ETBC: Interacting Hydrological and Biogeochemical Controls on Nitrogen Transformation Hot Spots and Hot Moments in a Eutrophic Reservoir

为了满足不断增长的全球人口对食物和能源的需求，人类的陆地固氮量增加了一倍多。由此导致的营养物质浓度升高破坏了许多淡水和沿海生态系统。与此同时，全球水坝数量的增加导致大陆地表水的平均存量增加了7倍。总之，这些人为变化相互作用，改变氮循环中的关键过程，包括反硝化（微生物介导的生物有效氮的去除）和相关的氧化亚氮的产生，氧化亚氮是一种能够消耗平流层臭氧的温室气体。水库中的氮处理可能是控制下游氮运输、氧化亚氮生产和生态系统功能的关键。然而，储层氮的处理尚不清楚，部分原因是反硝化作用已被证明难以测量。为了解决这一理解上的差距，该研究项目将：1)开发和测试新的跨学科方法来量化沉积物到水的N通量；2)使用这些新方法，结合成熟的方法，在一个小型污染水库中确定微生物N去除和氧化亚氮生产的热点和热点时刻；3)将这些热点和热点时刻与生物地球化学和物理过程联系起来。为了实现这些目标，该项目将整合水文测量（包括全水库温度分层和高分辨率近床水流）和生物地球化学测量（包括全水库和近床N积累和梯度，以及完整的岩心孵化）。用于定量二氮和一氧化二氮产量的已建立的质量平衡和完整岩心孵育方法将与更新颖的低氮气体积累和通量梯度方法相辅相成。通量梯度法的目的是在周和几十米的尺度上解决原位N通量，从而解决？热的时刻?然后呢?热点?快速反硝化和氧化亚氮生产。除了水库浅层、中层和深层区域之间的差异外，还将进行采样以解决氮处理的季节性变化。初步测量表明，秋季大坝放水是一个变化特别快的时期，因此将特别努力表征这段时间的氮动力学。新的通量估算技术，如果在这个项目中被证明是成功的，将来可以应用于水柱和沉积物之间循环的其他系统和其他化合物（如磷、硫和铁），并可能最终被纳入水库生物地球化学循环的确定性模型。由于无法在适当的时间和空间尺度上测量反硝化和氧化亚氮的产生，我们理解、预测和减轻人为加速全球氮循环影响的能力受到了一定程度的阻碍。本研究将通过开发广泛适用的量化沉积物-水N通量的新方法来解决这一迫切需求。研究结果还将有助于1)深入了解水库系统中氮循环的基本水文和生物地球化学控制，2)量化该系统中氮去除的热点和热点时刻的重要性，以及3)帮助确定水库放水可以提高系统氮去除效率的时间，从而减少下游氮的运输及其对下游生态系统的后续影响。最后，该项目将促进教学、培训和学习，支持研究生和本科生在跨学科背景下的专业发展。

To meet the food and energy demands of a growing global population, humans have more than doubled terrestrial nitrogen (N) fixation. Resulting elevated nutrient concentrations have damaged many freshwater and coastal ecosystems. Meanwhile, a global increase in the number of dams has caused a 7-fold increase in the average standing stock of continental surface waters. Together, these anthropogenic changes interact to modify key processes in the nitrogen cycle, including denitrification (the microbialy mediated removal of biologically available N) and associated production of nitrous oxide, a greenhouse gas capable of depleting stratospheric ozone. N processing in reservoirs is likely critical in controlling downstream N transport, nitrous oxide production, and ecosystem function. However, reservoir N processing is poorly understood, in part because denitrification has proven difficult to measure. To address this gap in understanding, this research program will: 1) develop and test novel, interdisciplinary methods to quantify sediment-to-water N fluxes, 2) use these novel methods, in conjunction with well-established approaches, to identify hot spots and hot moments for microbial N removal and nitrous oxide production in a small polluted reservoir, and 3) relate these hot spots and hot moments to biogeochemical and physical processes. To achieve these aims, the program will integrate hydrological measurements (including reservoir-wide temperature stratification and highly-resolved near-bed currents) and biogeochemical measurements (including reservoir-wide and near-bed N accumulation and gradients, as well as intact core incubations). Established mass-balance and intact core incubation approaches for quantifying dinitrogen and nitrous oxide production will be complimented with more novel hypolimnion gas accumulation and flux gradient approaches. The flux gradient approach aims to resolve in situ N fluxes on scales of weeks and tens of meters, thereby resolving the ?hot moments? and ?hot spots? of rapid denitrification and nitrous oxide production. Sampling will be conducted to resolve seasonal variability in N processing, in addition to variability between shallow, intermediate, and deep regions of the reservoir. Preliminary measurements indicate that an autumn dam release is a period of particularly rapid transformation, so special effort will be made to characterize N dynamics during this time. The novel flux estimation techniques could, if proven successful in this project, be applied in future to other systems and other chemical compounds that cycle between the water column and sediments (e.g. phosphorus, sulfur, and iron), and may eventually be incorporated into deterministic models of reservoir biogeochemical cycling. Our capacity to understand, predict, and mitigate the impacts of anthropogenic acceleration of the global N cycle has been hampered in-part by an inability to measure denitrification and nitrous oxide production at appropriate temporal and spatial scales. This study will address this pressing need by developing broadly applicable new methods for quantifying sediment-water N fluxes. Results will also 1) lend insight into the fundamental hydrologic and biogeochemical controls on N cycling within a reservoir system, 2) quantify the importance of hot spots and hot moments for N removal in this system, and 3) help pinpoint times of year when water release from reservoirs could enhance system N-removal efficiency, thereby reducing downstream N transport and subsequent effects on downstream ecosystems. Finally, this project will promote teaching, training, and learning by supporting the professional development of graduate and undergraduate students in an interdisciplinary context.