Variation of carbon and nitrogen cycling processes along a topographic gradient in a central Amazonian forest

It is well recognized in the literature that topography can influence soil nutrient stocks and dynamics in temperate regions, but for tropical forests, this source of variation has sometimes been ignored. The nature of such variations may depend upon the soil type, which in turn, is closely linked to local or regional topography. This study characterizes the soil and describes the status of carbon and nitrogen in vegetation, litterfall, litter-layer and soil upper layers along the main positions of a topographic gradient (plateau, slope and valley), 60 km north of Manaus, on Cuieiras Reserve watershed. Nitrogen concentrations in living leaves, fresh litterfall, litter-layer and soil upper layers were lower in the valley than in both slope and plateau plots. Carbon concentrations in plant material were not significantly different among the three topographic positions, resulting in higher C : N ratios in valley plots. Local topography (plateau, slope and valley) clearly was an influential factor in the nutrient distribution along the study locations. Lower rates of N cycling processes in the valley are probably related to its sandy soil texture and seasonal flooding.