Collaborative Research: Revealing the Role of Less-Mobile Porosity in Hyporheic Denitrification and Greenhouse Gas Production

溪流和河流对自然和人类产生的污染物具有显著的清洁功能，因为生活在河床上的微生物可以将这些污染物转化为危害较小的化合物。陆地和水生生态系统中过量的氮会降低水质，产生一种强大的温室气体，现在被认为是全球范围内对人类最大的威胁之一。这项研究将河流的净化功能与全球氮过剩问题联系起来。河床细菌可以分解活性氮化合物，主要释放非活性氮气，无害地返回到大气中。然而，一部分被释放为强温室气体一氧化二氮（N2O）。令人信服的数据表明，河床或微区中长期储存的水，创造了分解溶解氮和形成N2O所需的低氧条件。新的河床微带遥感技术将使我们能够更好地解决氮是如何通过河流运输而衰减和转化的问题，改进对流域营养减缓的评估，并有助于更好地预测未来的气候变化。此外，这项研究将通过与已建立的外展机构建立社区一级的伙伴关系，与STEM教育相结合。外展合作伙伴（印象5科学中心和MSUSiFest）专门为4-12岁的儿童和社区“生活学习者”开发、执行和评估STEM展览和活动，这两个群体都是STEM的关键人群。项目pi将与康涅狄格大学的本科生设计团队和外展合作伙伴联系，开发新的地下水和河床流动模型展览和基于探究的演示，旨在利用社会对实时传感和互动日益增长的兴趣。外展合作伙伴将使用这些产品来说明地下水流动、污染物运输和温室气体产生的原理，每年向15万多名学生和社区成员提供服务。该项目将通过双域质量输运原理将瞬态储存与河流沉积物的生物地球化学功能联系起来并进行量化，以揭示对低渗反硝化和河流N2O生成的新见解。这项工作是及时的，因为最近的全球评估表明，河流是N2O的主要生产者，但生产的机制和空间分布仍然未知。与现有的河流沉积物生物地球化学模型相反，假设硝酸盐还原为N2O主要发生在河床沉积物中，这些沉积物总体上是含氧的，但具有局部的、缺氧的、不太移动的孔隙空间。在过去的工作中，这些缺氧微位点被忽视了，必须从机械上理解，以便从点到到达到盆地尺度的淡水氮动力学。需要新的观测方法和基于过程的模型来解释缺氧微生物位点在流体交换和氮生物地球化学中的作用。最近，项目团队成员开发了电地球物理方法来推断不太可移动的参数，因为电场可以直接感知不太可移动孔隙中空间可变的溶质动力学。其他团队成员专注于开发标记15N示踪剂方法，以揭示反硝化的停留时间控制。这些技术将结合起来解锁缺氧微位点的存在和功能。该工作计划包括受控实验室实验、数值模拟和在美国马萨诸塞州伊普斯维奇流域一个已建立的研究地点进行的现场实验。我们的工作将直接将新的基于过程的理解与现有的河网硝酸盐模型联系起来，扩展和利用以前的NSF LINXII研究。具体而言，将表征低流动性孔隙空间的内在性质，量化厌氧微点的存在和厌氧微点中发生的反硝化作用，增强河流氮生物地球化学的多尺度格局。总的来说，这项工作将改变目前对潜流微场过程的理解，为潜流带在流域溶质运输、氮循环和温室气体产生中的作用提供新的机制模型。拟议的研究将通过量化我们流域中一些非常小的地方与以前未表征的低流动性孔隙空间的水动力交换来解决大问题。

Streams and rivers have a remarkable cleansing function for natural and human generated contaminants, as microbes living in the streambed can transform these contaminants into less harmful compounds. Excess nitrogen in our terrestrial and aquatic ecosystems is now considered one of the greatest global-scale threats to humanity by degrading water quality and producing a powerful greenhouse gas. This research couples the cleansing function of rivers to this global excess nitrogen issue. Streambed bacteria can break down the reactive nitrogen compounds, primarily releasing non-reactive nitrogen gas that returns harmlessly to the atmosphere. However, a fraction is released as the strong greenhouse gas nitrous-oxide (N2O). Compelling data indicates pockets of longer-term water storage in streambeds, or microzones, create the low-oxygen conditions needed to both break-down dissolved nitrogen and form N2O. New remote sensing techniques of streambed microzones will allow us to better resolve the how nitrogen is attenuated and transformed through river transport, improving evaluations of watershed nutrient mitigation and helping better predict future climate change. Further, this research will dovetail with STEM education via community level partnerships with established outreach institutions. Outreach partners (Impression 5 Science Center and MSUSiFest) specialize in developing, executing and evaluating STEM exhibits and activities for children ages 4-12 and community "life-learners", both of which are key STEM demographics. Project PIs will connect with UConn undergraduate design teams and outreach partners to develop novel groundwater and streambed flow model exhibits and inquiry-based demonstrations designed to harness society's increasing fascination with real-time sensing and interaction. Outreach partners will use these products to illustrate principles of groundwater flow, contaminant transport, and greenhouse gas production, reaching 150,000+ students and community members each year.This project will link and quantify transient storage via dual-domain mass transport principles with the biogeochemical functions of stream sediments to reveal new insights on hyporheic denitrification and stream N2O production. This work is timely because recent global assessments reveal that rivers are major N2O producers, but the mechanism and spatial distribution of production remain unknown. Contrary to existing biogeochemical models for stream sediments, it is hypothesized that nitrate reduction to N2O occurs predominantly within streambed sediments that are oxic in a bulk sense but have local, anoxic less-mobile pore spaces. Largely overlooked in past work, these anoxic microsites must be mechanistically understood in order to upscale freshwater nitrogen dynamics from point, to reach, to basin scales. New observation methods and process-based models are needed to account for the role of anoxic microsites in fluid exchange and nitrogen biogeochemistry. Recently, project team members developed electrical geophysical methods for inference of less-mobile parameters, as the electric field can directly sense spatially variable solute dynamics in less-mobile porosity. Other team members have focused on developing labeled 15N tracer methods to reveal residence time controls on denitrification. These techniques will be combined to unlock the presence and function of anoxic microsites. The workplan comprises controlled laboratory experiments, numerical modeling, and field experiments at an established research site in the Ipswich Watershed, MA, USA. Our work will directly connect new process-based understanding to existing river network nitrate models, extending and capitalizing on previous NSF LINXII research. Specifically, the intrinsic properties of less-mobile pore space will be characterized, the existence of anoxic microsites and denitrification occurring in anaerobic microsites will be quantified, and multi-scale patterns of river nitrogen biogeochemistry will be enhanced. Overall, this work will transform the current understanding of hyporheic microsite processes, providing new mechanistic models of the role of hyporheic zones on watershed solute transport, nitrogen cycling and greenhouse gas production. The proposed research will address big questions about some very small places in our watersheds by quantifying hydrodynamic exchange with previously uncharacterized less-mobile hyporheic pore space.