High CO2 fluxes from grassland on histic Gleysol along soil carbon and drainage gradients

Drained organic soils are anthropogenic emission hotspots of greenhouse gases (GHGs). Most studies have focused on deep peat soils and on peats with high organic carbon content. In contrast, histic Gleysols are characterized by shallow peat layers, which are left over from peat cutting activities or by peat mixed with mineral soil. It is unknown whether they emit less GHGs than deep Histosols when drained. We present the annual carbon and GHG balance of grasslands for six sites on nutrient-poor histic Gleysols with a shallow (30 cm) histic horizon or mixed with mineral soil in Northern Germany (soil organic carbon concentration (C-org)from 9 to 52%). The net GHG balance, corrected for carbon export by harvest, was around 4 t CO2-C-eq ha(-1) yr(-1) on soils with peat layer and little drainage (mean annual water table < 20 cm below surface). The net GHG balance reached 7- 9 t CO2-C-eq ha(-1) yr(-1) on soils with sand mixed into the peat layer and water tables between 14 cm and 39 cm below surface. GHG emissions from drained histic Gleysols (i)wereas high as those from deep Histosols, (ii) increase linearly from shallow to deeper drainage, (iii) but are not affected by C-org content of the histic horizon. Ecosystem respiration (R-eco) was linearly correlated with water table level even if it was below the histic horizon. The R-eco/GPP ratio was 1.5 at all sites, so that we ruled out a major influence of the inter-site variability in vegetation composition on annual net ecosystem exchange (NEE). The IPCC definition of organic soils includes shallow histic topsoil, unlike most national and international definitions of Histosols. Our study confirms that this broader definition is appropriate considering anthropogenic GHG emissions from drained organic soils. Countries currently apply soil maps in national GHG inventories which are likely not to include histic Gleysols. The land area with GHG emission hotspots due to drainage is likely to be much higher than anticipated. Deeply drained histic Gleysols are GHG hotspots that have so far been neglected or underestimated. Peat mixing with sand does not mitigate GHG emissions. Our study implies that rewetting organic soils, including histic Gleysols, has a much higher relevance for GHG mitigation strategies than currently recognized.