Nitrogen cycling in the tropical rain forest of French Guiana: comparison of two sites with contrasting soil types using delta N-15

The natural N-15 abundance method for estimating symbiotic biological N-2-f ixation was tested on legume trees from two rain forests on contrasting soi ls (oxisols and spodosols) in French Guiana. When possible, the significanc e of N-2-fixing species in the plant community was evaluated in terms of de nsity, biomass and contribution of N-2-fixation to the building up of the t otal nitrogen mass in the leaves. Of the two sites, the rain forest on spod osols was the less favourable for application of the delta(15)N method: the available soil nitrogen was isotopically similar to fixed-N-2. Hence, the results showed that a reliable estimate of N-2- fixation could not be obtai ned. A substantial contribution of fixed-N-2 to the nitrogen nutrition of l egumes was found on oxisols, with an average value of 54 % Ndfa (Nitrogen d erived from the atmosphere). The contribution of the N-2-fixing legumes to the biomass of the stand was estimated to be 2 t ha(-1) for the leaf biomas s and 136 t ha(-1) for the total above-ground plant biomass. With 7.5 % of trees in the stand able to fix N-2 (462 out of 6156), N-2-fixation was esti mated to be 7 kg ha(-1) y(-1). These results are the first use of the delta (15)N method to estimate nitrogen input by N-2-fixing legumes to a natural rain forest. The inter-site variability observed in the delta(15)N of the n on-fixing plants suggested different nitrogen-cycling processes in the two soils. The delta(15)N Of the non-N-2-fixing plants could be related to the soil nitrogen availability and be used as an indicator of efficient or non- efficient nitrogen-cycling rain forests. The spatial variability of the del ta(15)N in the plant-available soil nitrogen pool and the nitrogen balance in tropical rain forests are discussed.