Unraveling the paradox of dissimilatory nitrate reduction to ammonium in upland soils

氮是一种重要的营养物质，通常仅限于植物和微生物。含氮分子在土壤中转化为不同的化学形式，可以调节它是保留在生态系统中供植物和微生物吸收，还是从生态系统中流失，从而造成水和空气污染。该项目将增进对森林土壤中不同含氮分子产生和消耗的时间、地点和原因的理解。该项目具有广泛的社会意义，因为它有可能缓和人为氮输入对土壤浸出和气体排放造成的地面和地表水污染的影响。从这项研究中获得的知识可以指导未来的研究，以制定最佳管理实践，在农业生态系统中保留氮，提高作物产量，同时减少肥料使用带来的养分流失和释放。该项目支持一名早期职业女性科学家担任首席研究员，并有助于培养一名女性研究生。所有项目人员都将参与指导本科生和高中生的研究经验，包括招募在科学领域代表性不足的学生，并为伊利诺伊州4-H暑期学院开发一门新的土壤生物学短期课程，为各种高中学生提供服务。由于硝态氮是一种比铵态氮更具流动性的含氮分子，土壤微生物进行的异化硝态氮还原（DNRA）过程在调节生态系统氮保留与损失中起着关键作用。通过将硝酸盐转化为铵，DNRA在生态系统中保留氮以支持植物生产力，减少硝酸盐在地下水和地表水中的损失，并与反硝化过程竞争以减少气态二氮和一氧化二氮的损失。尽管它很重要，但DNRA在陆地上的非洪水生态系统中通常被忽视和研究不足，因为人们错误地认为该过程仅限于洪水环境中的典型条件。最先进的稳定同位素和分子技术将通过回答以下问题来解开现有的对DNRA的矛盾理解：(1)DNRA在哪里发生，为什么发生？(2)哪些微生物负责DNRA？(3)作为硝酸盐的命运，DNRA相对于反硝化有多重要，是什么控制了这两个途径之间的竞争？这项研究将利用美国国家科学基金会长期生态研究（LTER）和关键区域观测站（CZO）网络。选择的地点将代表一系列能够控制DNRA环境和遗传潜力的土壤特性、气候和微生物群落结构。实验室土壤微观实验将用于确定氧气和亚硝酸盐如何影响DNRA的速率以及与DNRA相互作用的其他氮转化过程，如硝化和反硝化。稳定同位素示踪剂和池稀释技术将用于测量这些过程的总速率。Illumina测序，定量聚合酶链反应和蛋白质组学分析将用于询问哪些微生物执行DNRA及其原因。

Nitrogen is a critical nutrient that is often limiting to plants and microbes. Transformations of nitrogen-containing molecules into different chemical forms in soil can regulate whether it is retained in ecosystems to be available for plant and microbial uptake or lost from ecosystems to contribute to water and air pollution. This project will improve understanding of when, where, and why different nitrogen-containing molecules are produced and consumed in forest soils. This project has broad societal implications because of the potential to moderate the effects of human-induced nitrogen inputs on ground- and surface water pollution from soil leaching and gas emission. Knowledge gained from this research can guide future studies to develop best management practices to retain nitrogen in agroecosystems to improve crop yields while reducing nutrient runoff and release from fertilizer use. The project supports an early career female scientist as the principal investigator and contributes to the training of a female graduate student. All project personnel will be involved in the mentoring of undergraduate and high school students in research experiences, including recruiting students who are members of under-represented groups in science and developing a new a short course on Soil Biology for the Illinois 4-H Summer Academy, serving a variety of high school students.The process of dissimilatory nitrate reduction to ammonium (DNRA), carried out by soil microbes, plays a pivotal role in regulating ecosystem nitrogen retention versus loss because nitrate is a more mobile nitrogen-containing molecule than ammonium. By converting nitrate to ammonium, DNRA retains nitrogen in ecosystems to support plant productivity, reduces nitrate loss to ground- and surface waters, and competes with the process of denitrification to decrease gaseous dinitrogen and nitrous oxide losses. Despite its importance, DNRA is generally disregarded and understudied in non-flooded ecosystems on land because of the misconception that the process is restricted to conditions typically found in flooded environments. State-of-the-art stable isotope and molecular techniques will be used to unravel the existing paradoxical understanding of DNRA by answering the following the questions: (1) Where can DNRA occur, and why? (2) which microorganisms are responsible for DNRA? and (3) How important is DNRA relative to denitrification as a fate of nitrate, and what controls the competition between these two pathways? The study will utilize sites across the NSF Long-Term Ecological Research (LTER) and Critical Zone Observatory (CZO) Networks. Sites will be selected to represent a range in edaphic properties, climate, and microbial community structure that can control the environmental and genetic potential for DNRA. Laboratory soil microcosm experiments will be used to determine how oxygen and nitrite affect rates of DNRA and other nitrogen transformation processes that interact with DNRA, such as nitrification and denitrification. Stable isotope tracer and pool dilution techniques will be used to measure gross rates of these processes. Illumina sequencing, quantitative polymerase chain reaction, and proteomic analyses will be used to interrogate which microbes perform DNRA and why.