Linking Biogeochemical Processes and Ecosystem Health in Restored Marshes of the Doñana National Park

Coastal marshes play a crucial role in global carbon (C) cycles as net C sinks, making them valuable for offsetting anthropogenic greenhouse gas emissions through C stock preservation or wetland restoration. However, the capacity of restored marshes to sequester organic C often falls below expectations, even decades post-restoration. This discrepancy highlights gaps in our understanding of biogeochemical processes that influence soil organic matter (SOM) dynamics and C sequestration potential. Specifically, the quality and molecular composition of SOM and its interaction with microbial communities remain underexplored. This study investigates the chemical composition of SOM and its relationship with biogeochemical processes and biome composition in marshes within the Doñana National Park, a globally significant biodiversity hotspot in Andalusia, Spain. The park has historically undergone significant human-driven transformations, necessitating large-scale restoration efforts.

Our research focuses on three representative sites within the Doñana. The first site, “Marisma Gallega” (MG), is a high marsh that remains in its natural state, having never been cultivated. The second site, the “Finca de Caracoles” (FC), is undergoing active restoration and includes two distinct zones: a northern zone (FC_N) with a lower degree of restoration due to a shorter time since cultivation ceased, and a southern zone (FC_S) that has been uncultivated for a longer period. The third site is a fully transformed paddy soil (PS) currently used for rice cultivation. Soil samples were collected from each site at three depths (0–10; 10–20; and 20–30 cm) to analyze soil organic carbon (SOC), soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), and other key nutrients. Microbial activity was assessed through soil respiration (MicroResp technique), whereas microbial diversity and abundance were evaluated using phospholipid fatty acid (PLFA) analysis and metagenomic shotgun sequencing of 16S rRNA genes. Bioinformatic analyses provided insights into microbial richness, alpha diversity indices, and the relative abundance of different microbial communities.

Our results indicate that MG and both FC zones exhibited higher SOC, SOM, TN, and TP contents compared to PS, suggesting higher C accumulation and improved SOM quality in the restoration sites, with values not significantly differing from MG. However, MG and both FC zones displayed lower concentrations of available P and NO₃⁻ relative to PS. These findings can be attributed to the conditions in degraded wetlands like PS, which disrupt N and P storage and cycling, promoting the mineralization of organic nutrients, such as organic N, and its transformation to NO₃⁻. Microbiological analyses revealed higher microbial respiration rates and greater fungal and bacterial biomass in MG and FC compared to PS, along with shifts in microbial community composition. These variations were influenced by the degree of naturalization and restoration. Accumulated organic C, combined with aerobic conditions, likely supports microbial growth, thereby accelerating microbial respiration.

By characterizing SOM properties and linking them to microbial and biogeochemical processes, this study provides insights into how restoration and land-use change impact C storage and ecosystem services in marshes. The findings will inform future restoration strategies and improve the monitoring of these ecosystems’ health and functionality.