Histosols cover about 8–10% of Lithuania’s territory and most of this area is covered with nutrient-rich organic soils (Terric Histosols). Greenhouse gas (GHG) emissions from drained Histosols contribute more than 25% of emissions from the Land Use, Land Use Change and Forestry (LULUCF) sector. In this study, as the first step of examining the carbon dioxide (CO₂) fluxes in these soils, total soil CO₂ efflux and several environmental parameters (temperature of air and topsoil, soil chemical composition, soil moisture, and water table level) were measured in drained Terric Histosols under three native forest stands and perennial grasslands in the growing seasons of 2020 and 2021. The drained nutrient-rich organic soils differed in terms of concentrations of soil organic carbon and total nitrogen, as well as soil organic carbon and total nitrogen ratio. The highest rate of total soil CO₂ efflux was found in the summer months. Overall, the rate was statistically significant and strongly correlated only with soil and air temperature. A trend emerged that total soil CO₂ efflux was 30% higher in perennial grassland than in forested land. Additional work is still needed to estimate the net CO₂ balance of these soils.

Carbon dioxide (CO₂) emissions from drained organic soils used for agriculture contribute significantly to the overall anthropogenic greenhouse gas budget in the land use, land use change, and forestry (LULUCF) sector. To justify the implementation of climate change mitigation measures on these lands, it is important to estimate at least the regional variation in annual net CO₂ fluxes. This study presents the first estimates of annual net CO₂ fluxes from drained nutrient-rich organic soils in croplands (8 sites) and grasslands (12 sites) in the hemiboreal region of Europe, represented by Estonia, Latvia, and Lithuania. The study sites represented both deep and shallow highly decomposed, organic soils, categorised based on the concentration of organic carbon in the top 20 cm soil layer. CO₂ flux measurements were conducted over at least 2 years at each site. To estimate annual net CO₂ fluxes, ecosystem respiration (R_eco) and soil heterotrophic respiration (R_het) were measured using a manual dark-chamber technique, and carbon (C) input into soil through plant residues was estimated. R_eco was strongly dependent on temperature, particularly soil temperature at 10 cm depth, but was rather independent of the soil water table level and soil moisture. The overall mean annual net soil CO₂ fluxes, calculated as the difference between annual output (R_het) and input (plant residues), were 4.8 ± 0.8 $\mathrm{t}{\mathrm{CO}}_{\mathrm{2}}\text{-}\mathrm{C}{\mathrm{ha}}^{-\mathrm{1}}{\mathrm{yr}}^{-\mathrm{1}}$ for croplands and 3.8 ± 0.7 $\mathrm{t}{\mathrm{CO}}_{\mathrm{2}}\text{-}\mathrm{C}{\mathrm{ha}}^{-\mathrm{1}}{\mathrm{yr}}^{-\mathrm{1}}$ for grasslands, while the means for “true” or deep organic soil were 4.1 ± 0.7 $\mathrm{t}{\mathrm{CO}}_{\mathrm{2}}\text{-}\mathrm{C}{\mathrm{ha}}^{-\mathrm{1}}{\mathrm{yr}}^{-\mathrm{1}}$ in croplands and 3.2 ± 0.6 $\mathrm{t}{\mathrm{CO}}_{\mathrm{2}}\text{-}\mathrm{C}{\mathrm{ha}}^{-\mathrm{1}}{\mathrm{yr}}^{-\mathrm{1}}$ in grasslands (mean ± standard error). Both the annual R_eco and net CO₂ fluxes for shallow highly decomposed organic soils, currently not recognised as organic soil by the Intergovernmental Panel on Climate Change (IPCC), were of similar magnitude to or even higher than those from deep organic soil, suggesting a need to separate them from mineral soils in emission estimation.

