Organic soils can be CO₂ sinks in both drained and undrained hemiboreal peatland forests

<p>Drainage of organic soils is associated with increasing soil carbon dioxide (<span class="inline-formula">CO₂</span>) efflux, which is typically linked to losses in soil carbon (C) stock. In previous studies, drained organic forest soils have been reported as both <span class="inline-formula">CO₂</span> sinks and <span class="inline-formula">CO₂</span> sources depending on, e.g., soil nutrient and moisture regime. However, most of the earlier research was done in the boreal zone, and both the magnitude of <span class="inline-formula">CO₂</span> efflux and the impact of soil moisture regime on soil C stock are likely to vary across different climatic conditions and ecosystems, depending further on vegetation. A 2-year study was conducted in hemiboreal forest stands with nutrient-rich organic soil (including current and former peatlands) and a range of dominant tree species (black alder, birch, Norway spruce, Scots pine) in the Baltic states (Estonia (EE), Latvia (LV), Lithuania (LT)). In this study, we analyzed the <span class="inline-formula">CO₂</span> balance of organic soil in drained (19) and undrained (7) sites. To assess the <span class="inline-formula">CO₂</span> balance, soil respiration was measured along with the evaluation of <span class="inline-formula">CO₂</span> influx into the soil through aboveground (aGV) and belowground (bGV) litter. To characterize the sites and factors influencing the <span class="inline-formula">CO₂</span> fluxes, we analyzed soil temperature, soil water table level (WTL), and physical and chemical parameters of soil and soil water. Irrespective of drainage status, the soils functioned as both <span class="inline-formula">CO₂</span> sinks and <span class="inline-formula">CO₂</span> sources. On average, a close-to-neutral soil <span class="inline-formula">CO₂</span> balance (<span class="inline-formula">+0.45</span> <span class="inline-formula">±</span> <span class="inline-formula">0.50</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">t</mi><mspace width="0.125em" linebreak="nobreak"/><msub><mi mathvariant="normal">CO</mi><mn mathvariant="normal">2</mn></msub><mtext>-</mtext><mi mathvariant="normal">C</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">ha</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">yr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="84pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="dd568f418b995b2f996a388906c3c2dc"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-4627-2025-ie00001.svg" width="84pt" height="16pt" src="bg-22-4627-2025-ie00001.png"/></svg:svg></span></span>) was observed in drained sites dominated by black alder, birch, or Norway spruce, while drained Scots pine sites showed soil <span class="inline-formula">CO₂</span> removals with a mean rate of <span class="inline-formula">+2.77</span> <span class="inline-formula">±</span> <span class="inline-formula">0.36</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M22" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">t</mi><mspace width="0.125em" linebreak="nobreak"/><msub><mi mathvariant="normal">CO</mi><mn mathvariant="normal">2</mn></msub><mtext>-</mtext><mi mathvariant="normal">C</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">ha</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">yr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="84pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="540989c6a85c99644841b532af9e168b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-4627-2025-ie00002.svg" width="84pt" height="16pt" src="bg-22-4627-2025-ie00002.png"/></svg:svg></span></span>. In undrained birch- and spruce-dominated sites, soil functioned as a mean <span class="inline-formula">CO₂</span> sink at <span class="inline-formula">+1.33</span> <span class="inline-formula">±</span> <span class="inline-formula">0.72</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M27" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">t</mi><mspace linebreak="nobreak" width="0.125em"/><msub><mi mathvariant="normal">CO</mi><mn mathvariant="normal">2</mn></msub><mtext>-</mtext><mi mathvariant="normal">C</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">ha</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">yr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="84pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="ccbc9935c7d7ad51defec09dd47ba659"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-4627-2025-ie00003.svg" width="84pt" height="16pt" src="bg-22-4627-2025-ie00003.png"/></svg:svg></span></span>, while the undrained black alder stands showed an uncertain <span class="inline-formula">CO₂</span> balance of <span class="inline-formula">+1.12</span> <span class="inline-formula">±</span> <span class="inline-formula">2.47</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M32" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">t</mi><mspace linebreak="nobreak" width="0.125em"/><msub><mi mathvariant="normal">CO</mi><mn mathvariant="normal">2</mn></msub><mtext>-</mtext><mi mathvariant="normal">C</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">ha</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">yr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="84pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="90b59eeeb063cf10c63ed34051b5138b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-22-4627-2025-ie00004.svg" width="84pt" height="16pt" src="bg-22-4627-2025-ie00004.png"/></svg:svg></span></span>. Variation in the soil <span class="inline-formula">CO₂</span> balance was related to soil macronutrient concentrations and pH: forest types characterized by lower nutrient availability showed greater soil <span class="inline-formula">CO₂</span> sink. The reported soil <span class="inline-formula">CO₂</span> balance values may be used as regional emission factors (EFs).</p>