Clear cutting effects on soil characteristics and C dynamics in Middle Taiga

The disturbances to soil horizonation and to organic matter pools caused by harvesting were assessed in a Middle Taiga zone. Soils were sampled before (pristine forest) and 5 years after clear-cutting in a Russian boreal forest near the village of Ust’Kulom (Komi Republic). After clear-cut, the logging residues were left in place, and forest allowed to naturally regenerate. At both sites, soils were bisequal: they showed a sequence of O horizons followed by E and spodic ones, and eluvial horizons depleted in clays overlying argic ones. At the clear-cut site the organic layers thickness was higher and an E/A horizon was also present, above the E one. The soils were characterised for the principal chemical and physical properties. Fractionation of humic substances was carried out on the horizons forming the Podzol sequum: after NaOH-extraction, humic (HA) and fulvic acids (FA) were separated by HCl acidification, purified and freeze-dried. Humic fractions were investigated for their chemical composition by elemental analysis, E4/E6 ratio and FT-IR spectroscopy. The results showed a translocation of organic matter, as expected, with an accumulation of carbon in the spodic horizons. At the clear-cut site, the mineral horizons showed generally lower carbon contents than in the pristine forest, although the organic layers were enriched in C. The C to N ratios were higher than those found at the pristine site, while the humification ratio was lower. In both soils, an accumulation of oxalate extractable-Fe (Feo) and Al (Alo) was visible in the mineral horizons enriched in organic carbon, and their trend with depth was related to the variations in the FA/HA ratio. The spodic index (Alo+0.5Feo), used as a taxonomic evidence for spodic horizons, pointed towards a more intense translocation of metallorganic complexes in the clear-cut site. In the pristine forest the Bhs was too shallow to allow this soil to be identified as a Podzol. According to the WRB, the soils were an Albeluvisol (pristine forest) and a Podzol (clear-cut). In the pristine forest, the fulvic acids were progressively richer in oxygen-containing functional groups and aromatic moieties with depth, as deduced by elemental analysis and E4/E6 ratios. In the clear-cut site the FA features along the profile were more homogeneous than in the pristine forest and highly acidic compounds were translocated at depth. Humic acids appeared less affected by harvest operations and similar trends were visible at the two sites with the more oxidised and aromatic compounds in the Bhs horizon. In the short-term, clear-cutting caused an increase of C in the organic layers due to the large inputs of plant residues, left after harvest operations. The higher decomposition following clear-cut led to the production of oxidised organic material, which leached at depth and favoured Me complexation, accelerating the podzolisation processes. The C depletion observed in all mineral horizons could thus be related to clear-cut, also through migration of the more mobile organic products below the Bhs horizon.