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

[摘要] Drained organic soils are anthropogenic emission hotspots of greenhouse gases(GHGs). Most studies have focused on deep peat soils and on peats with highorganic carbon content. In contrast, histic Gleysols are characterized byshallow peat layers, which are left over from peat cutting activities or bypeat mixed with mineral soil. It is unknown whether they emit less GHGs thandeep Histosols when drained. We present the annual carbon and GHG balance ofgrasslands for six sites on nutrient-poor histic Gleysols with a shallow(30 cm) histic horizon or mixed with mineral soil in Northern Germany (soilorganic carbon concentration (C_org) from 9 to 52%).

The net GHG balance, corrected for carbon export by harvest, was around 4 tCO₂–C–eq ha⁻¹ yr⁻¹ on soils with peat layer and littledrainage (mean annual water table < 20 cm below surface). The netGHG balance reached 7–9 t CO₂–C–eq ha⁻¹ yr⁻¹ on soilswith sand mixed into the peat layer and water tables between 14 cm and39 cm below surface. GHG emissions from drained histic Gleysols (i) were ashigh as those from deep Histosols, (ii) increase linearly from shallow todeeper drainage, (iii) but are not affected by C_org content ofthe histic horizon. Ecosystem respiration (R_eco) was linearlycorrelated 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 amajor influence of the inter-site variability in vegetation composition onannual net ecosystem exchange (NEE).

The IPCC definition of organic soils includes shallow histic topsoil, unlikemost national and international definitions of Histosols. Our study confirmsthat this broader definition is appropriate considering anthropogenic GHGemissions from drained organic soils. Countries currently apply soilmaps in national GHG inventories which are likely not to include histicGleysols. The land area with GHG emission hotspots due to drainage is likelyto be much higher than anticipated.

Deeply drained histic Gleysols are GHG hotspots that have so far beenneglected or underestimated. Peat mixing with sand does not mitigate GHGemissions. Our study implies that rewetting organic soils, including histicGleysols, has a much higher relevance for GHG mitigation strategies thancurrently recognized.

[学科分类] 地球化学与岩石