This work is part of the SOLOBIOMA project, focused on the biogeochemistry cycle of nutrients and diversity and function of the soil biota in a chronosequence of sites, from pasture to old growth forest, in the Southern Atlantic Forest of Brazil. The understanding of the carbon and nitrogen dynamics is one of the project ́s aims. To know the complexes interactions between land use, biological diversity, biogeochemistry and functional structure of ecosystems we need to employ adequate analytical techniques. The use of the Isotopic Ratio Mass Spectrometry (IRMS) is one of the powerfull techniques employed to help the understanding of these interactions. Small variations of the C and N isotopic natural abundance in the ecosystems can be measured with great precision, being these variations related to changes in the vegetation community. The aim of this work was to measure the natural abundance of 13C and 15N in the vegetation, soil and litter in a pasture-forest chronosequence in a Southern Brazilian Atlantic Forest. The chronosequence consisted in three repetitions of thefollowing vegetation communities: Pasture (P), Herbaceous Forest (H), Initial Arboreal Forest (A), Medium Forest (M) and Old Growth Forest (F), amounting to 15 study areas. The species selection in each stage of the chronosequense was made according the Importance Value Adjusted Index (IVIa), being chosen the five species with the greatest IVIa in each sucessional stage and the leguminous species occurring in the plot. The collection of the vegetal material (leaves) was carried out during January of 2008. Soil and litter samples were collected in June and July of 2007. In each plot were collected 15 composite soil samples at 0-5, 5-10, 10-20, 20-40 and 40-100 cm of depth, and 15 random litter samples using a square collector with 25 x 25 cm. In the P plots the presence of C4 plants changed the isotopic C constitution allowing to identify the contribution of these plants to the soil organic matter. But some years after the abandon of the pasture the δ13C values became again closest to that of the forest vegetation. The stages H, A and M showed lower values of δ15N when compared to stages P and F in the superficial layer of the soil, indicating that N supply from atmosphere was occurring as a demand of the system. The δ15N determination in the leaves showed to be a useful tool to discriminate the N sources to plants, and to evaluate the N cycling in the ecosystem when measured on soil and litter samples. The main N source to plants is the soil organic matter but competition between plants and the rising of the N demand direct then to use a suplementar source (N from atmosphere) to maintain the development of the forest succession. The biological N fixation plays an important role, after the pasture abandon, but it stays only until the reaching of the forest climax when this is not more relevant.
