Résumé
Standard C-13-labelled plant material was exposed over 2-3 yr at 8 sites in a north-south climatic gradient of coniferous forest soils, developed on acid and calcareous parent materials in Western Europe. In addition to soils exposed in their sites of origin, replicate units containing labelled material were translocated in a cascade sequence southwards along the transect, to simulate the effects of climate warming on decomposition processes. The current Atlantic climate represented the most favourable soil temperature and moisture conditions for decomposition. Northward this climatic zone, where the soil processes are essentially temperature-limited, the prediction for a temperature increase of 3 degrees C estimated a probable increase of C mineralisation by 20-25% for the boreal zone and 10% for the cool temperate zone. Southward the cool Atlantic climate zone, (the Mediterranean climate), where the processes are seasonally moisture-limited, the predicted increase of temperature by 1-2 degrees C little affected the soil organic matter dynamics, because of the higher water deficit. A significant decrease of C mineralisation rates was observed only in the superficial layers recognised in Mediterranean forest soils as 'xeromoder' and subject to frequent dry conditions. In the deeper Mediterranean soil organic horizons (the mull humus types), representing the major C storage in this zone, C mineralisation was not affected by a simulated 2 degrees C temperature increase. The temperature effect is probably counteracted by a higher water deficit.