Peat Collapse as a new threat to fen hydrological stability: the case of Nature reserve De Zegge (Belgium)

Fen peatlands are essential ecosystems that support high biodiversity, buffer hydrological extremes such as droughts and flooding, sequester carbon (C), and contribute to human well-being. However, increasing climate anomalies and anthropogenic disturbances are accelerating peat degradation, potentially triggering abrupt shifts in peat integrity and function – with significant implications for the global C cycle. Our study investigated the rapid peat subsidence observed in Belgium’s oldest nature reserve ‘De Zegge’, which represents an unheard form of fen ecosystems deterioration, and an environmental alarm. Thus, this case study may provide insights for land managers and researchers working in similar peat systems worldwide. To determine how hydrological stress, coupled with chronic hydro-chemical pressures − may push the system beyond a critical threshold, and lead to peat collapse,  we: (1) estimated the loses in elevation and C stocks using field-based digital elevation models, (2) compared peat characteristics between collapsed, adjacent non-collapsed and distant non-collapsed areas, and (3) experimentally assessed the effects of potential collapse triggers,- hydrological alterations and hydro-chemical additions (control, ditchwater, and sulfate [SO­₄^2-])- as on peat stability using a mesocosm experiment by measuring greenhouse gases (GHGs) emissions and porewater chemistry as indicators. Our findings demonstrate that fen peatland collapse led to significant lowering of the surface level (-12.8 ± 2.3 cm) accompanied by significant carbon losses (-21.6 ± 6.1 kg−C m^-2), alongside structural and functional shifts across biological, vegetative, physiochemical, and molecular dimensions (P < 0.05). In the mesocosm experiment, hydrologically perturbed peat exhibited reduced stability compared to undisturbed monoliths, particularly in regulating GHGs fluxes. During the successional phase, SO­₄^2- emerged as a key stressor, exerting pressure on system-wide stability. SO­₄^2- intrusion indirectly increased N₂O emissions during the re-saturation, with high spatial variability. Collectively, our study provides new insights into long-term pedological shifts affecting peat integrity and function in fen ecosystems.