Doctoral Dissertations

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Energy Science and Engineering

Major Professor

Richard J. Norby

Committee Members

David J. Weston, Colleen M. Iversen, Sarah L. Lebeis, Sean Schaeffer


Tropical forests, relative to other terrestrial ecosystems, exchange the largest amount of carbon with the atmosphere and also constitute a significant carbon sink. However, nutrient limitation, particularly of phosphorus (P), could limit growth of tropical forests and their function with the global carbon cycle. Thus, understanding root mechanisms to acquire P is necessary to representing the P cycle and corresponding interactions with plant growth. A large portion of total soil P in tropical forests occurs in organic forms, only accessible through root and microbial production of phosphatase enzymes. These phosphatase enzymes mineralize organic P into orthophosphate, the form of P most readily accessible to plants and microbes. My dissertation aims to understand the relationship between root and microbial phosphatase activity and available P in order to understand how phosphatase may contribute to soil P dynamics in tropical forests. In this work, I explore the variation of phosphatase with respect to tree species, soil depth, and elevated [CO2]. I found that tree species and available P strongly influenced phosphatase produced by roots. Although bacterial community composition was most influenced by available P, phosphatase activity of bacteria was regulated instead by host tree species. Variation in phosphatase activity due to tree species could be due to differences in root or soil factors. I determined that the phosphatase activity released from roots (root phosphatase) is predicted by specific root length and available P while microbially-produced phosphatase enzymes in the soil (soil phosphatase) is more regulated by fine-root mass distribution and total soil P concentration. A major uncertainty is how phosphatase activity may respond in elevated [CO2] conditions. In a greenhouse experiment, I found that root phosphatase increased in some tree seedlings exposed to elevated [CO2], but not in others, perhaps due to P limitation. Ultimately, phosphatase activity represents a necessary function to create a source of available P from soil organic P compounds. Phosphatase activity contributes to the capacity to intensely use sources of P from the rhizosphere representing an important facet of P acquisition, different, but complementary to root traits that increase root exploration of the soil volume.

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