Mass balance of major solutes in a rainforest catchment in the Central Amazon: Implications for nutrient budgets in tropical rainforests

A solute mass balance for a 23.4 ha catchment of undisturbed rainforest in the central Amazon Basin was computed from detailed measurements of water and solute fluxes via rainfall, streamflow, and subsurface outflow over an annual cycle. Annual atmospheric deposition fluxes are lower than previously reported among mass balance studies conducted in the Amazon. Nutrient export fluxes are lower than previously reported for the Amazon, despite the fact that export fluxes via flow paths not previously measured were included. Given that climatic conditions were representative of a one in 10 wet year, the ecosystem was expected to show a net loss of nutrients rather than net gain. Instead, an excess of nutrient inputs via rainfall over ecosystem outflows was detected, ranging in annual quantities from 0.30 to 0.50 of the measured input. Among several mechanisms that could reconcile this budget, two are supported by the information presently available while two others cannot be evaluated without further research. Interannual variability in the amount of water available for runoff at the spatial scale of small catchments varies by a factor of two, in contrast to rainfall variability of ±20%, and may be a critical control on the apparent changes in ecosystem storage detected by annual-scale nutrient budgets in rainforests. Entrainment of materials from the terrestrial ecosystem to the atmosphere, including particulates containing elements which do not exist as gases, may be a particularly important loss pathway in rainforests existing on deeply weathered or nutrient poor soils.