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

生物固氮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菌的微生物进行的，负责向未受管理的陆地生态系统输入新的氮的97%。目前的研究大多集中在与植物根共生的微生物的生物固氮方面。然而，自由生活的固氮重氮磷在许多土壤中都非常丰富，它们对生态系统氮循环的贡献还不是很清楚。虽然化肥等速效氮的输入会抑制生物固氮，但对生物固氮与顽固性氮素有效性或土壤中紧密结合的氮素有效性之间的关系知之甚少。这是一个重大的知识鸿沟，因为重氮在绝大多数N存在于顽固性池中的环境中非常常见。该项目将1)筛选重氮藻种类以确定可以进入难降解氮库的比例，2)研究难降解氮对重氮氮的影响，以及3)量化生物固氮与难降解氮的关系。由于N的有效性是大多数陆地生态系统中植物生长的主要限制因素，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，结果将被整合到教师的探究式学习模块中。顽固不化的N在自由生活的重氮类化合物调节脑源性神经营养因子中的作用完全未知。该项目提出并测试了一种不稳定大气顽固性(LAR)氮获取策略，这是一种基于成本效益原则的重氮菌获取细胞氮的新框架，在生态系统规模上具有重大影响。该项目将在低不稳定氮但高顽固氮利用率的条件下，测量纯培养条件下重氮托磷对氮的获取。在这些实验之后，将在田间土壤中进行试点培养，以调查自然生态系统中胞外酶活性和生物固氮之间的联系。这些实验将通过直接表征氮素固定在重氮菌整体氮素获取策略中的作用，有助于对生物固氮的过程水平的理解。这一结果最终将有助于建立生物固氮在陆地生态系统中的微生物显式预测模型。

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.