Balanço nutricional em uma floresta na Amazônia Central: o papel dos estoques e fluxos na ciclagem de nutrientes

Abstract

In general, soils of old growth tropical forests have low fertility, and as a result, nutrient cycling becomes of great importance for ecosystem functioning, once these elements are essential for plant metabolic processes and tissue construction. Understanding and quantifying the processes that involve nutrient acquisition, storage, and cycling in different plant tissues, and their relationship with forest productivity and biomass accumulation, is essential to characterize ecosystem nutrient dynamics and to project how global environmental changes, such as the increase in atmospheric CO2 can affect such processes. Therefore, in this study we investigated the nutrient dynamics of a terra firme forest in Central Amazonia, through the quantification of carbon and nutrient stocks, flows, retranslocation rates and nutrient use efficiency. Ecosystem net primary productivity (NPP), biomass stocks and the concentration of macronutrients (N, P, Ca, Mg and K), and micronutrients (Fe, Mn and Zn) in canopy, stems, fine roots, litterfall, coarse wood debris, and soils from the AmazonFACE (Free-Air CO2 Enrichment) project database, near Manaus, Amazonas, Brazil were used to calculate the nutrient budget of this forest. We aimed to test the hypothesis that the macronutrient cycling more efficiently in the ecosystem would potentially be the most limiting element for productivity. Our results showed that ecosystem nutrient flow was higher in canopy > fine litterfall > stems > fine roots > coarse wood debris > medium litter layer, while nutrient stocks were greater in soils > stems > coarse wood debris > fine litter layer > canopy > fine roots > medium litter layer. Among the studied compartments, soil was the one displaying higher nutrient storage, playing an important role as a nutrient source for plants in the long term. However, many of these nutrients are not immediately available for plant uptake. Large amounts of nutrients were also found stored in living stems, thus forming an important long-term nutrient reservoir. Phosphorus uptake and input into the soil were very slow and its retranslocation rate and use efficiency was high, suggesting a closed nutrient cycle of this element and a large potential to limit net primary production. Nitrogen use efficiency, on the other hand, was low, and both the stock and fluxes were high. Knowing how much nutrient is needed to allow biomass production in a year, and how much of it can be recycled or relocated, sheds light on how nutrient limitation may constrain ecosystem level responses to elevated atmospheric CO2, and help us to better predict the forest response to climate change.