Limited recovery of soil organic matter composition in fen peatlands after rewetting

Approximately half of Europe’s peatlands have been extensively drained for agriculture, forestry, and peat extraction, with lowland fens disproportionately affected due to their high nutrient availability and productivity post-drainage. Peatland rewetting has gained importance as a strategy to mitigate carbon losses and restore biodiversity; however, its effectiveness in reestablishing below-ground processes remains uncertain. Soil organic matter (SOM) dynamics play a crucial role in peatland restoration outcomes, influencing peat structural integrity and hydrological properties, as well as the availability of substrates and nutrients for microbial and plant communities. Here, we present a Europe-wide assessment of the effects of fen drainage and rewetting on SOM molecular composition, analyzing fens with contrasting hydrological status (undrained, drained, and rewetted) across three depths (0 − 5, 15 − 20, and 45 − 50 cm) using pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS). Despite considerable variability induced by site-specific heterogeneity, the molecular composition of undrained fens clearly reflected the dominant contributions of sedges, grasses, and brown mosses to peat formation in fens, with characteristic stratification. Drainage consistently led to a decline in components from plant biopolymers (carbohydrates and lignins) across sites, and an increase in microbially altered compounds (N-compounds, benzenes, and aliphatics), with the most pronounced effects at intermediate depths (15 − 20 cm). Rewetting partially reversed these alterations, with some fens exhibiting a shift in molecular composition reverting toward undrained conditions, particularly in sites rewetted for longer periods. However, significant differences remained between rewetted and undrained fens. Our findings suggest that while rewetting over time can facilitate partial recovery of carbon dynamics, the below-ground legacy of drainage-induced alterations can persist, at least for a timescale of several decades.