Progress and knowledge gaps in estimating CO2 emissions from organic-rich agricultural soils

Rewetting of organic-rich agricultural soils is a management strategy to reduce CO₂ emissions and meet policy targets for agricultural GHG emissions. Denmark has committed to support the rewetting of at least 100,000 ha of agricultural peat soils by 2030. The climate benefits of this initiative depend on emissions before and after rewetting, particularly CO₂ emissions, which have been assessed using national emission factors. However, recent studies suggest that modelling CO₂ emissions as a function of mean annual water table depth (WTD) provides a more accurate assessment than fixed emission factors. Despite these advances, large uncertainties and knowledge gaps remain in the estimation of GHG emissions from agricultural peat soils, particularly in relation to the effect of land use and environmental factors such as peat depth, organic carbon (OC) content, water table depth and soil moisture. 

Most controlled studies of GHG emissions from drained peat soils have focused on the topsoil where microbial activity is higher than in the subsoil. However, drained subsoils contribute to overall GHG fluxes, but the controlling factors need to be clarified. Some studies suggest that CO₂ emissions increase proportionally to WTD, i.e., with equal importance of drained topsoil and subsoil compartments, while other data suggest that deeper  WTD have progressively less effect on CO₂ fluxes. In addition, topsoil and subsoil may differ in their sensitivity to environmental changes, such as temperature and moisture fluctuations, influenced by climate change. Therefore, understanding how topsoil and subsoil differ in their contribution to GHG fluxes is critical for developing predictive models and supporting climate-smart peatland management strategies.

Drained OC-rich soils are hotspots for CO₂ emissions, but the continued microbial mineralization means that the OC content transitions to that of mineral soils. Yet, it remains unclear how the residual OC content controls the rate of CO₂ emissions. The Danish definition of organic soils includes soils with >6% OC, while other countries use thresholds of >12% OC or higher. Hence, soils with 6-12% OC are part of the Danish GHG inventory, but there is a lack of data on the CO₂ emissions from these soils. Recent studies suggest that soils with 6-12% OC can emit CO₂ at similar rates as soils with >12% OC, and that emissions from soils with 6-12% OC may be underestimated. The refinement of CO₂ emission estimates for 6-12% OC soils is critical for the accuracy of national inventories, but also for crediting the climate benefits of initiatives to rewet drained organic soils.

In brief, this poster invites discussion on the progress and knowledge gaps in estimating CO₂ emissions from agricultural peat soils, e.g., in relation to the role of subsoils and the OC content.