DISSERTATION RESEARCH: Biogeochemical controls of Mo and P availability for N2-fixing trees across the Amazon Basin

低地热带雨林是地球上生产力最高的生态系统之一，也是土壤固碳的关键场所。在全球气候变化时期，这些森林作为碳汇的长期未来是不确定的，因为对其他元素(如氮、磷和微量金属)在推动热带树木生产中所起的作用认识不完全，特别是那些能够通过与固氮微生物共生关系利用大气氮的树木。这项研究将有助于我们理解这些过程，回答有关磷(P)和钼(Mo)作为固氮树木限制养分的重要性以及土壤肥力如何影响其多样性的问题。该项目将为美国和巴西的本科生提供研究机会，并将作为与德克萨斯州和华盛顿特区的高中生对话的一部分，向更多的受众传播。最后，这项研究的结果将通过与参与为政府间气候变化专门委员会制定预测的建模小组共享数据，为国家和国际政策提供信息。这项研究将结合之前使用温室和田间施肥实验进行的工作，以及亚马逊盆地P和Mo的经验田间测量，以证明这些元素在推动固氮树产量方面所起的作用。使用亚马逊来测试机械假说是理想的，因为它的土壤类型有很大的多样性，而且作为陆地碳汇的重要性。为了验证这些假设，这项研究将检验三种不同肥力的不同土壤类型，以及它们的物理和地球化学性质。具体地说，这项研究将对比(I)年轻地表(即亚马逊西部丘陵)的高肥力和高风化速率；(Ii)旧的高粘土盾构表面(即亚马逊中部的圭亚那盾构土壤)上的低肥力和低风化速率；以及(Iii)被称为砂壤的白色沙土上的低肥力和低风化速率。对于每种土壤类型，将测量土壤(通过土壤剖面在不同深度采样)和植物组织(叶和根)中最常见的固定剂和非固定剂中的全磷和有效磷和钼。此外，这项研究将在俄勒冈州立大学测量土壤样本和植物组织的钼同位素，以首次尝试了解热带雨林中的钼循环和同位素组成。因此，这项研究将能够解决有关氮素固氮剂的钼磷限制以及土壤肥力如何影响氮固氮剂物种多样性的关键问题。这些分析还将允许建立第一个热带森林钼预算，并将检验热带森林固定者和非固定者养分限制的关键假设

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