Dynamics and Biological Interactions of Phosphorus Cycling in Central Amazonian Forests

Abstract

Soil nutrient availability is considered to constrain the productivity of terrestrial ecosystems, with phosphorus (P) considered to be the most limiting nutrient in tropical forests. Due the great importance of Amazon forests in carbon (C) cycling and the fact that the majority of Amazon forests grow in low-fertility soils, understanding how nutrient limitation may affect net primary productivity (NPP) in these ecosystems is crucial to predict C storage in response to future climate. The direct effects of nutrient limitation on above and belowground forest functioning can only be tested through experimentation and up to now, the few large scale fertilisation experiments installed in lowland tropical forests indicate that multiple nutrients may limit different aspects of tropical forests, with inconsistent evidence for P limitation. Since much less is known about the potential effects of nutrient limitation on belowground forest functioning, this research aimed to analyse the main belowground mechanisms involved in P cycling, and how roots and soil microorganisms adapt to different conditions of soil fertility in central Amazon forests. I investigated how root morphological traits, mycorrhizal colonisation as well as enzyme exudation both from roots and soil microbes were expressed in natural low-fertility soils and how these traits responded to the short-term addition of P, nitrogen (N) and cations, as part of the first large-scale soil nutrient manipulation experiment in a central Amazon lowland forest near Manaus, Brazil. I show that in natural low-fertility soils, roots display a range of adaptations to increase P-uptake efficiency and investments in root morphological and physiological adaptations as well as association with fungi symbionts are complementary towards maintaining forest productivity in a central Amazon forest. With nutrient addition, I found support for the hypothesis of P-limitation, since trees were able to rapidly adapt their root morphological traits, reduce investments in enzyme exudation and increase association with mycorrhizal fungi. Such responses were also affected by cation addition, reinforcing the idea that multiple nutrients may control the expression of root traits. The soil microbial community was also affected by the short-term addition of nutrients, with a reduction in enzyme production with the addition of phosphorus, indicating a rapid alleviation of phosphorus limitation, but this reduction was eliminated when cations were also added. My results suggest that plants and soil microorganisms can rapidly respond to changes in soil nutrient Abstract 4 availability by changing their investments in nutrient uptake mechanisms, ultimately impacting plant productivity. These responses are crucial if we are to better understand how these forests function and how they may respond to global change.