Soil organic matter composition along altitudinal gradients in permafrost affected soils of the Subpolar Ural Mountains

• SOM composition was measured for permafrost soils along altitudinal gradient.
• 13C NMR and H:C ratios of HAs showed similar organic layer quality among soils.
• In forests, mineral soil HAs are more aromatic than in mountain tundra soils.
• HAs of mineral soils were more aliphatic in waterlogged tundra than in forest zone.
• Permafrost melt might induce oxidative SOM loss in drained mountain soils.

Soil organic matter (SOM) in high-latitude soils is assumed to be highly vulnerable to climate changes. Relative little information exists from soils of mountain ecosystems which might respond differently to permafrost melt than in flat terrain due to a better drainage. In this study, we measured SOM composition of six typical soils along an altitudinal gradient of the remote Subpolar Urals, reaching from alpine tundra to the forest zone. The SOM characteristics was estimated by applying 13C nuclear magnetic resonance (13C NMR) spectroscopy, electron paramagnetic resonance (ESR), elemental analysis and amino acid composition of humic acid (HA) extracts from soils. Result showed that SOM stocks ranged between 8 and 13 kg C m− 2 but reached up to 40 kg C m− 2 in a Stagnic Podzol in the alpine tundra. In the mineral soil, 13C NMR indicated that the contribution of alkyl-C was 60% in the forest and 50% in the tundra, while aromaticity was 5% in the tundra, but 19% in the forest. This shows that SOM of mineral soils in alpine tundra was more aliphatic but less aromatic than in the Podzols of the forested zone. In contrast to mineral soils, SOM characteristic in organic layers was very similar among all soil types despite different vegetation types. Consequently, we suggest that the large difference in SOM quality in the mineral soil between tundra and forest can primarily be attributed to abiotic soil conditions in the deeper soil with a stronger waterlogging and a lower permafrost depth in the tundra soils. The low status of oxidative SOM degradation in the mineral soil of the tundra is also an indication that SOM of tundra is highly vulnerable to an improved aeration associated with permafrost melt in drained mountain soils.