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

湿地和河岸带是氮污染的重要自然过滤器，通常被称为景观之肾。它们通过微生物过程（反硝化作用）将污染性硝酸盐氮转化为无害的氮气，从而从土壤和地下水中去除氮。反硝化作用通常发生在潮湿和低氧的土壤条件下。然而，极端潮湿可能是太多的好事，并可能改变微生物平衡，有利于通过一个称为异化硝酸盐还原为铵（DNRA）的过程将氮保留在土壤中（作为铵态氮）。反硝化作用提供了一个生态系统服务，通过去除多余的N，而DNRA计数it.How和哪些土壤条件提示从反硝化平衡DNRA还没有得到很好的理解，是本研究的重点。对这些过程和条件的清楚了解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的氮去除。这项研究还将有助于更好地决策，现有的milldams，可以创造有害的潮湿和死水条件的溪流和河岸带的环境影响。这里学到的经验教训也将提高我们的理解，氮污染的景观受到盐渍化的道路盐和海平面上升。这项工作将支持两名博士生的教育和三名初级和两名高级科学家的专业发展。 本研究将确定调节DNRA反硝化二分法和缺氧土壤中铵态氮产生的关键条件。关键假设是：（a）水文停滞（低地下水混合）有利于DNRA;（B）DNRA将增加（相对于反硝化作用）当电子供体的特定浓度比（有机碳、亚铁和硫化物）与电子受体（硝态氮）超标;（c）盐渍化通过非生物置换土壤吸附的铵态氮而增加了河岸地下水中铵态氮的浓度，以及刺激DNRA的亚铁离子和硫化物离子的盐度驱动释放。这些控制措施将通过四种独立但互补的方法的创新组合和整合进行评估：（a）设计的围隔因子实验，使用15 N标记的硝酸盐-N来比较DNRA和反硝化过程的速率和所涉及的因素;（B）在现有的milldam研究地点对河岸土壤和水氮的原位测量;（c）围隔生态系统和河岸土壤中DNRA基因（nrfA）的微生物定量;以及（d）为实验室围隔生态系统实验建立反应性输运模型和校准，然后按比例放大并对河岸站点进行测试。该奖项反映了NSF的法定使命，并被认为值得支持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。

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.