Topsoil dominance in CO2 fluxes from agricultural peat soils across moisture and temperature gradients: evidence from an incubation study

Abstract

Drainage of peatlands for agriculture reverses their natural carbon sink function and leads to substantial CO2 emissions. However, the relative contributions of topsoil and subsoil layers to these emissions remain poorly quantified, limiting the accuracy of CO2 flux models in national greenhouse gas inventories. In this incubation study, we quantified CO2, N2O and CH4 fluxes from intact peat cores collected at 5–13 cm (topsoil) and 25–33 cm (subsoil) depth from arable and permanent grassland sites at a cultivated Danish bog. Fluxes were measured at five temperatures (2.5–23.4 °C) and across four soil water tensions ranging from full rewetting to pF 2.5. Topsoil consistently showed higher CO2 fluxes than subsoil, averaging a threefold difference. Temperature exerted a strong exponential control on emissions, with Q10 values ranging from 1.7 to 4.0 depending on depth, site and moisture status. Rewetting reduced CO2 fluxes from the topsoil at elevated temperatures but had little effect on the subsoil, which showed weaker temperature sensitivity and lower microbial activity. These findings align with asymptotic depth–flux relationships indicating that topsoil dominates the short-term response of CO2 fluxes to rewetting. Fluxes of N2O were negligible except in rewetted arable subsoil, where high fluxes suggested incomplete denitrification under micro-oxic and acidic conditions. Fluxes of CH4 were also negligible, indicating insufficiently anaerobic conditions for methanogenesis during the short-term incubation. Our results emphasise the importance of incorporating depth-specific CO2 dynamics in emission models and indicate that rewetting strategies must target the near-surface peat to optimise mitigation outcomes.