Biotic and Abiotic Controls on Nitrous Oxide Dynamics in Denitrifying Bioreactors

化肥被用于各种农业实践，以提高农作物的产量，以养活不断增长的美国和全球人口。然而，这种肥料的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（WBRs）是一种低成本的方法来减少这些来源的氮污染，但它们也会产生一氧化二氮（N2O），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水反应堆，以减少N污染，同时最大限度地减少N2O作为废物的排放。该项目将为环境工程博士生提供培训，并为STEM领域的本科生研究助理提供研究经验。社区外展活动将通过卡尤加湖水上课堂进行，这是一个基于船只的平台，供K-12学生和社区成员参与有关纽约芬格湖水资源和水质的问题。如果成功，该项目将提供并完善一种低成本的方法来保护国家的水安全。陆地-水边界的工程生态系统和自然生态系统对于缓解非点源氮污染至关重要，但在生物去除活性氮和N2O排放之间可能存在权衡。本项目的目标是推进水-空气质量传递和微生物转化之间耦合的机制理解，以水-水界面环境中N2O命运的控制，并选择wbr作为代表性系统进行研究。本研究的工作假设是，多孔介质的地下水位波动导致气相在孔隙结构中被困，而由于O2抑制微生物N2O还原动力学和N2O向气相分配，流动水和不流动气相之间的氧气（O2）和N2O交换是wbr内部N2O积聚和输出的关键影响因素。该项目采用互补技术，包括溶解气体和稳定同位素示踪剂测量和建模、定量成像和新一代测序，以解决气体传输中微生物消耗对水中N2O平衡的影响，并解释两相（空气-水）系统中的N2O行为。具体工作包括：1)利用非平衡平流-弥散-传质模型解释溶解气体示踪实验数据，量化水文条件对多孔介质中N2O和O2两相输运的影响；2)通过溶解O2成像和15N同位素测量，评价水文驱动的O2交换对微生物N2O还原的抑制作用；3)通过宏基因组和基因表达分析，评估新发现的“II枝”N2O还原微生物在wbr中调控N2O命运的作用。该研究结果将为整合水陆界面N2O排放的生物和非生物过程控制提供统一的机制框架，并将为wbr和其他非点源氮拦截工程系统的设计和水文管理提供信息。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。

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.