Soils and soil organic matter transformations during the two years after a low-intensity surface fire (Subpolar Ural, Russia)

With an rise in temperature in the circumpolar region, an increase in the frequency of fires can be expected. We studied the effects of fire in larch (Lárix sibírica Lebed.) forests on the organic matter composition of topsoils of Leptosols in the Subpolar Ural Mountains (northern taiga, Komi Republic, Russian Federation). We compared topsoils before the fire (BF), during the low-intensity surface fire (DF), one (1AF) and two years (2AF) after the fire. The low intensity surface fire burnt only the upper part of the organic horizon, leading to the formation of a thin pyrogenic horizon. The soil alteration included alkalization of the solution by three pH units, threefold increase in the content of exchangeable forms of cations, a nine times increase in the specific surface area of the solid phase, increase in total carbon and nitrogen contents, and a five-fold increase in the aromaticity of the organic matter. The latter is mirrored by a strong increase in benzene polycarboxylic acids (BPCAs) as marker for pyrogenic carbon (PyC), representing 82 to 97 mg BPCA C g⁻¹C. Two years after the fire, arrival of fresh litter, intensified water regime and soil acidification led to a decline of the surface area of organic horizons. Thus, it was revealed that two years after a low-intensity fire, the soils differed from the initial ones in a total carbon and nitrogen contents, the organic matter aromaticity and the BPCA contents. The content of aromatic fragments in pyrogenic horizon evaluated according ¹³C NMR spectroscopy decreased after two years after fire, but at the same time, the PyC concentration, estimated on the carbon of BPCAs, remains stable. This indicates that periodically repeating low-intensity fires in boreal northern forests can contribute to the accumulation of stable forms of PyC in soils.