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Title
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Towards a better understanding of nutrient cycling in the lowland tropical rainforests of French Guiana
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Author
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Abstract
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| Tropical forests are among the most productive ecosystems in the world and play a central role in global biogeochemical cycles due to the high exchange of carbon between biosphere and atmosphere. Nutrient cycles, especially of nitrogen and phosphorus, exert an important control on this exchange and by consequence also on forest growth and dynamics. How nutrients cycle between atmosphere, biosphere and pedosphere in tropical forests remains thus far poorly understood, mainly because a lack of data and a high diversity of tropical forest ecosystem functioning make generalizations difficult. To complicate matters, although tropical forests are very productive, they typically grow on impoverished soils with especially low mineral nutrient availabilities. This begs the question where the required nutrients are coming from. In this thesis, various sampling schemes are coupled with chemical measurements to evaluate how nutrient poor these soils are, and how much nutrients are coming into this system every year. For this, we focused on two topographic gradients with expected soil nutrient heterogeneity situated in two distinct tropical field sites in French Guiana. We measured soil nutrient distribution along the topographic gradients and found that soil phosphorus content differs with topography, a characteristic driven mainly by the soil texture. Additionally, we measured ecosystem nitrogen input through asymbiotic nitrogen fixation and found that these inputs are very low compared to other tropical forest regions, mainly because of the low phosphorus availability. Another nutrient input, namely atmospheric deposition, was also measured and results showed that organic nitrogen, an often-ignored part of the deposition, represented the main fraction of nitrogen deposition in our forest. Phosphorus deposition amounted to only 0.5 kg ha-1 y-1, and was most likely primarily derived from African dust transported across the Atlantic. This arguably low amount of phosphorus deposition, however, amounted to one third of the yearly amount of phosphorus recycled through litterfall, meaning that the amount deposited from the atmosphere is an important contribution to the ecosystem. Finally, lab and field fertilization studies were conducted that showed that any P added to the soil is rapidly taken up by plant roots, and that plots receiving additional P showed a large increase in nitrogen fixation, indicating that phosphorus availability was severely limiting this process. From these results it becomes clear that phosphorus is in short supply in these forests and that its availability is severely limiting both microbial processes and plant growth. |
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Language
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English
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Publication
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Antwerpen
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Universiteit Antwerpen, Faculteit Wetenschappen, Departement Biologie
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2020
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Volume/pages
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237 p.
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Note
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Janssens, Ivan [Supervisor]
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Boeckx, Pascal [Supervisor]
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Weedon, James [Supervisor]
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Full text (open access)
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