Abstract
Abstract. This study investigated soil organic matter (OM) composition of differently stabilized soil OM fractions in the active layer of a polygonal tundra soil in the Lena Delta, Russia, by applying density and particle size fractionation combined with qualitative OM analysis using solid state 13C nuclear magnetic resonance spectroscopy, and lipid analysis combined with 14C analysis. Bulk soil OM was mainly composed of plant-derived, little-decomposed material with surprisingly high and strongly increasing apparent 14C ages with active layer depth suggesting slow microbial OM transformation in cold climate. Most soil organic carbon was stored in clay and fine-silt fractions (< 6.3 μm), which were composed of little-decomposed plant material, indicated by the dominance of long n-alkane and n-fatty acid compounds and low alkyl/O-alkyl C ratios. Organo-mineral associations, which are suggested to be a key mechanism of OM stabilization in temperate soils, seem to be less important in the active layer as the mainly plant-derived clay- and fine-silt-sized OM was surprisingly "young", with 14C contents similar to the bulk soil values. Furthermore, these fractions contained less organic carbon compared to density fractionated OM occluded in soil aggregates – a further important OM stabilization mechanism in temperate soils restricting accessibility of microorganisms. This process seems to be important at greater active layer depth where particulate OM, occluded in soil aggregates, was "older" than free particulate OM.
Highlights
RaivnedrbLaanpkteevroSSseicoaniseh(nCelhcfaedrkesirniveest from peral., 2011; tion in cold climate
Most soil organic carbon was stored in Gustafsson et al, 2011), including an increasing portion of clay and fine-silt fractions (< 6.3 μm), which were composed old, strongly transformed DOC and particulate organic carbon (POC) most probably exof little-decomposed plant material, indicated by the domi- ported from thawing permafrost soils (Guo et al, 2007; van nance of long n-alkane and n-fatty acid compounds and low Dongen et al, 2008; GOustcafessaonneSt acl.,ie20n11c)e
In this study we investigated the composition of differently stabilized soil OM compartments in the shallow active layer and the uppermost still-frozen permafrost in a polygonal rim on Samoylov Island (Lena Delta, Siberia)
Summary
RaivnedrbLaanpkteevroSSseicoaniseh(nCelhcfaedrkesirniveest from peral., 2011; tion in cold climate. Organo-mineral associations, which of these studies are identifying terrestrial OM from analyare suggested to be a key mechanism of OM stabilization in ses of river and marine sediments as well as of suspended temperate soils, seem to be less important in the active layer material from both systems Few investigations have been conducted so in soil aggregates – a further important OM stabilization far of the active layer in permafrost soils, which thaws durmechanism in temperate soils restricting accessibility of mi- ing summer, as a potential source of terrestrial OM to Arctic croorganisms. This process seems to be important at greater rivers and the ocean. Physical and chemical stabilization processes, which are considered to play an important role for OM stabilization in soils and sediments (Arnarson and Keil, 2007; Kogel-Knabner et al, 2008; Marschner et al, 2008), have not been investigated in the active layer of permafrost soils
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