Anthropogenic disturbance and effects on the organic matter migration in a clear-cut boreal forest soil.

The practice of clear cutting in boreal forests is often based on heavy caterpillar harvesting machinery, strongly damaging forest soils. Large areas of European Northern Russian countries face these disturbances and the risk of loss of productive forest soils. The objective of this work was to evaluate the effect of high intensity human disturbance on soil carbon balance, and on the migration capacity of the organic matter. A 35-40 years old clear-cut Russian boreal forest was studied (Ust-Kulom, Komi Republic). In this area, the forest road built to collect wood was still visible from the concentration of birch trees. Two soil profiles were selected: one was representative of low level anthropogenic disturbance (clear-cut forest), and the other was opened in the most disturbed area (forest road). In the most disturbed profile, the presence of a B horizon enriched in C clearly documented the migration of organic matter. The soils were sampled and analysed for the main chemical and physical characteristics. The total extractable carbon (TEC) was obtained by NaOH, and fractionated into humic (HA) and fulvic acids (FA), which were purified and freeze-dried. The carbon content (CHA and CFA) in the fractions was determined by wet oxidation. The spectroscopic analyses, the determination of the E4/E6 ratio, and the elemental composition were carried out on the humic fractions. In the less disturbed site, the C stock calculated down to 60 cm in soil horizons was 4.4 kg m-2, while in the most disturbed soil, the amount was 8.0 kg m-2. The effect of disturbance on the organic fraction was evident also on the distribution of C in the profile: in the clear-cut site the carbon content decreased with depth, while in the profile of the forest road a translocation of organic matter with an accumulation of carbon in the deep horizons (EBh and Bh) was evident. The C to N ratios were different between the soils. In fact, at depth, N was twice as much in the FA from forest road than in the FA from the forest profile, and the molar C/N ratios were very low. In the forest profile, the FA from the clay-depleted surface horizons were richer in oxygen-containing functional groups than those from the clay-richer deepest horizons, as deduced by the elemental analyses and by the E4/E6 ratios. In the most disturbed profile the FA were generally more oxidised and richer in aromatic moieties with depth. The FT-IR spectra confirmed the higher aromaticity of the deepest FAs from the EBh and Bh horizons, as evident from the decrease of the 1720 to 1630 cm-1 bands ratio. From the quantitative point of view, the CFA/CHA ratio indicated an increase of the low molecular weight humic substances with depth in the forest profile, while in the road profile the FA increase was evident down to horizon overlying the EBh horizon. The characteristics of organic matter in the two sites clearly indicated that migration has been more effective in the most disturbed site. On the other hand, the differences in the humic material present in the deep horizons of the two sites suggest that other processes may have contributed to accumulation of organic matter in the forest road.