Unravelling drivers of organic matter dynamics in Baltic coastal wetlands using the TBI method

Coastal wetlands provide valuable ecosystem services, including carbon storage, but are threatened by anthropogenic pressures and changes in management practices. This study aimed to identify the primary environmental drivers of organic matter decomposition in Baltic coastal wetlands across different plant communities and seasons. We used the Tea Bag Index (TBI) method to determine the decomposition rate ( k ) and stabilization factor ( S ) and measured longer-term mass loss over three seasons in five coastal wetlands. Soil samples were collected to analyse physicochemical properties. Results showed no significant correlation between k and S . Abandoned areas encroached by juniper (Scrub and developing Woodland - SW) exhibited the highest decomposition rates ( k ), whereas areas encroached by reed (Reed Swamp - RS) and Open Pioneer (OP) at managed sites had lower rates. An interplay of soil moisture and water permeability as well as soil C:N ratio and water permeability, were identified as key edaphic factors influencing the Stabilization factor ( S ), while salinity was a key factor influencing the decomposition rate ( k ). Longer-term mass loss varied significantly among plant communities, with plant communities characteristic of the cessation of management (RS and SW) showing distinct seasonal decomposition patterns between them and compared with the other plant communities. Over the year, elevation was an additional factor affecting mass loss. These results demonstrate that abandonment creates distinct biogeochemical environments that alter decomposition patterns and thus carbon storage compared with traditionally managed coastal wetland systems. Ecosystem management is therefore crucial for maintaining the ecological functions and carbon sequestration capacity of these protected semi-natural wetland systems.