Soil carbon fractionation as a tool to monitor coastal wetland rehabilitation

Coastal wetland rehabilitation can provide nature-based solutions for climate change mitigation. The high carbon accumulation rate and carbon secured, potentially for several millennia, as soil organic carbon (SOC), is among the reasons. Measuring SOC storage and accrual over time are the main tools to understand rehabilitation success. However, SOC is partitioned among different organic matter fractions with varying physio-chemical properties and stabilities. In this research, we separated different organic matter fractions based on density (free light fraction: f-LF, occluded light fraction: o-LF and heavy fraction: HF) and solubility (dissolved fraction: DF) from soils taken from a wetland under rehabilitation and a pristine mangrove forest in Queensland, Australia. The f-LF and o-LF contain particulate organic carbon (POC_f-LF and POC_o-LF), whereas HF consists of mineral-associated organic carbon (MAOC_HF). Mangroves are superior to wetlands under rehabilitation and terrestrial forests in terms of C storage in each fraction. Soils from both mangroves and wetlands under rehabilitation are dominated by MAOC_HF. However, MAOC_HF from mangrove soils are relatively physio-chemically stable, while wetlands under rehabilitation are leaching aged-SOC (>1000 years) from the HF to DF as dissolved organic carbon (DOC_DF). Therefore, reducing the risk of mobilisation of aged-SOC can be a key to achieve rehabilitation success.