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最具生产力的生态系统之一，也是土壤中碳固存的关键场所。在全球气候变化时期，这些森林作为碳汇的长期前景是不确定的，因为人们对其他元素（如氮、磷和微量金属）在推动热带树木生产方面所起的作用还不完全了解，特别是那些能够通过与固氮微生物的共生关系利用大气氮的树木。这项研究将有助于我们对这些过程的理解，通过回答关于磷(P)和钼（Mo）作为固氮树的限制性养分的重要性以及土壤肥力如何影响它们的多样性的问题。该项目将为美国和巴西的本科生提供研究机会，并将作为“问生态学家”对话的一部分传播给更多的受众，该对话涉及德克萨斯州和华盛顿特区的高中生。最后，该研究的结果将通过与参与政府间气候变化专门委员会预测的建模小组共享数据，为国家和国际政策提供信息。本研究将整合之前的温室和田间施肥实验，以及亚马逊流域的P和Mo的实地测量，以证明这些元素在推动固氮树木生产中的作用。利用亚马逊来测试机械假设是理想的，因为它的土壤类型非常多样化，而且它作为陆地碳汇的重要性。为了验证这些假设，这项研究将检查三种不同的土壤类型，它们的肥力不同，以及它们的物理和地球化学性质。具体而言，本研究将对比(1)年轻地表（即亚马逊西部山麓）的高肥力和高风化率；旧的高粘土盾面（即亚马逊中部的圭亚那盾面土壤）肥力低，风化率低；（三）被称为砂质土的白色沙土的低肥力和低风化率。对于每种土壤类型，将测量土壤（通过土壤剖面在不同深度取样）和植物组织（叶子和根）中最常见的固定物和非固定物的总磷和有效磷和钼。此外，本研究将测量俄勒冈州立大学土壤样品和植物组织的Mo同位素，首次尝试了解热带雨林中的Mo循环和同位素分数。因此，本研究将能够解决Mo和P对n2固定物的限制以及土壤肥力如何影响n2固定物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Mo预算，并将检验热带森林中固定生物和非固定生物营养限制的关键假设

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