Degraded coastal agricultural lands hold significant soil carbon stocks and are potential candidates for blue carbon restoration

Blue carbon ecosystems, including mangroves, offer nature-based solutions against the impacts of climate change. Degraded coastal agricultural lands that were previously coastal wetlands harbour ancient (>1000 years) soil organic carbon (SOC) pools and can be potential candidates for coastal wetland restoration. We investigated the SOC storage capacity of coastal wetlands under rehabilitation (CWUR) by measuring the SOC stocks of coastal wetlands under different stages of rehabilitation (Phase 1 and Phase 2 sites) and comparing their stocks to those from well-established mature mangroves (reference sites). The location of our study is on Maroochy River floodplain on the Sunshine Coast, Queensland, Australia. In addition, the contribution of different sources to SOC in mature mangroves and CWUR was investigated using Bayesian stable isotope mixing models. Dating of sediments by ²¹⁰Pb and ¹³⁷Cs radionuclides was used to estimate the historical sedimentation and carbon accumulation rates for mature mangroves. CWUR had SOC stocks ranged from 242.6 to 322.2 Mg OC ha^-1 m^-1 whereas the average SOC stocks of mature mangroves were 461.4 Mg OC ha^-1 m^-1. Not only are the carbon stocks in mature mangroves higher than those for CWUR but they are also comparable to those from mangroves in subtropical Australia and estuarine settings. Unexpectedly, CWUR had higher SOC stocks compared to SOC stocks for pasturelands from previous studies. The suspended particulate matter (SPM) supplied by tidal and fluvial processes is a main constituent of SOC pools in both mature mangroves and CWUR that we studied. Furthermore, mangrove roots contributed significantly to peat and organic-rich sediment formation in mature mangroves on Maroochy River floodplain. Presence of remnant SOC (>1000 years) and availability of accommodation space for sediment deposition in CWUR are advantageous for blue carbon restoration.