Underestimation of carbon dioxide emissions from organic-rich agricultural soils

Organic-rich soils such as peatlands store vast amounts of carbon and have formed under water-logged, low-oxygen conditions that inhibit aerobic mineralisation. Agricultural drainage and management (e.g., liming, fertilisation, tillage) increase oxygen supply and stimulate microbial decomposition, resulting in high CO₂ emissions from these soils. Many organic soils are transitioning from true peat to organo-mineral soils with lower OC content. Rewetting is promoted to reduce CO₂ emissions, though it can increase CH₄ under anaerobic conditions. National GHG inventories under UNFCCC/IPCC guidelines typically assign emission factors only to soils with >12% OC; soils with 6–12% OC are often excluded or treated like mineral soils. Denmark uniquely assigns tentative lower emission factors to 6–12% OC soils (assumed half of >12% OC), but evidence from limited field studies suggests soils with ~5–10% OC can emit CO₂ at rates comparable to >12% OC soils. This inconsistency, coupled with variable national definitions of “organic” soils, creates uncertainty and potential underestimation in LULUCF reporting. To address this gap, the study tests whether area-scaled CO₂ emissions are controlled by OC content or OC density across a wide range (6.2–52.1% OC) using controlled aerobic incubations of undisturbed topsoil cores from across Denmark.

Prior syntheses show drained peatlands are major CO₂ sources and that rewetting generally yields net climate benefits despite possible CH₄ increases. IPCC guidelines focus on >12% OC as “organic soils,” while Denmark defines >6% OC as organic; soils with 6–12% OC can be excluded from reporting or treated like mineral soils under IPCC. Limited field studies in Germany and elsewhere indicate that soils with ~5–10% OC can have CO₂ emissions similar to soils with >12% OC, challenging the practice of applying substantially lower emission factors to the 6–12% OC class. Studies on disturbed samples often report weight- or OC-specific respiration increases with OC content or changes with OC quality; however, these metrics can mislead for area-based emission factors due to bulk density effects. Water table depth frequently emerges as the dominant field control on CO₂ fluxes.

Study area and sampling: Denmark (temperate to cold climate; mean annual precipitation 759 mm). Organic topsoils (>6% OC) were sampled at 103 sites across all regions, with georeferenced positions, peat depth, groundwater depth, land use, and decomposition degree recorded. From each site, two undisturbed cores (100 cm³ stainless rings; 3.5 cm height, 6.1 cm diameter) were collected from 10–15 cm depth (total n = 206 cores) and stored at 2 °C for incubation. Bulk soils were air-dried and sieved (2 mm) for total C and N by dry combustion; soils lacked carbonates by HCl test, so total C = OC. Soil pH measured in 1:5 soil:water. Incubations: Cores were trimmed to 100 cm³ and incubated either at a standardised matric potential of ψ = −100 hPa (pF 2) via controlled wet/dry equilibration on sandboxes (2 weeks) or at their in situ water content. Each core was placed in an airtight 1-L glass jar (effective headspace 0.9 L), bottom sealed, and incubated in the dark at 15 °C for 60 days. Headspace gas sampling (10 mL) was performed every 5–7 days and analyzed for CO₂ (and CH₄) on an Agilent 7890 GC, with jars opened for 10 minutes post-sampling to reset baselines. Four empty jars served as controls. Oxygen checks after 7 days indicated 19.4 ± 0.4% O₂. Relative water loss over 60 days was ~4.7% and not corrected to avoid preferential rewetting. Calculations: Interval CO₂ emissions (µg CO₂-C) were computed as (Ct − C0) × Vh × M × Vm⁻¹ with Ct/C0 in µL L⁻¹, Vh = 0.9 L, M = 12 g mol⁻¹, Vm = 23.6 L mol⁻¹ at 15 °C. Cumulative ΣCO₂ over 60 days was summed across intervals. Emission rates were then derived as: (i) area-scaled mg CO₂-C h⁻¹ m⁻² = ΣCO₂ / time / A, with time = 1440 h and A = 29.2 cm²; (ii) soil weight-specific mg CO₂-C h⁻¹ kg⁻¹ soil = ΣCO₂ / time / soil dry weight; (iii) OC-specific mg CO₂-C h⁻¹ kg⁻¹ OC = ΣCO₂ / time / OC mass per 100 cm³ core. Bulk density was determined from dry weights at the end. Statistics: Differences among OC content classes were assessed by one-way ANOVA (means) with diagnostics for normality (Shapiro–Wilk) and homoscedasticity (Levene) and transformations (log/sqrt) as needed; post hoc pairwise comparisons used emmeans with Tukey-adjusted P values. Medians were compared via Kruskal–Wallis tests (χ²-based P values). Ordinary least squares regressions (with transformations as needed) related emissions to explanatory variables (OC content, OC density, volumetric water content), reporting r² and P. Correlations among soil parameters employed Spearman rank correlation.

