Organic matter composition and potential trace gas production of permafrost soils in the forest tundra in northern Siberia

Abstract

The amount, quality and bioavailability of organic matter stored in permafrost soils are important factors determining the response of high-latitude soils to climate warming. In this study, we investigated the storage and composition (isotopic composition, lignin, pyrogenic carbon) of organic matter in mineral soils which are differently affected by permafrost, and we determined the potential CO 2 emission, CH 4 exchange and N 2O emission of these soils at different temperature (5 °C and 15 °C) and moisture (60% of the maximum water-holding capacity [WHC] and completely water saturated) in a laboratory incubation experiment. Soil samples were collected in the summer of 2002 and 2003 from the mineral soils of the Grawijka Creek catchment in the Siberian forest tundra and for comparison, from a fertilized grassland in Germany. The depth of the seasonal thaw layer of the Siberian soils ranged from 15 cm to > 90 cm and was greater in soils located on slopes than in soils of plane areas where drainage was poor and soils showed gleyic properties. The soil organic carbon (SOC) concentration ranged from 14 to 74 g kg − 1 in the upper 20 cm and from 5 to 128 g kg − 1 in the subsoil. The total SOC and N accumulation in the upper 30–40 cm were larger in soils with a seasonal thaw layer < 40 cm (up to 23 kg C m − 2 and 1.3 kg N m − 2) than in soils without permafrost in the upper 90 cm (approximately 8 kg C m − 2 and 0.6 kg N m − 2). The concentration of lignin-derived CuO oxidation products in soil OC were larger in a soil without permafrost than in the permafrost soils in which lignin oxidation appears to be more advanced. All soil samples from the forest tundra contained considerable amounts of black carbon (up to 57 g C kg − 1 SOC), which indicates the importance of fire in this ecosystem. Water logging in the permafrost soils seems to restrain the decomposition of black carbon. Soil organic carbon mineralization in the gleyic permafrost soils increased by a factor of approximately 4 if soil temperature was raised from 5 to 15 °C and soil moisture reduced from complete water saturation to 60% WHC. Emission of N 2O was negligible from all Siberian soils but very high from the fertilized grassland soil at complete water saturation. At 60% WHC, all forest tundra soils were a net-sink for atmospheric methane with significantly larger CH 4 uptake in the A horizon of the soil without permafrost (∼ 0.1 ng CH 4–C h − 1 g − 1) than in the A horizon of the permafrost soils (< 0.02 ng CH 4–C h − 1 g − 1). The results show that permafrost distribution is an important factor determining storage and composition of SOC in the Grawijka Creek area and that permafrost distribution may considerably affect current and future net fluxes of the greenhouse gases CO 2 and CH 4 in this region.