Lowland tropical rainforests are among the most productive ecosystems on the planet, and are critical sites of carbon sequestration in soils. The long-term future of these forests as carbon sinks during times of global climate change is uncertain due to an incomplete understanding of the role other elements (such as nitrogen, phosphorus, and trace metals) play in driving tropical tree production, particularly trees that are capable of using atmospheric nitrogen through a symbiotic relationship with nitrogen-fixing microbes. This research will contribute to our understanding of these processes by answering questions about the importance of phosphorus (P) and molybdenum (Mo) as limiting nutrients for nitrogen-fixing trees and how soil fertility affects their diversity. This project will provide research opportunities for US and Brazilian undergraduate students, and will be disseminated to a larger audience as part of an Ask an Ecologist dialogue that engages high school students in Texas and Washington D.C. Finally, results of this research will inform national and international policy through data-sharing with modeling groups involved in developing predictions for the Intergovernmental Panel on Climate Change.
This study will integrate previous work using greenhouse and field fertilization experiments with empirical field measurements of P and Mo across the Amazon Basin to demonstrate the role these elements play in driving production of nitrogen-fixing trees. Using the Amazon to test mechanistic hypotheses is ideal due to its great diversity in soil types and its importance as a terrestrial carbon sink. To test these hypotheses, this study will examine three contrasting soil types that differ in fertility, and their physical and geochemical properties. Specifically, this study will contrast (i) high-fertility and high-weathering rates on young surfaces (i.e., foothills of western Amazonia); (ii) low-fertility and low-weathering rates on old high clay shield surfaces (i.e., Guyana Shield soils in central Amazonia); and (iii) low-fertility and low-weathering rates on white sand soils called Arenosols. For each soil type, total and available P and Mo in soil (sampled at a variety of depths through the soil profile) and plant tissue (leaves and roots) will be measured for the most common fixers and non-fixers. In addition, this study will measure Mo isotopes of the soil samples and plant tissues at Oregon State University in the first attempt to understand Mo cycling and isotopic fraction in a tropical rainforest. As a result, this research will be able to address critical questions about Mo vs. P limitation on N2-fixers and how soil fertility affects N2-fixer species diversity. These analyses will also allow the construction of the first tropical forest Mo budget, and will test critical hypotheses of nutrient limitation on fixers and non-fixers in tropical forests
