Biotic and Abiotic Controls on Nitrous Oxide Dynamics in Denitrifying Bioreactors

化肥被用于各种农业实践中，以增加农作物的产量，以养活日益增长的美国和全球人口。然而，这种化肥使用导致环境中活性氮(N)过量，影响水质，降低水体的商业和娱乐价值。进入水体的氮大部分来自农业和雨水径流等来源。木片生物反应器(WBR)是减少这些来源的氮污染的一种低成本方法，但它们也会产生一氧化二氮(N2O)，这是一种主要的温室气体和臭氧消耗物质。该项目将探索如何设计和运行WBR，以减少氮污染，同时最大限度地减少作为废物的N2O的排放。该项目将为环境工程博士生提供培训，并将为STEM领域的本科生研究助理提供研究经验。社区外展将通过卡尤加湖浮动教室进行，这是一个船基平台，供K-12学生和社区成员参与与纽约指湖水资源和质量有关的问题。如果成功，该项目将提供并改进一种保护国家水安全的低成本方法。陆地-水域边界的工程和自然生态系统对于缓解非点源N污染至关重要，但在生物去除活性N和N2O排放之间可能存在权衡。该项目的目标是促进对水-空气传质和微生物转化之间的耦合作为控制陆地-水界面环境中N2O命运的机制的理解，并选择WBRS作为一个具有代表性的系统进行研究。本研究的工作假设是，多孔介质中的地下水位波动导致空气相被困在孔结构中，而流动水和固定气相之间的氧气(O2)和N2O的交换是影响WBR内部和外部N2O积累和输出的关键因素，这是因为O2抑制了微生物N2O还原动力学和N2O分配到气相中。该项目采用补充技术，包括溶解气体和稳定同位素示踪剂测量和建模、定量成像和下一代测序，以弄清气体转移中微生物消耗对N2O水平衡的影响，并解释两相(空气-水)系统中的N2O行为。具体任务包括：1)用非平衡平流-弥散-传质模型解释溶解气体示踪实验数据，以量化水文状况对N2O和O2在多孔介质中两相传输的影响；2)通过溶解O2和15N同位素测量成像，评估水文驱动的O2交换在抑制微生物N2O还原方面的作用；以及3)通过元基因组和基因表达分析，评估最近发现的“第二类”N2O还原微生物在调节WBR中N2O命运方面的作用。这项研究的结果将推进一个统一的机制框架，以整合生物和非生物过程控制水陆界面N2O排放，并将为WBR和其他截留非点源氮的工程系统的设计和水文管理提供参考。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Fertilizer is used in various agricultural practices in order to increase the yield of crops to feed the growing US and global population. However, this fertilizer use has led to an excess of reactive nitrogen (N) in the environment that impacts water quality and reduces the commercial and recreational value of water bodies. A majority of the N entering water bodies is from sources such as agriculture and stormwater runoff. Woodchip bioreactors (WBRs) are a low-cost method to reduce N pollution from these sources, but they can also produce nitrous oxide (N2O), a major greenhouse gas and ozone-depleting substance. This project will explore how to design and operate WBRs to reduce N pollution while minimizing the release of N2O as a waste product. This project will provide training for an environmental engineering Ph.D. student and will provide research experiences for undergraduate research assistants in a STEM field. Community outreach will be conducted through the Cayuga Lake Floating Classroom, a boat-based platform for engagement of K-12 students and community members on issues relating to New York Finger Lakes water resources and quality. If successful, this project will provide and refine a low-cost method for protecting the Nation's water security. Engineered and natural ecosystems at land-water boundaries are critical for the mitigation of nonpoint source N pollution, but there can be tradeoffs between biological removal of reactive N and N2O emissions. The goal of this project is to advance mechanistic understanding of couplings between water-air mass transfer and microbial transformation as controls of N2O fate in environments at terrestrial-aquatic interfaces, with WBRs chosen as a representative system for investigation. The working hypothesis for this study is that water table fluctuations in porous media lead to entrapment of air phases in the pore structure, and that exchange of oxygen (O2) and N2O between the mobile water and immobile gas phase is a key influence on N2O accumulation inside and export from WBRs due to O2 inhibition of microbial N2O reduction kinetics and N2O partitioning into gas phases. This project employs complementary techniques including dissolved gas and stable isotope tracer measurements and modeling, quantitative imaging, and next-generation sequencing to disentangle the effects of microbial consumption from gas transfer on aqueous N2O balances and to account for N2O behavior in two-phase (air-water) systems. Specific tasks include: 1) Interpretation of dissolved gas tracer experimental data with nonequilibrium advection-dispersion-mass transfer models to quantify the impact of hydrologic regime on two-phase transport of N2O and O2 within porous media; 2) Evaluating the role of hydrologically-driven O2 exchange on inhibiting microbial N2O reduction via imaging of dissolved O2 and 15N isotope measurements; and 3) Evaluating the role of recently-identified "clade II" N2O-reducing microbes in regulating N2O fate in WBRs via metagenomic and gene expression analysis. Results of this research will advance a unified mechanistic framework to integrate biotic and abiotic process controls on N2O emissions from aquatic-terrestrial interfaces, and will inform the design and hydrologic management of WBRs and other engineered systems for the interception of nonpoint source nitrogen.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.