Contrasting stages of agricultural abandonment reflected in carbon pools, greenhouse gas fluxes, and soil biogeochemistry at two drained peatlands

Peatland drainage has led to significant soil carbon (C) losses, contributing to global greenhouse gas emissions. While rewetting is a key climate mitigation strategy, many abandoned, previously drained peatlands undergo secondary succession by shrubs and trees, forming mature woodland on peat soil (wet woodlands). The climatic and biogeochemical implications of this encroachment remain poorly understood, particularly on drained, nutrient-enriched peat soils. This study investigated C stocks and turnover processes in a temperate peatland using a space-for-time comparison of two sites representing contrasting stages of abandonment: a recently abandoned grassland and a site abandoned 75 years ago, now encroached by dense willow shrubland. We assessed above- and belowground biomass and soil C stocks, in situ CO 2 and CH 4 fluxes, soil temperature, water table depth (WTD), and soil and groundwater geochemistry. Laboratory soil incubations evaluated site-specific responses of soil C flux to moisture and temperature. The willow shrub site had substantially greater biomass C storage (approximately sevenfold greater) and higher soil C stocks than the grassland (56.2 ± 4.92 vs. 36.5 ± 1.70 kg C m⁻²), as well as a deeper peat layer. While both sites remained deeply drained on an annual basis (annual mean WTD >30 cm), the willow shrub exhibited lower soil respiration under variable soil moisture content than the grassland did in the laboratory incubations. This indicated lower moisture sensitivity of soil C fluxes at the willow shrub site. Lower porewater N and higher Fe and P concentrations at the willow shrub indicated contrasting nutrient conditions between the sites. Additionally, an explorative estimate of ecosystem respiration (R eco ) suggested lower R eco at the willow shrub than at the grassland (2.83 ± 0.72 vs 3.50 ± 0.40 kg C m −2 yr −1 ). These findings show contrasting C storage and flux responses between the two studied sites. However, where abandoned soils stay deeply drained, such as the willow shrub where the water table approached terrain only four times during the measurement year, more intensive rewetting efforts remain essential to restore peatland functioning as a C sink.