Annual net CO₂ fluxes from drained organic soils used for agriculture in the hemiboreal region of Europe

Abstract. 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.