The Role of Plant-Soil Interactions for Carbon Cycling in Baltic and North Sea Coastal Wetlands

Abstract

Plants in coastal wetlands act as ecosystem engineers, as they shape ecosystem development through bio geomorphic feedbacks and exert strong control over carbon cycling in these dynamic systems and globally important carbon sinks. High primary productivity supplies large amounts of organic matter to the soil, while water saturation slows soil organic matter (SOM) decomposition. The fate of the sequestered carbon further depends on plant-soil interactions, as plants modify soil conditions such as redox potential and pH - key drivers of microbial activity. In addition, plants influence microbial decomposition by supplying substrates of differing quality. Compared to upland ecosystems, plant-mediated control over microbial activity is particularly pronounced in wetlands, where roots not only provide organic inputs but also release oxygen, altering redox conditions through radial oxygen loss. These interactions are particularly important for methane dynamics - a greenhouse gas with greater radiative forcing potential than carbon dioxide - which can be high in wetlands due to the waterlogged soils. Despite their disproportionate role as carbon sinks, the effects of global warming on ecosystem carbon responses in wetlands remain poorly understood. The net climate-carbon feedback of wetlands is strongly dependent on plant-soil interactions and their warming-induced alterations, as both plant carbon assimilation (inputs) and SOM decomposition (outputs) are temperature-sensitive. Warming-driven increases in SOM decomposition are likely to enhance plant-available nitrogen, potentially alleviating nitrogen limitation and promoting greater biomass production and carbon retention. However, if decomposition outpaces plant carbon inputs, this will result in substantial SOM loss, creating a positive climate-carbon feedback accelerating climate change. This thesis investigates carbon cycling and storage under current and warmer climate conditions in coastal wetlands of the North Sea and Baltic Sea, with a particular focus on plant-soil interactions. Four complementary studies were conducted. First, carbon stocks (chapter 2) and methane emissions (chapter 3) were quantified across German coastal wetlands, addressing both large-scale differences (between coasts) and small-scale variation (within sites and plant communities). Second, two studies (chapter 4 and 5) were conducted in a mesocosm warming experiment and investigated warming effects on transplanted vegetated soil-sods originating coastal wetlands from Denmark, Sweden, and Finland spanning distinct soil morphologies and plant communities. Here we assessed warming effects on aboveground biomass, soil organic matter (SOM), and microbial communities. Across these studies, several key findings emerged. Low-energy Baltic Sea salt marshes contained higher soil organic carbon (SOC) stocks than high-energy North Sea marshes. Livestock grazing increased SOC stocks in the North Sea by enhancing soil compaction but showed mixed effects in the Baltic, driven by changes in plant biomass. Contrary to expectations, grazing effects on soil compaction and plant communities did not increase methane emission but varied strongly among sites. Grazing-induced shifts in plant community composition and subsequent alterations in belowground biomass and hence redox potentials explained methane emission. Grazing often excluded high-emission species such as Phragmites australis, highlighting complex underlying plant-soil interactions that vary between species and connected functional traits. The studies conducted in the mesocosm warming experiment revealed that elevated temperatures increased plant-available nitrogen and enhanced aboveground biomass especially in SOM-rich systems, though systems with nitrogen-fixing plants and lower SOM content showed weaker responses. SOM losses occurred in soils initially rich in SOM, suggesting that ecosystem carbon responses to warming depend strongly on initial resource status. Warming also induced significant restructuring of soil microbial communities in SOM richer tall-grass communities but not in the SOM-poorer tall-grass community, likely reflecting differences in successional stage and microbial specialization. Taxa that increased under warming were linked to nutrient cycling and organic matter breakdown, whereas those that decreased under warming were associated with cold and low-oxygen adaptation. Together, these studies demonstrate the pivotal role of plant-soil-microbe interactions in regulating carbon dynamics in coastal wetlands under both current and future climate conditions. Future research should prioritize two key pathways: (1) developing a mechanistic understanding of plant-soil interactions and how they are altered under warming, understanding particularly the influence of plant traits such as radial oxygen loss and root exudation on microbial activity and SOM decomposition; and (2) assessing SOM as a central variable in ecosystem carbon responses to warming, supported by further experimental studies to test the generality of this pattern.