Exploring recalcitrant N regulation of free-living nitrogen fixation in terrestrial ecosystems

生物固氮（BNF）是将大气中的氮气转化为可供植物吸收的形式。 这一过程是由一种被称为重氮营养菌的微生物进行的，97%的新氮输入到未受管理的陆地生态系统中。 目前的研究主要集中在与植物根系共生的微生物的生物固氮上。然而，自由生活的固氮重氮菌在许多土壤中是非常丰富的，它们对生态系统氮循环的贡献还没有很好的了解。 虽然投入的速效氮，如化肥，已知抑制生物固氮，很少有人知道生物固氮和柑橘N的可用性，或在土壤中紧密结合的N的可用性之间的关系。这是一个主要的知识空白，因为重氮营养菌在绝大多数N存在于柠檬酸池中的环境中非常常见。 本项目将1）筛选重氮营养物物种以确定可进入柠檬酸氮库的比例，2）研究柠檬酸氮对重氮营养物生物固氮的影响，3）量化生物固氮与柠檬酸氮降解之间的关系。 由于氮的可用性是大多数陆地生态系统中植物生长的主要限制因素，因此该项目对管理施肥和农业生产力具有重要意义。 该项目将涉及本科生和高中生，其结果将被整合到一个基于探究的学习模块的教师。在调节生物固氮的自由生活diazatrophs的作用是完全未知的。 该项目提出并测试了一种拉比尔-大气-顽拗性（LAR）N-获取策略，这是一种基于成本效益原则的固氮生物如何获得细胞氮的新框架，在生态系统尺度上具有重大意义。该项目将在低不稳定氮但高可利用氮的条件下测量纯培养中重氮营养菌的氮获取。 这些实验之后将在田间土壤中进行试验性培养，以调查自然生态系统中胞外酶活性与生物固氮之间的联系。 这些实验将有助于直接表征固氮微生物在整体氮获取策略中的固氮作用，从而有助于对生物固氮过程的理解。 研究结果将最终有助于陆地生态系统中生物固氮的微生物明确的预测模型。

Biological nitrogen fixation (BNF) is the conversion of atmospheric nitrogen gas to a form that is available for uptake by plants. This process, which is carried out by a type of microorganism known as diazatrophs, is responsible for 97% of new N inputs into unmanaged terrestrial ecosystems. Most current research focuses on BNF of microbes that live in symbiotic association with plant roots. However, free-living N fixing diazatrophs are highly abundant in many soils, and their contribution to ecosystem N cycling is not well understood. While inputs of readily available N such as fertilizer are known to inhibit BNF, very little is known about the relationship between BNF and recalcitrant N availability, or availability of N that is tightly bound in the soil. This is a major knowledge gap, because diazatrophs are very common in environments where the vast majority of N exists in recalcitrant pools. This project will 1) screen diazatroph species to determine the proportion that can access recalcitrant N pools, 2) study the effects of recalcitrant N on diazatroph BNF, and 3) quantify the relationships between BNF and degradation of recalcitrant N. Because N availability is a major limitation for plant growth in most terrestrial ecosystems, this project has significant implications for managing fertilization and agricultural productivity. The project will involve undergraduate and high school students, and the results will be integrated into an inquiry-based learning module for teachers.The role of recalcitrant N in regulating BNF by free-living diazatrophs is entirely unknown. This project proposes and tests a Labile-Atmospheric-Recalcitrant (LAR) N-acquisition strategy, a novel framework for how diazotrophs obtain cellular nitrogen based on cost-benefit principles, with large implications at the ecosystem scale. The project will measure N acquisition of diazatrophs in pure culture under conditions of low labile N but high recalcitrant N availability. These experiments will be followed by pilot incubations in field soils to investigate links between exoenzyme activity and BNF in natural ecosystems. The experiments will contribute to a process-level understanding of BNF by directly characterizing the role of N fixation in the overall N acquisition strategies of diazotrophic microbes. The results will ultimately contribute to microbially-explicit predictive models of BNF in terrestrial ecosystems.