Amazonian forest has a prominently huge stock of carbon, and has a large effect on the global carbon dynamics. Fine roots (less than 2mm in diameter) are one of the most dynamic parts of trees, which account for 30-50% of the net primary production of forest ecosystems. However, their local distribution is not homogeneous, and their variation should be carefully concerned on the evaluation of their contribution to the forest dynamics.

We have studied the fine root biomass, its local variation, and the cause of it in two undisturbed tropical rain forests of the Brazilian Amazon at Manaus (ZF-2 research forest) and at São Gabriel da Cachoeira (SGC). Fine root biomass was sampled by 100-cm3 cylinders at 3 depth classes (0-5, 5-10, 10-20cm) at ZF-2 and at SGC. The soil total nitrogen, soil total carbon and the above-ground biomass and the density of the trees were also measured, and their effects on the density of the fine root biomass were analyzed. The soil total carbon and soil total nitrogen, the above-ground biomass were positively correlated with the fine root density, but their effects differed among sites.