Limitação de fósforo e aumento de CO2na atmosfera: uma perspectiva dos mecanismos de aquisição de nutrientes na floresta Amazônica

Abstract

The Amazon rainforest is the largest continuous tropical forest in the world and plays an important role in the global carbon (C) dynamics, particularly as a carbon sink. The highly weathered soils found in approximately 60% of these forests have low phosphorus (P) availability, which has the potential to limit net primary productivity (NPP) at various scales, including the ecosystem level. Global climate models, which simulate the effects of climate change, including increased concentrations of carbon dioxide (CO2) in the atmosphere, suggest that the forest has a great capacity to assimilate carbon into its biomass and thereby mitigate the effects of climate change. However, this capacity may be overestimated as most models do not include a nutrient response, particularly phosphorus in the Amazon. Therefore, a greater understanding and representation of the processes involving nutrient cycling and multiple nutrient acquisition strategies will be extremely relevant for a better understanding of how the Amazon will respond to the increase in atmospheric CO2 concentrations, reducing uncertainties in projections related to the response of tropical forests to climate change. The cycling and decomposition of fresh aboveground organic matter (leaves, woody debris) are among the main sources of nutrients, especially phosphorus, in these areas. The high diversity of tree species commonly found in tropical forests has developed multiple strategies to enhance the efficiency of nutrient availability and acquisition. However, there are knowledge gaps related to these strategies, particularly in the Amazon forest considering the increase in atmospheric CO2 concentrations. In order to enhance the understanding of these different processes, two different experiments were proposed for this thesis. In Chapter 1, we evaluated the effect of the presence of fine roots and increased phosphorus availability on the decomposition of woody debris from different species. We observed a positive effect of root presence and a reduction in the C:P ratio in the decomposition and nutrient release, particularly for the lower density debris. However, the effect of root presence on nutrient release was only observed in the later stages of the experiment. These results suggest that the proliferation of fine roots and microbial decomposition may contribute to alleviating nutrient limitation in the Amazon, but under natural conditions of phosphorus limitation, nutrient mobilization may occur over decades, directly influencing carbon fluxes. In Chapter 2, to understand the effect of increased atmospheric CO2 concentrations on the phosphorus cycle and nutrient acquisition mechanisms in the Amazon, an experimental increase of 200ppm in CO2 concentrations relative to ambient conditions was simulated in the forest understory. We evaluated multiple nutrient acquisition mechanisms related to the fine root dynamics in the litter layer and soil, decomposition and nutrient release from leaf litter, microbial activity, and soil nutrient concentrations. We observed different nutrient acquisition strategies in the fresh litter and soil in response to increased CO2. In the litter layer, plants increased investment in direct phosphorus acquisition through phosphatase exudation and changes in root morphological traits without changes the productivity. In the soil, however, we observed a reduction in fine root productivity, decreased root length and specific area, accompanied by an increase in the arbuscular mycorrhizal fungi colonization. Furthermore, the reduction in total P concentration in the litter without changes in decomposition rate under increased CO2 conditions indicates an increase in biochemical mineralization. The same process may be related to the reduction of organic phosphorus in the soil under increased CO2 conditions, which may indicate an increase in phosphorus uptake by plants, considering that this ix reduction in the organic fraction did not result in changes on the inorganic fractions or microbial P mobilization. In summary, our results indicate that woody debris and leaf litter represent a crucial nutrient source for plants, and the presence of fine roots acts as an efficient mechanism to enhance direct nutrient acquisition in the litter layer, directly influencing the availability and interception of these nutrients. Additionally, with the increase in atmospheric CO2 concentrations, we observed that plants exhibit plasticity in directing the extra carbon use towards different acquisition strategies in the litter layer and soil, optimizing efficiency in acquisition and availability, especially of phosphorus. Under conditions of higher atmospheric CO2 concentration, this ability of plants to alter P availability and increase efficiency through multiple strategies is an important mechanism that can influence the resilience of the Amazon rainforest to climate change, impacting the global C cycle.