Storage and transformation of organic matter fractions in cryoturbated permafrost soils across the Siberian Arctic

[摘要] In permafrost soils, the temperature regime and the resulting cryogenicprocesses are important determinants of the storage of organic carbon (OC)and its small-scale spatial variability. For cryoturbated soils, there is alack of research assessing pedon-scale heterogeneity in OC stocks and thetransformation of functionally different organic matter (OM) fractions, suchas particulate and mineral-associated OM. Therefore, pedons of 28 Turbelswere sampled in 5 m wide soil trenches across the Siberian Arctic tocalculate OC and total nitrogen (TN) stocks based on digital profilemapping. Density fractionation of soil samples was performed to distinguishbetween particulate OM (light fraction, LF, < 1.6 g cm⁻³),mineral associated OM (heavy fraction, HF, > 1.6 g cm⁻³),and a mobilizable dissolved pool (mobilizable fraction, MoF). Across allinvestigated soil profiles, the total OC storage was 20.2 ± 8.0 kg m⁻²(mean ± SD) to 100 cm soil depth. Fifty-four percent of thisOC was located in the horizons of the active layer (annual summer thawinglayer), showing evidence of cryoturbation, and another 35 % was present inthe upper permafrost. The HF-OC dominated the overall OC stocks (55 %),followed by LF-OC (19 % in mineral and 13 % in organic horizons). Duringfractionation, approximately 13 % of the OC was released as MoF, whichlikely represents a readily bioavailable OM pool. Cryogenic activity incombination with cold and wet conditions was the principle mechanism throughwhich large OC stocks were sequestered in the subsoil (16.4 ± 8.1 kg m⁻²;all mineral B, C, and permafrost horizons). Approximately 22 %of the subsoil OC stock can be attributed to LF material subducted bycryoturbation, whereas migration of soluble OM along freezing gradientsappeared to be the principle source of the dominant HF (63 %) in thesubsoil. Despite the unfavourable abiotic conditions, low C / N ratios andhigh δ¹³C values indicated substantial microbial OMtransformation in the subsoil, but this was not reflected in altered LF andHF pool sizes. Partial least-squares regression analyses suggest that OCaccumulates in the HF fraction due to co-precipitation with multivalentcations (Al, Fe) and association with poorly crystalline iron oxides and clayminerals. Our data show that, across all permafrost pedons, themineral-associated OM represents the dominant OM fraction, suggesting thatthe HF-OC is the OM pool in permafrost soils on which changing soilconditions will have the largest impact.

[学科分类] 地球化学与岩石