Carbon Cycling in Estuarine Marshes: A Focus on DOC Stabilization and Mobilization Pathways

Coastal wetlands are vital to global carbon cycling because they can store large amounts of Soil Organic Carbon (SOC). These ecosystems are influenced by complex interactions between salinity, flooding frequency and vegetation, which affect the formation, stabilization and mobilization of Dissolved Organic Carbon (DOC). Stabilization mechanisms, including mineral association and aggregation, are critical for long-term SOC storage, with Mineral-Associated Organic Matter (MAOM) being the dominant mechanism. However, the mechanisms driving DOC mobilization in estuarine marshes, particularly spatial and seasonal variabilities and the effects of climate and vegetation, remain poorly understood.

This study addresses these gaps by examining how seasonal fluctuations driven by biotic factors impact DOC concentrations in marsh soils along salinity and flooding gradients. As a part of 12 months field study, pore-water samples are being collected monthly using suction cups in nine marsh zones along the Elbe Estuary, representing a salinity gradient (salt, brackish, and freshwater marshes) and flooding gradients (pioneer, low, and high zones) at depths of 10 cm and 30 cm. The collected samples are analyzed for Non-Purgeable Organic Carbon (NPOC), anions, and Iron (Fe) concentration. Preliminary results revealed that NPOC concentrations were consistently higher in salt marshes compared to brackish and freshwater marshes. Pioneer zones exhibited the highest NPOC concentrations, particularly at 30 cm depth, highlighting the interaction of site and elevation as key factors driving spatial variability. Seasonal trends showed elevated NPOC levels during summer, followed by declines in autumn, likely driven by increased organic matter decomposition during warmer periods. Our results indicate a negative correlation between NPOC and Fe concentrations, suggesting that redox-driven mechanisms, such as Fe reduction, play a critical role in DOC stability and release. In conclusion, DOC mobilization in the Elbe Estuary is strongly influenced by salinity and flooding gradients, with higher concentrations in salt marshes and during summer. Understanding DOC dynamics in tidal marshes is essential for predicting the impacts of climate change on carbon cycling within estuarine ecosystems. As global sea levels rise and salinity gradients shift, this research provides important baseline knowledge to inform strategies for protecting the carbon sinks of coastal wetlands.