Decomposition in Drained Coastal Peatlands: The Slow Waltz of Decay and Subsidence

Peatlands, while covering only about 3% of the Earth’s surface, store nearly a third of global soil carbon. This carbon accumulated under waterlogged, oxygen-poor conditions that slow microbial decomposition. However, widespread drainage for agriculture and development has altered these conditions, turning peatlands from carbon sinks into significant greenhouse gas (GHG) sources. Drained peatlands now account for 4–6% of global anthropogenic CO₂ emissions and are undergoing irreversible land subsidence, particularly in coastal areas where the risk of flooding and land loss is increasing. Microbial activity is central to both GHG emissions and subsidence. When peat is drained, oxygen exposure accelerates microbial decomposition, leading to carbon loss and volume reduction of the soil, which manifests as land subsidence. This creates a feedback loop: as the land lowers, drainage must continue, exposing deeper peat layers to oxygen, further accelerating decomposition. Current mitigation efforts—primarily water table management—are insufficient, and better strategies are needed to address the underlying processes. This thesis aims to improve the mechanistic understanding of peat decomposition, particularly in the deeper, water-saturated zones that have been largely overlooked. It explores how oxygen availability, botanical composition of peat, historical drainage, redox conditions, and physical compaction affect decomposition. Both field and laboratory experiments were conducted to investigate microbial processes across peat types and environmental conditions, with a focus on refining models of GHG emissions and land subsidence. Key findings from the research include: Peat origin matters: Plant material from sedges and reeds decomposed more rapidly under both aerobic and anaerobic conditions than wood- and sphagnum-derived peat. Botanical composition is a strong determinant of GHG emissions. Transition zones are critical: The fluctuating zone between saturated and unsaturated peat exhibits dynamic redox conditions. Experiments showed that adding iron and sulphate in these zones stimulated CO₂ production under anoxic conditions, indicating that anaerobic decomposition is more significant than previously assumed. Compaction affects decomposition: Mechanical compaction reduced porosity and slowed decomposition, although the extent varied with peat degradation status. This highlights how physical changes influence both microbial processes and land subsidence rates. Rewetting has mixed effects: Rewetting oxidised peat reduced CO₂ but increased CH₄ emissions, while oxygen exposure of deeper layers increased CO₂ emissions. Results were highly site-specific, depending on local substrate and redox conditions, indicating that restoration outcomes are variable. Improved emission models: The study demonstrated that including redox potential and lab-derived respiration data in emission models enhances prediction accuracy. Anaerobic processes, especially in the transition zone, contribute substantially to total CO₂ flux, challenging the assumption that decomposition is negligible in saturated layers. The thesis concludes with a conceptual model linking microbial and physical processes to GHG emissions and subsidence. The findings offer practical insights for improving national carbon inventories and developing more effective restoration and management strategies for drained peatlands, particularly in coastal regions under threat from climate change and sea-level rise.