Impact of changing forest management on soil organic matter in low mountain acid media

The impacts of changes in vegetation cover from native deciduous forest to Douglas fir (Pseudotsuga menziesii Franco) and of human activity on soil organic matter (SOM) characteristics were studied in two low mountain areas of east-central France. No striking difference in soil type (Dystric Cambisol) was found between the two sites. Humus-rich horizons were of the "Dysmull" and "Moder" types, regardless of the nature of the bedrock. Contrary to a common affirmation concerning other coniferous species, Douglas fir had no negative effect on soil pH and humification degree of SOM, with respect to the native beech vegetation. Pruning and partial clearing slightly improved humification, especially the decomposition of the leaf litter. Special attention was placed on the characteristics of water-soluble SOM, to suggest chemical and isotopic methods that would allow the tracking and estimation of soluble transfers from soils to streams. Differences arose between forested watersheds, according to the predominance of either deciduous or coniferous vegetation; the significance of such differences is discussed.