NUTRIENT ACQUISITION STRATEGIES IN TROPICAL RAIN FORESTS: NITROGEN AND PHOSPHORUS INTERACTIONS THROUGH PLANTMICROBIAL MUTUALISMS

Symbiotic dinitrogen (N2) fixation is a fundamental biological process that allows some plants to overcome nitrogen (N) limitation by converting atmospheric N2 into biologically available forms. As a result, N2 fixers (and N2 fixation) should theoretically have a competitive advantage in relatively low N, high phosphorus (P) environments, but be less competitive in high N, low P environments. Yet, N2 fixing trees are relatively rare in many N poor temperate and high-latitude forests, but abundant in the N rich lowland tropics. This paradox raises the question: Why are symbiotic N2 fixers (and N2 fixation) so abundant in tropical rain forests? In light of the low P status of many tropical rain forests, previous attempts to identify the mechanisms that promote high abundances of N2 fixers have focused on interactions between N2 fixation and two soil P acquisition strategies: extracellular phosphatases that mineralize organic P and arbuscular mycorrhizal (AM) fungi that scavenge for inorganic P. However, a detailed understanding of the nature of these N and P interactions, and their broader implications, are lacking. Thus, the overall goal of my dissertation research was to understand the relationships between symbiotic N2 fixation and soil P acquisition, as well as their roles in plant-plant interactions and plant responses to environmental change. In a series of observational and experimental studies in multiple lowland tropical rain forest sites in Costa Rica and Panama, I found (1) N2 fixers have higher phosphatase activities and AM colonization than non-N2 fixers in the forest; (2) Differential investment in these two soil P acquisition strategies among N2 and non-N2 fixers lead to the exploitation of different soil chemical P compounds (i.e., soil P partitioning) when seedlings were experimentally grown in isolation; (3) Despite soil P partitioning, N2 fixers outcompeted non-N2 fixers for soil P when seedlings were experimentally grown together; and (4) Coupled N and P acquisition in N2 fixers elicit greater growth than non-N2 fixers when seedlings were experimentally grown in low nutrient soil and under elevated atmospheric carbon dioxide concentrations. Overall, my dissertation provides an intriguing set of interactions between symbiotic N2 fixation and soil P acquisition that may enable N2 fixing trees to be competitively superior to non-N2 fixing trees in acquiring soil P. I argue that the ability to fix atmospheric N2, which in turn contributes to the acquisition of soil P, could drive, in part, the relative of abundance of N2 fixers in lowland tropical rain forests