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

Saltmarshes provide key ecosystem services, including atmospheric CO 2 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.