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

氮是一种关键的营养物质，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造成水和空气污染。该项目将提高对森林土壤何时、何地以及为什么产生和消费不同的含氮分子的理解。该项目具有广泛的社会影响，因为它有可能缓解人类造成的氮输入对土壤淋溶和气体排放造成的地下水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，以制定在农业生态系统中保留氮素的最佳管理做法，以提高作物产量，同时减少养分径流和化肥使用释放。该项目支持一名职业早期的女科学家担任首席研究员，并帮助培训一名女研究生。所有项目人员将参与指导本科生和高中生的研究经验，包括招收科学领域代表性不足群体的学生，并为伊利诺伊州4-H夏季学院开发一门新的土壤生物学短期课程，服务于各种高中生。土壤微生物进行的异化硝酸盐还原为铵(DNRA)的过程在调节生态系统氮保持与损失方面发挥关键作用，因为硝酸盐是一种比铵更具流动性的含氮分子。通过将硝酸盐转化为铵，DNRA在生态系统中保留氮素以支持植物生产力，减少地下水和地表水中硝酸盐的损失，并与反硝化过程竞争以减少气态氮和一氧化二氮的损失。尽管DNRA很重要，但在非洪水淹没的陆地生态系统中，DNRA通常被忽视和研究不足，因为人们错误地认为，这一过程仅限于洪水环境中常见的条件。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对DNRA的现有矛盾的理解：(1)DNRA发生在哪里，为什么？(2)哪些微生物对DNRA负责？以及(3)作为硝酸盐的命运，DNRA对于反硝化有多重要，是什么控制了这两条途径之间的竞争？这项研究将利用NSF长期生态研究(LTER)和临界区观测站(CZO)网络中的站点。选择的地点将代表一系列土壤属性、气候和微生物群落结构，这些结构可以控制DNRA的环境和遗传潜力。实验室土壤微宇宙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相互作用的其他氮素转化过程，如硝化和反硝化。将使用稳定的同位素示踪剂和水池稀释技术来测量这些过程的毛利率。免疫测序、定量聚合酶链式反应和蛋白质组学分析将被用来询问哪些微生物执行DNRA以及为什么。

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.