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 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 (Corg) and nitrogen (NT) 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 Corg and NT 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 Corg stocks in the top 1 m ranged from 32 to 57 t Corg ha-1, with higher values in vegetated sediments. However, only 5-12% of Corg was stored during the rewilding period. Corg burial rates averaged 69 g Corg m-2 y-1, and NT stocks were 55% higher in rewilded sediments than in wild ones (3.6 vs. 1.6 t NT ha-1). Despite vegetation recovery, burial rates of Corg and NT did not increase clearly, suggesting that long-term storage may be influenced by factors beyond rewilding. Less than 8% of sedimentary Corg 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 Corg and NT recovery.

This study was funded by the CEIMAR Foundation through a project for young researchers (CEIJ-005, 2020), AQUA&AMBI (0750_AQUA&AMBI_2_5_P) funded by Interreg Spain–Portugal, RICAS (TED2021-132439B-I00) funded by MICIU/AEI/10.13039/501100011033 and by the European Union NextGenerationEU/PRTR, and the REWRITE project (101081357 H2020/HORIZON-CL5-2022-D1-02-05) funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union. Neither the European Union nor the granting authority can be held responsible for them. SH was funded by a regional postdoctoral fellowship (grant DGP_POST_2024_00524), supported by the Junta de Andalucía / CUII and the European Social Fund Plus (FSE+).

Peer reviewed