NUTRIENT DYNAMICS IN A CHRONOSEQUENCE OF DOUGLAS-FIR (PSEUDOTSUGA-MENZIESII (MIRB) FRANCO) STANDS ON THE BEAUJOLAIS-MOUNTS (FRANCE) .2. QUANTITATIVE APPROACH

Nutrient fluxes were determined in a chronosequence of Douglas-fir sta nds in the Beaujolais Mounts (France). Annual and seasonal variations occurred during the 3 years of investigation; fluxes were generally hi ghest in autumn-winter. Atmospheric inputs were among the mean values from a monitoring network of forest ecosystems in France. Nutrient out puts from the soil profile were higher than average and occurred mainl y during vegetation dormancy. Mean input-output budgets were negative for N, S, K, Ca and Mg, characterising an imbalance of the site chemis try dynamics. The P budget was positive. Most of the nutrient output f rom the ecosystem occurred as losses in the drainage water. These loss es were related to excess nitrification and consecutive cation mobilis ation throughout the soil profile. Surface water, however, had a neutr al pH and very low nitrate and aluminium contents, which may have been buffered by the subsoil, Budgets differed between stands and tended t o be more negative in the youngest stand. Part of this behaviour was r elated to stand age and part to the former land use of plots. Theoreti cal budgets were calculated for forest rotation lengths of 20, 40 and 60 years; it was concluded that shorter rotations would increase nutri ent losses. The trend of a decrease in budget deficits with stand age suggests that the effect of vegetation change will be reduced at the n ext rotation but the impact of stand development may remain. Predicted nutrient budgets for a second 60 year Douglas-fir rotation suggested that available Ca in the soil would be depleted and that this depletio n would be even more drastic if whole tree harvesting were adopted. In conclusion, Douglas-fir stands introduced changes in soil function th at may impoverish the soil if present trends remain the same over the next forest rotations. The maintenance of sustainability will require nutrient input by fertilisation.