Nitrogen input by cyanobacterial biofilms of an inselberg into a tropical rainforest in French Guiana

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 (δ ¹⁵N). At the inselberg peak cyanobacterial biofilms on bare rocks and soils within small vegetation islands had similar δ ¹⁵N values of ‑1.9‰ and ‑1.3‰ while δ ¹⁵N of soils progressively increased towards the primary rainforest up to 6‰. 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 δ ¹⁵N. The distribution of N%, δ ¹⁵N and δ ¹³C values suggest that the main N supply for soils at and nearby the inselberg is derived from cyanobacterial N ₂ fixation through leaching processes.