Quantity and chemistry of water-extractable organic matter in surface horizons of Arctic soils under different types of tundra vegetation – A case study from the Fuglebergsletta coastal plain (SW Spitsbergen)

Abstract

Water-extractable organic matter (WEOM) is the most labile, dynamic, and bioavailable fraction of soil organic matter (SOM). WEOM is a very important component of soil because it is responsible for the mobilization and translocation of many elements and pollutants in the soil environment and plays a crucial role in the biogeochemistry of carbon (C) and nitrogen (N), mineral weathering as well as pedogenesis. The main objectives of this study were: 1) to determine the quantity and chemistry of WEOM in the surface horizons of High Arctic soils under different types of tundra vegetation in the eastern part of the Fuglebergsletta coastal plain in the Hornsund area of southwestern Spitsbergen; 2) to determine relationships between quantity of WEOM and physical and chemical soil properties; and 3) to verify the hypothesis that soil surface horizons covered with different types of tundra vegetation and showing different physical and chemical properties exhibit different quantity and quality of WEOM. Soil samples (uppermost 10cm) in six replicates from surface horizons were randomly collected at sites differing in terms of tundra vegetation type (wet moss, ornithocoprophilous, lichen-heath, polygonal). The obtained results indicate that the highest concentration of water-extractable organic carbon (WEOC) and water-extractable total nitrogen (WETN) occurs in surface horizons of soils covered with wet moss and ornithocoprophilous tundra vegetation. Soils covered with lichen-heath and polygonal tundra vegetation exhibit a markedly and significantly lower concentration of WEOC and WETN. On average, WEOC constitutes maximally up to 8.0% of total soil organic carbon (SOC) and WETN constitutes maximally up to 1.5% of total nitrogen (Ntot) content in the studied soil surface horizons. The lowest proportion of WEOC to total SOC and WETN to Ntot occurs in soils containing the highest content of SOM. On the other hand, the highest proportion of WEOC to total SOC and WETN to Ntot is present in soils characterized by the lowest content of SOM. Concentration of WEOC and WETN is highly positively and significantly correlated to the content of SOC, Ntot, total phosphorus (Ptot), and the C/N ratio as well as negatively and significantly correlated to pH and ash content. Fourier-transform infrared attenuated total reflectance (FTIR-ATR) spectra indicate clear differences in the chemistry of WEOM between the studied soils. The WEOM in the surface horizons of soils under wet moss and ornithocoprophilous tundra vegetation is characterized by a prevalence of aromatic compounds over aliphatic compounds. On the other hand, WEOM in surface horizons of soils covered with polygonal tundra vegetation exhibits a prevalence of aliphatic compounds over aromatic compounds. WEOM in surface horizons of soils under lichen-heath tundra vegetation contains an almost equal concentration of aromatic compounds and aliphatic compounds.