Fe(III) reduction co-dominated the overall mineralization in the control treatment. Individual loading of Fe(III) and saltwater into the soils induced the predominance of Fe(III) and sulfate reduction, respectively. Under the combined treatment, microbial Fe(III) and sulfate reduction co-dominated the overall mineralization, but restrained each other. The shifts in the partitioning of mineralization pathways among treatments were primarily influenced by porewater Cl⁻ and SO₄²⁻, followed by Fe(III), and then DOC. Together, the results suggest that the projected modest levels of saltwater intrusion and Fe(III) loading may simultaneously accelerate the belowground C input and mineralization rates, shifting anaerobic pathways away from CH₄ production towards Fe(III) and sulfate reduction.