NITROGEN MINERALIZATION AND NITRIFICATION IN DIFFERENT COLONIZING STAGES OF SUB-ALPINE RHODODENDRON-FERRUGINEUM L HEATHLANDS

Potential nitrogen mineralization and nitrification was measured in so ils from 3 different colonizing stages (meadow open heathland, closed heathland) of Rhododendron ferrugineum subalpine heathlands. This stud y was conducted on 2 sites: the Collet-d'Allevard (CA; altitude = 1 65 0 m) and the Cote des Salieres (CS; altitude = 1 860 m). The CA popula tions have a greater productivity and a faster dynamics than the CS po pulations. If annual soil nitrogen production investigated in soil inc ubation war no more than 1.5 g.m(-2) (15 kg.ha(-1)) in all situations, the peak of mineralization did not. occur at the same time and the N- NO3-/N-NH4+ ratio was dependent on the sites. At the CS, the greatest concentrations of soil mineral nitrogen happened 1 month later than th ey happened at the CA. Moreover the N-NO3-/N-NH4+ ratio from CS was ne ar 1 while from CA it was near 0. However, closed heathlands of the 2 sites behaved differently because there was no mineralization at CS an d nitrification occurred at CA during august period. This allows us to hypothesize about the relationships between available soil nitrogen, the biological and ecological performances of the Rhododendron populat ions.