Drainage of organic soil is associated with increasing soil carbon (C) efflux, which is typically linked to losses in C stock. In previous studies, soil in drained peatland forests has been reported as both a C sink and source depending on, e.g., soil nutrient and moisture regimes. However, most of the earlier research was done in boreal sites, and the impact of soil moisture regime on soil C stock is likely to vary across different climatic conditions and ecosystems, depending further on vegetation. In this study, we examined the soil and forest floor (including ground vegetation) C balance in drained and undrained hemiboreal forests to evaluate drainage impact on C balance. A two-year study was conducted in 26 drained and undrained forest stands with nutrient-rich organic soil in the Baltic states (Estonia, Latvia, Lithuania). To assess the C balance, measurements of soil heterotrophic and total respiration were carried out, along with the evaluation of C influx into the soil through litter, including fine foliar litterfall, herbaceous ground vegetation, and fine roots of trees. The CO₂ emissions did not significantly differ between the study countries; therefore, one emission factor can be applied to characterize soil emissions in the Baltic States. It was observed that C influx into the soil through litter can compensate for the C losses caused by heterotrophic soil respiration, and neither drained nor undrained soils were proven to be losing their C stock. Comparing the C balances in drained and undrained sites, it was found that drainage of organic soils reduces their C sequestration by 0.43±2.69 t C ha⁻¹ year⁻¹.

Conversion of tropical peatland forests to oil palm plantations has been associated with elevated nitrous oxide (N₂O) emissions and nitrate (NO₃^-) leaching, yet the biogeochemical mechanisms driving these nitrogen (N) losses remain poorly understood. To address this gap, soil N₂O fluxes, gross N transformation rates, microbial gene abundances, annual NO₃^- leaching rates, and bulk soil δ¹⁵N signatures were quantified along transects in Malaysia that included intact tropical peat swamp forest and drained oil palm plantation. Oil palm plantation sites had higher topsoil oxygen (O₂) concentrations than forest sites, indicating that drainage shifts soils from predominantly anaerobic to aerobic conditions. These redox shifts, together with fertilizer inputs, increased nitrifier gene abundances and enhanced gross nitrification rates, which exceeded ammonification rates by up to 186%, promoting soil NO₃^- accumulation. The elevated soil NO₃^- concentrations stimulated N₂O production, probably through coupled nitrification-denitrification, and increased NO₃^- leaching. Overall, these results showed that oil palm plantation sites had higher gaseous and hydrological N losses than peat forest sites. Additionally, nitrification rates, hydrological N export, and gaseous N losses were positively correlated with more enriched bulk soil δ¹⁵N values along the land use gradient. These findings demonstrated that bulk soil δ¹⁵N patterns may have the potential to serve as indicators of long-term N cycling and losses in tropical peatlands associated with human-induced land use changes.

Carbon dioxide (CO₂) emissions from drained organic soils used for agriculture contribute significantly to the overall anthropogenic greenhouse gas budget in land use, land-use change and forestry (LULUCF) sector. To justify the implementation of climate change mitigation measures on these lands, it is important to estimate at least the regional variation in annual net CO₂ fluxes. This study presents the first estimates of annual net CO₂ fluxes from drained nutrient-rich organic soils in cropland (8 sites) and grassland (12 sites) in the hemiboreal region of Europe, represented by Estonia, Latvia and Lithuania. The study sites represented both deep, and shallow highly decomposed, organic soils, categorized based on the concentration of organic carbon in the top 20-cm soil layer. In each site, CO₂ flux measurements were conducted at least over two years. To estimate annual net CO₂ fluxes, ecosystem respiration (R_eco) and soil heterotrophic respiration (R_het) were measured using a manual chamber technique, and carbon (C) input to soil through plant residues was estimated. R_eco was strongly dependent on temperature, particularly soil temperature at 10 cm depth, but rather independent of soil water-table level and soil moisture. The overall mean annual net CO₂ fluxes, calculated as the difference between annual CO₂ output (R_het) and annual C input (plant residues), was 4.8 ± 0.8 t CO₂-C ha^–1 yr^–1 in cropland and 3.8 ± 0.7 t CO₂-C ha^–1 yr^–1 in grassland, while the means for “true” or deep organic soil were 4.1 ± 0.7 t CO₂-C ha^–1 yr^–1 in cropland and 3.2 ± 0.6 t CO₂-C ha^–1 yr^–1 in grassland. Both the annual R_eco and net CO₂ fluxes for shallow highly decomposed organic soils, currently not recognized as organic soil by the Intergovernmental Panel on Climate Change (IPCC), were of similar magnitude or even higher than those from deep organic soil, suggesting a need to separate them from mineral soils in emission estimation.