Greenhouse gas fluxes from nutrient-rich organic soils in Estonia

<p>Nutrient-rich organic soils are one of the largest key sources of greenhouse gas (GHG) emissions in cool moist climate regions in Europe, and around 15 Mha of wetlands are drained for forestry across the world's temperate and boreal areas. Drainage promotes the decomposition of the organic material stored in these naturally water-saturated organic soils, turning the wetland from a carbon sink into an emitter of CO<span xml:lang="EN-US"><span><sub>2</sub></span></span>. Lower soil water content in drained histosols leads to reduced CH<sub>4</sub> emission, while N<span xml:lang="EN-US"><span><sub>2</sub></span></span>O emission can increase due to increased mineralization and more favorable conditions for nitrification. However, detailed information of GHG emissions from drained organic soils under different land use and management in the hemiboreal zone is still scarce.  </p><p xml:lang="EN-US"><span xml:lang="EN-US">We conducted a full-year study at drained peatland sites with different land uses to assess the impact of drainage and land-use on GHG fluxes in Estonia. We investigated ten sites: (I) five forests with different tree species, (II) three grasslands with different water regimes, (III) cropland and (IV) natural wetland (fen). The GHG fluxes were measured twice per month using the manual static (CH</span><sub>4</sub> and N<sub>2</sub>O) and dynamic (heterotrophic respiration (CO<span xml:lang="EN-US"><span><sub>2</sub></span></span>)) closed chamber method from Jan 2020 to Dec 2021. Additionally, groundwater level, soil temperature and moisture were measured hourly with automatic loggers to determine soil conditions.   </p><p xml:lang="EN-US"><span xml:lang="EN-US">Our preliminary results show that all drained forest soils were annual CH<sub>4</sub> sinks (−59.4 ± 2.5 µg m</span><span xml:lang="EN-US"><span><sup>-2</sup></span></span> h<sup>-1</sup>, mean ± SE). However, CH<span xml:lang="EN-US"><span><sub>4</sub></span></span> uptake from the studied fen, crop and grasslands were lower, –13.2 ± 4.4, -12.2 ± 2.0 and -8.2 ± 3.3 µg m<span xml:lang="EN-US"><span><sup>-2</sup></span></span> h<sup>-1</sup>, respectively, while grassland with poor drainage soil was a less source of CH<sub>4 </sub><span xml:lang="EN-US">emission. Most of the sites were annual emitters of N</span><sub>2</sub>O; forest sites were higher emitters (15.9 ± 2.3 µg m<sup>-2</sup> h<sup>-1</sup>) than cropland (12.7 ± 4.1 µg m<span xml:lang="EN-US"><span><sup>-2</sup></span></span> h<sup>-1</sup>) and fen soils (6.3 ± 1.1 µg m<span xml:lang="EN-US"><span><sup>-2</sup></span></span> h<sup>-1</sup>). N<sub>2</sub>O fluxes from grasslands depend on drainage intensity and the site with poor drainage emitted less. Higher N<span xml:lang="EN-US"><span><sub>2</sub></span></span>O emissions and temporal variability were associated with sites where the water level had high seasonal fluctuations. Soil CO<sub>2</sub> fluxes (heterotrophic respiration) were highest from grasslands and peaked over all the study sites during the summer. Methane flux had a statistically significant correlation with water level and soil moisture, while N<sub>2</sub>O flux was controlled by soil temperature, having higher emissions in a warmer season. <span xml:lang="EN-US">The results provide insights into GHG fluxes over temporal and spatial scales and indicate the need for mitigation measures and further enhancement of modeling tools for climate-friendly land management practices in nutrient-rich organic soils.  </span> </p><p xml:lang="EN-US"><span xml:lang="EN-US">This research was supported by the LIFE programme project “Demonstration of climate change mitigation potential of nutrients rich organic soils in Baltic States and Finland”, (2019-2023, LIFE OrgBalt</span>, LIFE18 274CCM/LV/001158)<span> </span></p>