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被植物吸收的形式。这一过程是由一种被称为地氮菌的微生物进行的，它为未受管理的陆地生态系统提供了97%的新氮输入。目前大多数研究都集中在与植物根系共生的微生物的BNF上。然而，在许多土壤中，自由生活的固氮重氮菌非常丰富，它们对生态系统氮循环的贡献尚不清楚。虽然已知肥料等速效氮的输入会抑制BNF，但对于BNF与顽固性氮有效性或土壤中紧密结合的氮有效性之间的关系知之甚少。这是一个主要的知识缺口，因为在绝大多数N存在于顽固性池的环境中，透氮藻非常常见。本项目将1)筛选复氮物种，确定可进入抗性氮库的比例；2)研究抗性氮对复氮土壤BNF的影响；3)量化复氮土壤BNF与抗性氮退化之间的关系。在大多数陆地生态系统中，氮有效性是植物生长的主要限制因素，该项目对施肥管理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，结果将整合到教师的探究性学习模块中。顽固性氮在自由生活的透氮菌调节BNF中的作用是完全未知的。该项目提出并测试了一种不稳定-大气-顽固性（LAR） n获取策略，这是一种基于成本效益原则的重氮营养体如何获取细胞氮的新框架，在生态系统规模上具有重要意义。该项目将测量在低稳定氮、高抗性氮有效度条件下，在纯培养中地氮萎蔫菌的氮获取情况。这些实验之后将在田间土壤中进行试点孵化，以研究自然生态系统中外酶活性与BNF之间的联系。这些实验将通过直接表征氮固定在重氮营养微生物整体氮获取策略中的作用，有助于从过程层面理解BNF。这些结果最终将有助于建立陆地生态系统中微生物明确的生物多样性预测模型。

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.