Inselbergs are isolated rock outcrops displaying high heterogeneity in both soil formation and microclimatic condition with high variation in plant biodiversity. Vegetation patterns on inselbergs in the humid tropics range from rocks covered with dense biofilms predominated by cyanobacteria to high forest on deep soils. Along a similar transect, we investigated N supply to the vegetation using element and isotopic analyses of soil and biofilm samples from an inselberg in French Guiana. An increase in N content related to total dry weight (N%) in soils from the inselberg peak to surrounding habitats was related to changes in stable isotope composition (delta N-15). At the inselberg peak cyanobacterial biofilms on bare rocks and soils within small vegetation islands had similar delta N-15 values of -1.9 parts per thousand and - 1.3 parts per thousand while delta N-15 of soils progressively increased towards the primary rainforest up to 6 parts per thousand. From the peak towards the base of the inselberg, the density of higher plants and soil depth increased. Hence, soil N cycling and N losses to the environment resulted in a progressive increase of soil delta N-15. The distribution of N%, delta N-15 and delta C-13 values suggest that the main N supply for soils at and nearby the inselberg is derived from cyanobacterial N-2 fixation through leaching processes. (c) 2007 Elsevier GmbH. All rights reserved.