How do sand addition, soil moisture and nutrient status influence greenhouse gas fluxes from drained organic soils?

Drainage turns peatlands from natural carbon sinks into hotspots of greenhouse gas (GHG) emissions from soils due to alterations in hydrological and biogeochemical processes. As a consequence of drainage-induced mineralisation and anthropogenic sand addition, large areas of former peatlands under agricultural use have soil organic carbon (SOC) contents at the boundary between mineral and organic soils. Previous research has shown that the variability of GHG emissions increases with anthropogenic disturbance. However, how and whether sand addition affects GHG emissions remains a controversial issue. The aim of this long-term incubation experiment was to assess the influence of hydrological and biogeochemical soil properties on emissions of carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄). Strongly degraded peat with sand addition (peat-sand mixtures) and without sand addition (earthified peat) was systematically compared under different moisture conditions for fen and bog peat. Soil columns originating from both the topsoil and the subsoil of ten different peatlands under grassland use were investigated. Over a period of six months the almost saturated soil columns were drained stepwise via suction to −300 hPa. The CO₂ fluxes were lowest at water-saturated and dry soil moisture conditions, resulting in a parabolic dependence of CO₂ fluxes on the water-filled pore space (WFPS) peaking at 56–92% WFPS. The highest N₂O fluxes were found at between 73 and 95% WFPS. Maximum CO₂ fluxes were highest from topsoils, ranging from 21 to 77 mg C m⁻² h⁻¹, while the maximum CO₂ fluxes from subsoils ranged from 3 to 14 mg C m⁻² h⁻¹. No systematic influence of peat type or sand addition on GHG emissions was found in topsoils, but CO₂ fluxes from subsoils below peat-sand mixtures were higher than from subsoils below earthified peat. Maximum N₂O fluxes were highly variable between sites and ranged from 18.5 to 234.9 and from 0.2 to 22.9 μg N m⁻² h⁻¹ for topsoils and subsoils, respectively. CH₄ fluxes were negligible even under water-saturated conditions. The highest GHG emissions occurred at a WFPS that relates – under equilibrium conditions – to a water table of 20–60 cm below the surface in the field. High maximum CO₂ and N₂O fluxes were linked to high densities of plant-available phosphorus and potassium. The results of this study highlight that nutrient status plays a more important role in GHG emissions than peat type or sand addition, and do not support the idea of peat-sand mixtures as a mitigation option for GHG emissions.