Impact of peat depth on soil conditions, plant growth, and greenhouse gas emissions on a boreal agricultural drained peatland

Drained agricultural peatlands are significant sources of greenhouse gas (GHG) emissions, particularly carbon dioxide (CO₂) and nitrous oxide (N₂O). Peat thickness typically varies widely between sites due to cultivation history, but its effect on soil conditions, plant growth, and GHG emissions are poorly understood and seldom considered in national GHG inventories and emission mitigation strategies. Here, we present a three-year study of a drained agricultural peatland in northern Finland, with peat depth ranging from 20 to 80 cm. We measured soil properties, plant growth and GHG fluxes, to evaluate how peat depth influences ecosystem processes and emissions. Plots with deeper peat exhibited more stable and shallow water table depth (WTD), and had higher peaks in soil nitrate concentrations, but these differences did not result in significant changes in soil moisture, plant growth, or GHG fluxes. However, we observed periods when drier conditions in mull soil limited ecosystem respiration (R e ), while WTD indicated a stronger negative effect on R e and a positive effect on CH₄ fluxes in plots with deeper peat layer. Our results suggest that peat depth alone does not improve GHG inventories since shallow-peated drained peatlands can have as high GHG emissions as the thicker ones, and hydrological parameters should be prioritized instead. However, mitigation measures such as rewetting can still be targeted towards fields with thicker peat layer, as larger soil organic carbon stocks can be affected, and rewetting is likely easier due to more stable WTD and higher water holding capacity. • Three-year study on Finnish agricultural peatland explores GHG fluxes across peat depths. • Nitrate concentrations were positively correlated with peat depth. • Water table dynamics differed between shallow and deep peat soils. • Peat depth did not significantly affect soil moisture, plant growth or GHG fluxes. • Categorizing peatlands by depth may not improve accuracy of GHG inventories.