Collaborative Research: Saturated, suffocated, and salty: Hotspots of ammonium-N & dissimilatory nitrate reduction to ammonium-denitrification dichotomy in anoxic riparian soil

湿地和河滨（河岸）带是氮(N)污染的重要天然过滤器，通常被称为景观的肾脏。它们通过微生物过程（反硝化）从土壤和地下水中去除氮，将污染的硝酸盐氮转化为无害的氮气。反硝化通常发生在潮湿和低氧土壤条件下。然而，极端潮湿可能是一件好事，可能会改变微生物平衡，有利于通过一种称为异化硝酸盐还原为铵（DNRA）的过程保留土壤中的N（作为氨氮）。反硝化作用通过去除多余的氮来提供生态系统服务，而DNRA则对其进行反击。如何以及哪些土壤条件使反硝化与DNRA之间的平衡尚未得到很好的理解，这是本研究的重点。清楚地了解这些过程和条件将使流域管理者和环境机构能够更好地管理河岸带和湿地以去除N。这项研究还将有助于更好地制定有关现有水坝对环境影响的决策，这些水坝可能在溪流和河岸地带造成有害的潮湿和死水状况。这里的经验教训也将增强我们对受道路盐和海平面上升影响的景观中的氮污染的理解。这项工作将支持两名博士研究生的培养和三名初级科学家和两名高级科学家的专业发展。本研究将确定在缺氧土壤中调控dnra -反硝化二分法和氨氮生成的关键条件。主要假设是：(a)水文停滞（低地下水混合）有利于DNRA；(b)当电子给体（有机C，亚铁和硫化物）与电子受体（硝酸盐- n）的特定浓度比超过时，DNRA会增加（可见-à-vis反硝化作用）；(c)盐碱化通过土壤吸收的氨氮的非生物位移和盐驱动的亚铁离子和硫化物离子的释放来刺激DNRA，从而增加了河岸地下水中氨氮的浓度。这些控制将通过四种独立但互补的方法的创新组合和整合来评估：(a)设计中尺度因子实验，使用15N标记的硝酸盐- n来比较DNRA和反硝化过程速率和相关因素；(b)在现有的水坝研究地点就地测量河岸土壤和水氮；(c)中生态系统和河岸土壤中DNRA基因（nrfA）的微生物定量；(d)实验室中尺度实验的反应输运建模和校准，随后对河岸场地进行放大和测试。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Wetlands and streamside (riparian) zones are important natural filters for nitrogen (N) pollution and are often referred to as the kidneys of the landscape. They remove N from soils and groundwaters through a microbial process (denitrification) that converts polluting nitrate-N to harmless nitrogen gas. Denitrification typically happens under wet and low-oxygen soil conditions. However, extreme wetness can be too much of a good thing and could alter the microbial balance in favor of retaining N in soils (as ammonium-N) through a process called dissimilatory nitrate reduction to ammonium (DNRA). Denitrification provides an ecosystem service by removing excess N while DNRA counters it. How and which soil conditions tip the balance from denitrification to DNRA is not well understood and is the focus of this study. A clear understanding of these processes and conditions will allow watershed managers and environmental agencies to better manage riparian zones and wetlands for N removal. This study will also allow better decision making with regard to environmental impacts of existing milldams that can create detrimental wet and stagnant water conditions in streams and riparian zones. Lessons learnt here will also enhance our understanding of N pollution in landscapes subject to salinization from road salts and sea level rise. This work will support the education of two PhD students and the professional development of three junior and two senior scientists. This study will identify the key conditions that regulate the DNRA-denitrification dichotomy and the production of ammonium-N in anoxic soils. The key hypotheses are: (a) hydrologic stagnation (low groundwater mixing) favors DNRA; (b) DNRA will increase (vis-à-vis denitrification) when specific concentration ratio for electron donors (organic C, ferrous iron, and sulfide) versus the electron acceptor (nitrate-N) is exceeded; and (c) salinization increases the concentrations of ammonium-N in riparian groundwater through - abiotic displacement of soil-sorbed ammonium-N, and salinity-driven release of ferrous and sulfide ions that stimulate DNRA. These controls will be evaluated through an innovative combination and integration of four independent, but complementary, approaches: (a) designed mesocosm factorial experiments that use 15N labeled nitrate-N to compare DNRA and denitrification process rates and factors involved; (b) in-situ measurements of riparian soil and water N at existing milldam study sites; (c) microbial quantification of DNRA genes (nrfA) in mesocosm and riparian soils; and (d) reactive transport modeling and calibration for laboratory mesocosm experiments followed with scaling up and testing for riparian sites.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.