Short-time effect of clear cutting on soil carbon stock and dynamics in a boreal forest

Carbon accumulation in soil is an important process of the carbon cycle in forest ecosystems. Anthropogenic disturbances could quickly affect soil carbon stocks and dynamics, especially where organic substances drive the soil-forming processes, such as in Podzols. To evaluate the variations of soil processes and organic matter pools in short-term period, caused by clear cutting, a pristine boreal forest (Komi Republic, Russian Federation) and a recently clear cut site (5 year-old) were selected. In both soils carbon accumulation in the Bhs horizons arising from podzolisation was visible. The presence and the quality of logging residues induced a carbon increase in the organic layer from 0.94 in the pristine forest to 1.46 kg m-2 at the clear cut site. At the clear cut site, the organic materials were characterised by a low nitrogen incorporation and by less oxidised lignin-derived phenols, as expected from the larger abundance of woody tissues. However, slightly degraded organic matter was also present in the pristine forest, but the lack of disturbances allowed a selection process in the mineral horizons with the more oxidised and mobile fractions accumulating in the deeper horizons. Five years after clear cutting, a larger portion of this mobile fraction seemed to have migrated through the profile down to the Bhs and the underlying EB horizon. Undisturbed podzolisation induced an accumulation of acidic vanillyl units in the Bhs horizon of the pristine forest, while at the clear cut site the process was much less expressed. As a consequence of both addition of large amounts of organic residues and enhanced downwards migration of poorly chemically selected compounds, the stocks of carbon markedly increased also in deeper mineral horizons after clear cutting, thus limiting carbon losses at least in the short-time.