Impact of soil legacy on soil organic carbon partitioning in mangrove wetlands: a density-based fractionation and X-ray photoelectron spectroscopy study

Mangroves store a large proportion of their ecosystem C belowground in the form of soil organic carbon (SOC). Understanding the partitioning of mangrove SOC pool is important in order to explain the mechanistic responses to different environmental conditions and stressors. We separated the different SOC fractions from soils in a pristine mature mangrove forest, and an immature mangrove stand (described as young mangrove site in this study) under rehabilitation (previously degraded agricultural land) in Queensland, Australia, using density-based fractionation. Density-based separation resulted in three different SOC fractions: particulate organic carbon in a free-light fraction (POC f-LF ), particulate organic carbon in an occluded-light fraction (POC o-LF ), and mineral-associated organic carbon in a heavy fraction (MAOC HF ). Mature mangroves had the highest C concentration per unit mass of soil in each fraction. MAOC HF dominated the SOC pool in both mature and young mangroves. However, a significantly higher POC:MAOC ratio in mature mangroves indicated that favourable conditions facilitated litter accumulation and burial. X-ray photoelectron spectroscopy (XPS) analyses provided evidence for Al- and clay-mediated (aluminosilicates) stabilisation of SOC at the mineral-soil solution interface. In addition, mineral surface area occupied by organo-carbon species (MSA-OCS) obtained from XPS serves as an indicator to understand SOC dynamics in restoring coastal wetlands. Surface elemental compositions estimated using XPS also revealed that the aggerate stability improves as mangrove restoration continues. Therefore, XPS can be a useful tool to provide insights into soil carbon sequestration and stabilization processes in coastal wetlands. • Mineral-associated organic carbon (MAOC) dominates mangrove soils • Mature mangroves have higher particulate organic carbon (POC) and MAOC contents • Mineral surface area occupied by organo-carbon species manifests restoration success • Aggregate stability improves as mangrove restoration progresses • X-ray photoelectron spectroscopy is a useful tool for understanding soil C dynamics