Sedimentary organic carbon and nitrogen storage in a recovered saltmarsh: Rewilding as a nature-based solution for...

Saltmarshes provide key ecosystem services, including atmospheric CO₂ sequestration and nitrogen burial in sediments. In recent decades, these blue carbon ecosystems have faced significant degradation from natural and anthropogenic stressors. In this study, rewilding of a desiccated saltmarsh in Cadiz Bay (SW Spain) was assessed as a nature-based solution to restore carbon (C_org) and nitrogen (N_T) storage. The rewilding process began in 2004 after breaching an external tidal wall. We evaluated changes in vegetated and unvegetated areas using Landsat satellite imagery (1994-2024) and quantified C_org and N_T stocks and burial rates in wild and rewilded sediments, including vegetated saltmarsh (Sarcocornia sp.) and bare sediments colonized by microphytobenthos (MPB). Vegetated saltmarsh cover increased by 85% over 20 years, at an average recovery rate of 5 ha y^-1, concurrent with a decrease in unvegetated tidal flats. Average C_org stocks in the top 1 m ranged from 32 to 57 t C_org ha^-1, with higher values in vegetated sediments. However, only 5-12% of C_org was stored during the rewilding period. C_org burial rates averaged 69 g C_org m^-2 y^-1, and N_T stocks were 55% higher in rewilded sediments than in wild ones (3.6 vs. 1.6 t N_T ha^-1). Despite vegetation recovery, burial rates of C_org and N_T did not increase clearly, suggesting that long-term storage may be influenced by factors beyond rewilding. Less than 8% of sedimentary C_org originated from saltmarsh vegetation, indicating the dominance of allochthonous sources. These findings highlight the complexity of biogeochemical recovery in rewilded saltmarshes and underscore the need for long-term monitoring to determine how much time is truly required for C_org and N_T recovery.