Organic-rich soils, particularly drained peatlands, represent a substantial terrestrial carbon pool. Their conversion to agricultural land through drainage significantly increases CO₂ emissions due to enhanced aerobic microbial mineralization. Globally, drained peat soils contribute substantially to greenhouse gas emissions. Rewetting drained organic soils is a proposed mitigation strategy, but accurate assessment of its climate benefits requires precise CO₂ emission estimates. Current national greenhouse gas inventories, such as Denmark's submission to the UNFCCC, often employ emission factors based on OC content, with soils containing 6-12% OC receiving lower or no emission factors compared to those with >12% OC. This classification may lead to significant underestimation of emissions. This study aims to investigate the relationship between OC content and area-scaled CO₂ emissions to improve the accuracy of these estimates and support better assessment of rewetting initiatives.

Existing literature highlights the importance of drained peatlands as significant CO₂ emission sources and the role of drainage in accelerating organic matter decomposition. Studies have shown that rewetting can reduce CO₂ emissions but may increase methane emissions. However, there is a global lack of data on CO₂ emissions from soils with 6-12% OC, leading to inconsistencies in national GHG reporting. Different countries and organizations use varying thresholds for defining organic-rich soils, further complicating the issue. The IPCC guidelines allow for the exclusion of soils with 6-12% OC from GHG reporting, treating them similarly to mineral soils. However, some field studies suggest that soils with significantly lower OC content can still emit CO₂ at rates comparable to those with higher OC content, indicating the potential for underestimation in current inventories.

This study used a comprehensive laboratory incubation experiment with 103 undisturbed soil cores collected from various locations across Denmark, representing a wide range of OC contents (6.2–52.1%). Soil cores were incubated under controlled aerobic conditions at a standardized water potential (pF 2) and also at in-situ water contents. CO₂ emissions were measured over a 60-day period. Area-scaled, soil weight-specific, and OC-specific CO₂ emission rates were calculated. Statistical analyses, including ANOVA, Kruskal-Wallis tests, and linear regressions, were performed to assess the relationships between CO₂ emissions and OC content, OC density, and soil volumetric water content (VWC).

The key finding is that area-scaled CO₂ emissions were not significantly different among various OC content classes (6% intervals), including soils with 6-12% OC and >12% OC. Linear regression analysis confirmed a weak and non-significant association between OC content and area-scaled CO₂ emission rates. Similarly, OC density showed a weak and non-significant relationship with area-scaled emissions. In contrast, soil weight-specific CO₂ emission rates increased systematically with increasing OC content, but this was attributed to the negative correlation between OC content and soil bulk density. OC-specific CO₂ emission rates showed a negative correlation with OC content and density, suggesting that OC in more degraded soils is more susceptible to mineralization. Soil volumetric water content (VWC) was the most significant predictor of area-scaled CO₂ emissions, with higher VWC correlating with increased emissions. The study found no significant methane emissions during the incubations.

The finding that area-scaled CO₂ emissions are not strongly correlated with OC content, particularly within the 6-12% range, challenges current methodologies used in national GHG inventories. The observed relationship between weight-specific emission rates and OC content is primarily driven by the influence of soil bulk density, making this metric less suitable for estimating area-scaled emissions. The negative correlation between OC-specific emissions and OC content suggests a potential positive feedback mechanism, where more degraded soils exhibit increased mineralization rates. However, the role of OC quality needs further investigation. The significant influence of soil water content highlights the importance of considering water availability when assessing CO₂ emissions. This study's controlled conditions provide valuable insights but highlight the need for field research to refine emission factors for soils with 6-12% OC.

This study demonstrates that using OC content as the primary factor for determining CO₂ emission factors in organic-rich agricultural soils can lead to significant underestimation of emissions, particularly for soils in the 6-12% OC range. The results suggest that current Danish national greenhouse gas inventories may underestimate emissions by approximately 40%. The strong influence of soil water content on CO₂ emissions warrants further consideration. Future research should focus on refining emission factors for the 6-12% OC range through targeted field studies that also consider OC quality and other relevant soil parameters. Improved soil mapping and more comprehensive data are needed to accurately account for GHG emissions from organic soils globally.

This study was conducted under controlled laboratory conditions, which may not fully capture the complexity of field conditions. The relatively short incubation period (60 days) may not reflect long-term emission dynamics. The study focused primarily on topsoils and did not consider deeper soil layers. While soil water content was controlled in part of the experiment, the findings may be limited in their applicability to soils with water content outside of the experimental range.