Vulnerability of soil organic matter of anthropogenically disturbed organic soils

Abstract. Drained peatlands are hotspots of carbon dioxide (CO₂) emissions from agriculture. As a consequence of both drainage-induced mineralisation and anthropogenic mixing with mineral soils, large areas of former peatlands under agricultural use now contain soil organic carbon (SOC) at the boundary between mineral and organic soils and/or underwent a secondary transformation of the peat (e.g. formation of aggregates). However, low carbon organic soils have rarely been studied since previous research has mainly focused on either mineral soils or true peat soils. The aim of the present study was to evaluate the soil organic matter (SOM) vulnerability of the whole range of organic soils including very carbon rich mineral soils (73 g kg⁻¹ < SOC < 569 g kg⁻¹) and to identify indicators for mineralisation of such anthropogenically disturbed organic soils. Using a large sample pool from the German Agricultural Soil Inventory, 91 soil samples were selected covering a broad range of soil and site characteristics. Fen and bog samples were grouped into disturbance classes according to their pedogenetic features. Potential CO₂ production by aerobic incubation was then measured. Specific basal respiration rates (SBR) per unit SOC showed the highest potential emissions for heavily disturbed fen (12.1 ± 5.0 µg CO₂-C g SOC⁻¹ h⁻¹) and moderately disturbed bog samples (10.3 ± 5.2 µg CO₂-C g SOC⁻¹ h⁻¹). Surprisingly, SOM vulnerability increased with an increasing degree of disturbance and a decreasing SOC content, indicating positive feedback mechanisms as soon as peat soils are disturbed by drainage. Furthermore, with increasing degree of disturbance the variability of the SBR increased drastically, but correlations between soil properties and SBR could not be identified. Respiration rates increased more strongly with an increasing degree of disturbance in bog than in fen samples. Peat properties that positively influenced the turnover of SOM in less disturbed soil samples were mainly pH value and nitrogen content, while phosphorus was important for the mineralisation of increasingly disturbed samples and bog peat in general. Furthermore, a narrow carbon-to-nitrogen ratio correlated strongly with potential emissions. Given the high potential of CO₂ emissions from organic soils with a low SOC content, mixing with mineral soil does not seem to be a promising option for decreasing emissions.