Impact of hydrological change on long-term soil organic matter persistence and collapse of structure in temperate peatlands: Evidence from paired 1942–2024 soil profiles

Temperate peatlands provide long-term storage of soil organic matter (SOM), however widespread drainage for agriculture has caused severe peat degradation in many regions. Determining whether SOM loss is driven primarily by land use, vegetation change or hydrological alteration is critical for predicting peatland trajectories and informing management.

This study uses a unique paired dataset of peat soils originally described and analysed in 1942 then resampled in 2024 in the South East of South Australia. The sites include undrained alkaline fens; spring-influenced agricultural peatlands; and drained, hydrologically disconnected peat under long-term pasture. Laboratory analyses, combined with historical subsidence measurements, enable direct assessment of changes in peat thickness, bulk density and SOM content over an 82-year period.

Results demonstrate that major SOM loss and collapse of peat structure occurred where drainage caused sustained drying of the profile. In these sites, thickness declined from more than 2 m to less than 40 cm, accompanied by strong oxidation, increased bulk density and whole-profile SOM depletion. In contrast, sites converted to agriculture but remaining hydrologically connected to karst springs preserved deep peat structure bodies and high SOM at depth, despite surface disturbance and compaction following more than eight decades of grazing.

These findings demonstrate that long-term SOM persistence in peatlands is controlled primarily by hydrological regime rather than vegetation type or land use alone. Peat typologies and vegetation shifts reflect hydrological conditions but do not determine peat vulnerability in the absence of profile-scale drying. As such hydrological thresholds impact resilient from degraded peatland SOM trajectories