The effect of clear cutting on podzolisation and soil carbon dynamics in boreal forests (Middle Taiga zone, Russia)

Boreal forests are one of the most important terrestrial carbon sink, and a large portion of C is allocated in soil for long-term storage. However forest harvesting may quickly affect soil carbon stocks and dynamics, especially where organic substances drive the soil-forming processes, such as in Podzols. To evaluate the effects of clear cutting on carbon dynamics and podzolisation process over a short time period, a pristine boreal forest (Komi Republic, Russian Federation) and a recently clear cut site (5 year-old) were selected. Soils are polygenic: podzolisation occurs within the clay-depleted eluvial horizon, formed by a previous lessivage process. Because podzolisation can start only after the eluvial horizon has reached a sort of threshold, bisequal soils allow to individuate comparable pedogenic conditions prior to anthropogenic disturbances. After harvesting, C storage tended to increase in the upper part of the soil profile (organic layer and podzolic sequum) from 2.2 to 5.0 kg m-2. The abundance of woody materials on the forest floor together with an increase in soil water saturation, discernible by the vegetation survey and iron fractionation, prevented litter degradation and allowed organic matter accumulation at the soil surface. Fulvic acids (FA) in the organic layer of the pristine site showed a low incorporation of polysaccharide and proteinaceous moieties, confirming a higher degradation of the humified fraction than at the clear cut site. The lack of disturbances allowed a selection of FA with the more oxidised and mobile fractions accumulating in the deeper horizons, as currently observed in Podzols. Almost no differences were instead found in the chemical composition of FA along the profile from the clear cut site. A larger portion of FA showed the tendency to migrate through the profile after clear cutting even below the Bhs horizon (C-fulvic acid/C-humic acid >1) with a marked increase in the FA-carbon stocks with respect to the pristine forest soil (0.66 and 0.30 kg m-2 down to 30 cm, respectively). Clear cutting also affected Al and Fe dynamics. The reducing conditions acted upon soil mineral surfaces and enhanced Fe mobilisation probably both in the ionic form and complexed with organic matter. The Al dynamics was instead more related to short term transformations of the layer silicate phases. Traces of a poorly crystalline chlorite were detectable in the Bhs in the pristine forest, but at the clear site only hydroxy-interlayered vermiculite was present. The high amounts of organic acids that migrated through the Bhs after clear cutting may have partially complexed the Al from pedogenic chlorite, giving rise to hydroxy-interlayered behaviour, as normally occurs in Podzol eluvial horizons from where the organic Al-complexes migrate. Our findings suggested that if this trend proceeds further the whole podzolic sequum may migrate downwards. This may have important implication on C budget, as organic carbon will be transferred deeper in the soil profile limiting its losses at least over a short time period.