NUTRIENT DYNAMICS IN A CHRONOSEQUENCE OF DOUGLAS-FIR (PSEUDOTSUGA-MENZIESII (MIRB) FRANCO) STANDS ON THE BEAUJOLAIS MOUNTS (FRANCE) .1. QUALITATIVE APPROACH

Nutrient dynamics along a rotation of Douglas-fir was studied by chemi cal characterisation of solutions that are transferred through the eco system. The chronosequence approach was used to represent the developm ent stages of the forest rotation. Rainfall composition classified the present site as a pristine area. The transfer of precipitations throu gh canopies was characterised by an enrichment of the ionic charge in the solutions, which was related to the washoff of elements deposited on branches and needles (dry deposits) or to the ion-exchange processe s between the elements in the internal plant parts and those in the ra infall (canopy leaching). The main source of NO3-N, NH4+-N, SO42--S, C l-, Na+, Ca2+, Mg2+, Al3+ and H+ was considered to be the washoff from plant surfaces. Total organic carbon (TOC), K+ and Mn2+ were consider ed to originate mainly from leaching. The foliar uptake of nitrogen in the young stand and the parallel leaching of K+ from the canopy chara cterised differences in solution composition between the stands. The c oncentration increase was more significant in stemflow solutions than in throughfall as a consequence of the input of elements from the bole surface washoff. Differences between stands were related to stem size and to exposure of stems to the deposition of particles and gases. SO 42--S, NO3--N and Cl- controlled the transfer of nutrients in rainfall , throughfall and stemflow to the same extent. A higher participation of SO42--S in the ionic charge was mainly observed during winter. Soil solution chemistry was the result of a complex combination of factors like soil organic matter content, mineralisation and nitrification ca pacity and plant nutrient requirement, and was greatly influenced by s tand structure and seasonal patterns. The soil organic pool and its po tential for mineralisation were the main factors that determined the s olution chemistry in all layers, but to different extents. Nitrificati on was the major process acting on the release of cations from the ion -exchange sites and NO3--N was generally the main anion in the soil so lutions. SO42--S, occasionally controlled charge neutrality when the N O3--N concentration was too low. K+ that leached from canopy and from the organic litter influenced the quality of the soil solutions in the young stand down to a depth of 15 cm. Soil organic matter content, an d probably the type of organic matter and/or microbiota in each stand were the main Factors responsible for differences in the soil solution composition between stands. Although the differences in the soil solu tion composition were more related to soil characteristics than to sta nd age, the great differences in the quality of throughfall, stemflow and forest floor solutions were directly related to tree growth and st and characteristics.