DISSERTATION RESEARCH: Interactions among nitrogen and phosphorus through plant-microbial mutualisms in tropical rain forests

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The tropics contain some of the largest, fastest growing and carbon-rich forests on Earth. As they grow, tropical trees store large amounts of atmospheric carbon dioxide in their biomass, slowing the increase in atmospheric carbon dioxide and hence the pace of climate change. Yet, all trees require large amounts of soil nutrients, especially nitrogen and phosphorus, to grow, and it remains unclear whether the supply of soil nutrients will be sufficient to meet the demands of predicted tree growth in the future. For example, soil phosphorus concentrations are relatively low in tropical forest soils, and some research suggests that low phosphorus supply may limit increases in tree growth and carbon storage. However, previous work suggests that some tropical trees may be better than others at obtaining scarce nutrients because of their relationships with soil microorganisms that enhance the ability to scavenge nutrients. This research will explore how different tropical trees access soil nutrients and whether these microbial partnerships offer some trees an advantage over their competitors. Overall, this research will improve the understanding of how tropical forests may respond to global environmental change.Trees capable of symbiotic nitrogen (N) fixation, a plant-microbial mutualism that converts atmospheric N into biological forms, are abundant in tropical forests. Previous work suggests that N fixing trees have an advantage over non-N fixing trees in acquiring soil phosphorus (P) via the production of N-rich phosphatase enzymes that mineralize organic P and by hosting arbuscular mycorrhizal fungi that scavenge for inorganic P. This advantage could lead to competition for soil P in which N fixing trees outperform non-N fixing trees. This project will explore the interactions between N fixing and non-N fixing trees via an experimental seedling study on the Osa Peninsula, Costa Rica in which multiple species from each plant group will be grown alone or in competition under five P treatments that vary by the P compound being delivered. In addition, previous work suggests that this N and P interaction may not be constrained to trees that fix N, but may be evident in trees that have high foliar N regardless of fixation. Thus, foliar N may be an indirect control of P acquisition on an individual plant basis, as well as on a landscape scale if plants with similar foliar N concentrations are aggregated. Therefore, this research will also explore the relationships between foliar N, P acquisition, and the biogeochemical consequences across a set of individual emergent canopy trees and forest plots characterized by high and low foliar N. The results from both projects have the potential to elucidate complex plant-microbially mediated N and P interactions that operate above- and belowground, and on multiple spatial scales.