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. <br/><br/>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.<br/><br/>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.