Carbon Storage Dynamics and Conditions Following Clear-Cutting, in a Montane Dystric Cambisol Planted with Douglas-Fir.
Francis Andreux1, Jean Leveque1, Florence Roux1, and Jacques Ranger2. (1) Microbiologie et Géochimie des Sols - INRA Université de Bourgogne, 6 bd Gabriel, DIJON, 21000, France, (2) INRA-UR Biogéochimie des Ecosystèmes Forestiers, CHAMPENOUX, 54280, France
The impact of Douglas-fir (Pseudotsuga menziesii Franco) on soil biogeochemical functioning was recently studied in detail on three plantations aged 20 (B20), 40 (B40) and 60 (B60) years. The site was located on a slight slope, in a low mountain area (altitude 700 m) of Beaujolais (east-central France). The bedrock was an andesitic tuff and the soil was a Dystric Cambisol. At the end of 1998, the B60 stand was clear-cut, the trees and main branches were removed, and a new generation of Douglas-fir was planted in April 1999. Meanwhile, a pioneer heliophilic vegetation invaded the site and covered the soil in less than one year. The organic rests remaining from the plantation and those brought by the secondary vegetation were allowed to decompose on a 300-m2 area with 32 georeferenced sampling points. Changes in litter and soil Organic Matter (OM) contents were monitored during 30 months, based on 8 sampling campaigns. The litter material collected around each sampling point was dried and weighed, then aliquots were taken and powdered, prior to Carbon (C) and Nitrogen (N) determination. On the same sampling points, soil samples were taken from the 0-0.05, 0.05-0.10 and 0.10-0.15 m layers, using a calibrated steel cylinder, for purpose of soil bulk density measurements and organic C and N analyses by dry combustion. The mean amount of C in the litter material passed from 1.7 kg m-2 in Spring following the clear-cut, to 0.5 kg m-2 two years later. Mathematical modeling showed that the litter material from Douglas-fir followed an exponential decay function, whereas the residues from the secondary vegetation increased slowly, according to a nearly linear law. Both OM sources supplied additional energy to soil heterotrophic microorganisms, but did not compensate the total litter C loss, even after three years. Carbon storage determined on three profiles showed a significant decrease from B40 to B60, especially in the 0-0.05 m layer (1.8 and 1.4 kg m-2, respectively). Between the Summer following clear-cutting and the end of the first year, the mean C storage in this layer first increased to 3.5±0.2 kg m-2 with increasing bulk density, then decreased progressively to about 2.2±0.3 kg m-2 at the time of the final campaign. Conversely, at clearing time, C storage in the two deeper layers was much lower than in the upper layer (1.6±0.1 m-2 at 0.05-0.10 m and 1.3±0.1 kg m-2 at 0.10-0.15 m). Thereafter, it increased slightly, to final values of 2.0±0.3 kg m-2 and 1.4±0.2 kg m-2, respectively. Altogether, clear-cutting the Douglas-fir plantation resulted in an immediate, but probably reversible decrease in surface soil organic C storage, in spite of the slight slope and adverse climatic conditions. After 30 months, a marked decrease in C storage was noticed only in the 0-0.05 m layer, but it was almost compensated by a slight increase in the deeper layers. This was probably the consequence of the incorporation of decaying OM brought from the upper layer, as well as from the litter. The rapid initial decay of the Douglas-fir litter, assisted by the contribution of the secondary ground vegetation, maintained almost constant the level of organic C stored in the upper layers. Organic N followed a very similar pattern as for C, resulting in relatively stable values of the C/N ratio (most frequently between 14 and 18), and suggesting that soil vital processes had been scarcely affected by clear-cutting. Nevertheless, OM storage studies would require monitoring over several years, due to the long-lasting break of soil nutrient cycles, as suggested by other parameters, such as slight compaction and acidification.