Topsoil removal for Sphagnum establishment on rewetted agricultural bogs

Rewetting drained agricultural peatlands aids in restoring their original ecosystem functions, including carbon storage and sustaining unique biodiversity. 30–60 cm of topsoil removal (TSR) before rewetting for Sphagnum establishment is a common practice to reduce nutrient concentrations and greenhouse gas emissions, and increase water conductivity. However, the topsoil is carbon-dense and preservation in situ would be favorable from a climate-mitigation perspective. The effect of reduced TSR on Sphagnum establishment and nutrient dynamics on degraded and rewetted raised bogs remains to be elucidated. We conducted a two-year field experiment under Sphagnum paludiculture management with three TSR depths: no-removal (TSR0), 5–10 cm (TSR5), and 30 cm (TSR30) removal. We tested the effects of TSR on Sphagnum establishment and performance, nutrient dynamics, and hotspot methane emissions. After two years, TSR5 produced similar Sphagnum biomass as TSR30, while vascular plant biomass was highest in TSR0. All capitula nitrogen (N > 12 mg/g) indicated N-saturation. Phosphorus (P) was not limiting (N/P < 30), but a potential potassium (K) limitation was observed in year one (N/K > 3). In TSR0, ammonium concentrations were > 150 µmol/l in year one, but decreased by 80% in year two. P-concentrations remained high (c. 100 µmol/l) at TSR0 and TSR5, and remained low at TSR30. TSR30 and TSR5 reduced hotspot methane emissions relative to TSR0. We conclude that all TSR practices have their own advantages and disadvantages with respect to Sphagnum growth, nutrient availability and vegetation development. While TSR5 may be the most suitable for paludiculture, its applicability for restoration purposes remains to be elucidated. Setting prioritized targets when selecting the optimal TSR with peatland rewetting is pivotal.