- Area-scaled CO₂ emissions did not differ significantly among OC content classes (6% intervals) at pF 2: medians P = 0.14 (χ² = 8.36, df = 5); means P = 0.17 (F = 1.60, df = 5). - No significant difference between soils with 6–12% OC and >12% OC: medians P = 0.21 (χ² = 1.55, df = 1); means P = 0.31 (F = 1.02, df = 1). Mean emissions for 6–12% OC were only 8.9% lower than >12% OC, with strongly overlapping 95% CIs (9.4–14.1 vs 11.8–14.3 mg CO₂-C h⁻¹ m⁻²). - Linear regressions showed weak, non-significant associations for area-scaled emissions with OC content (r² = 0.008, P = 0.38) and with OC density (r² = 0.014, P = 0.228). - Soil weight-specific CO₂ emissions increased with OC content (r² = 0.271, P < 0.001), reflecting the inverse relationship between OC content and bulk density; OC-specific emissions were negatively related to OC content (r² = 0.091, P = 0.002) and strongly negatively related to OC density (r² = 0.459, P < 0.001). - Volumetric water content was the strongest single predictor of area-scaled CO₂ emissions across both in situ moisture and pF 2-standardised conditions (r² = 0.34, F(1,101) = 51.41, P < 0.001) in the observed VWC range (27–87%). No CH₄ emissions were observed during incubations. - Findings challenge inventory practices that assign lower emission factors to 6–12% OC soils: Danish national CO₂ emissions from organic croplands and grasslands may be underestimated by up to 40% if 6–12% OC soils have similar emission factors as >12% OC soils. - Broader implication: Countries with many transitioning organic soils (from peat to organo-mineral) likely also underestimate area-based CO₂ emissions for 6–12% OC soils; LUCAS data suggest 6–12% OC soils (n = 477) may be more common than >12% OC (n = 196) across EU sites.

The study directly addresses whether residual OC content or OC density governs area-scaled CO₂ emissions from organic topsoils under aerobic conditions. Results demonstrate that neither OC content nor OC density predicts area-based CO₂ emission rates, and soils with 6–12% OC emit at rates statistically indistinguishable from soils with >12% OC. This undermines the rationale for assigning substantially lower emission factors to 6–12% OC soils in inventories. Instead, soil water availability emerged as a key driver within the examined moisture range, consistent with field evidence that water table and aeration control CO₂ efflux. The divergence between area-scaled and weight- or OC-specific rates is explained by bulk density differences; thus, weight-specific metrics, while useful for mechanistic insights, are not appropriate for deriving area-based emission factors. The findings imply that accurate national GHG accounting and prioritisation of rewetting interventions must reconsider 6–12% OC soils, which are widespread and currently may be mischaracterised. The study also suggests that other rate-limiting properties (geochemical, microbiome, and physical site conditions) likely modulate emissions beyond simple OC quantity or density proxies.

Area-scaled CO₂ emissions from organic agricultural topsoils with >6% OC are not controlled by OC content or OC density, and soils with 6–12% OC exhibit emissions comparable to >12% OC soils under aerobic incubation. Consequently, using reduced emission factors for 6–12% OC soils likely underestimates national CO₂ emissions (by up to ~40% in Denmark). Water availability is a more influential predictor of emissions than OC measures in these drained organic soils. The study provides a robust benchmark from controlled conditions, highlighting the need to refine emission factors and to improve mapping and accounting of 6–12% OC soils. Future work should prioritise targeted field studies across moisture regimes, integrate water table dynamics, and examine geochemical and microbiome controls to translate laboratory benchmarks into inventory-ready field emission factors.

- Laboratory incubations under controlled aerobic conditions (15 °C, pF 2 or in situ moisture) may not capture full field variability (e.g., seasonal temperature and moisture fluctuations, water table dynamics, plant influences). - Only topsoil layers (10–15 cm depth) were examined; subsoil processes and profile integration were not addressed. - Danish sites dominate the sampling; extrapolation to other regions and climates requires caution. - Absence of CH₄ emissions in the incubations reflects aerobic conditions and does not represent potential anaerobic field states. - Adjusting official UNFCCC emission factors requires broader, multi-country evidence and targeted field validation beyond the scope of this study.