A Review of Fe–S–C Dynamics in Blue Carbon Environments: Potential Influence of Coastal Acid Sulfate Soils

Abstract

Vegetated blue carbon environments have the potential to sequester large amounts of carbon due to their high productivity and typically saturated, anaerobic soils that promote carbon accumulation. Despite this, and the coupling of Fe–S–C cycling processes, the influence of iron (Fe) in acid sulfate soils (ASSs) on carbon sequestration in blue carbon environments has yet to be systematically explored. To address this knowledge gap, this review provides an overview linking the current state of blue carbon studies with the influence of Fe on soil organic carbon (SOC), as well as the potential influence ASSs have on carbon sequestration. A systematic literature review on SOC stock in blue carbon studies using the Web of Science database yielded 1477 results. Studies that investigated the drivers of carbon accumulation in blue carbon studies were restricted to vegetation species/structure and geomorphic setting, and few focused on soil properties and type. Iron both protects and enhances SOC decomposition depending on its redox state. Under oxic conditions, Fe oxyhydroxides can protect SOC via adsorption, co-precipitation and by acting as a cement in soil aggregates. Iron can also increase SOC decomposition under oxic conditions due to Fenton reactions. However, under anoxic conditions, SOC mineralisation can also occur as Fe acts as an electron transporter in dissimilatory reductions. ASSs contain a range of Fe minerals, but the oxidation of Fe sulfides can result in soil acidification (pH < 4) and subsequent impacts, such as a decline in vegetation health, poor water quality and infrastructure damage. Therefore, potential SOC protection by Fe under oxic conditions may come at the cost of soil acidification in ASSs, while maintaining anoxic conditions prevents acidification but may enhance SOC decomposition. Future studies on the influence of ASSs on Fe–S–C cycling and carbon sequestration in blue carbon environments are important, particularly for ‘hotspots’ such as Australia.