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，人类对陆地氮的固定增加了一倍以上。由此导致的营养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蓄积量增加了7倍。总之，这些人为变化相互作用，改变了氮循环中的关键过程，包括反硝化作用(微生物介导的生物可利用氮的去除)和与之相关的一氧化二氮的产生，一氧化二氮是一种能够消耗平流层臭氧的温室气体。水库中的氮素处理在控制下游氮素运输、一氧化二氮生产和生态系统功能方面可能是至关重要的。然而，人们对水库氮的处理知之甚少，部分原因是反硝化作用被证明很难测量。为了解决这一认识上的差距，这项研究计划将：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，确定小型污染水库中微生物脱氮和一氧化二氮产生的热点和热点；3)将这些热点和热点与生物地球化学和物理过程联系起来。为了实现这些目标，该计划将整合水文测量(包括水库范围的温度分层和高分辨率的近床流)和生物地球化学测量(包括水库范围和近床N的积累和梯度，以及完整的岩心培养)。已建立的用于量化氮气和一氧化二氮产量的质量平衡和完整的核心孵化方法将与更新的次离子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数周和数十米尺度上解析就地N通量，从而解决热点时刻。然后呢？热点地区？快速反硝化和一氧化二氮的生产。将进行采样，以解决氮处理过程中的季节性变化，以及水库浅、中、深区域之间的变化。初步测量表明，秋季泄洪是一个特别迅速的转变时期，因此将特别努力地描述这一时期的氮素动态。新的通量估计技术如果在该项目中被证明是成功的，未来可以应用于其他系统和其他在水柱和沉积物(如磷、硫和铁)之间循环的化合物，并最终可能被纳入水库生物地球化学循环的确定性模型中。我们理解、预测和缓解人为加速全球氮循环的影响的能力受到了阻碍，部分原因是无法在适当的时间和空间尺度上测量反硝化和一氧化二氮的产生。这项研究将通过开发广泛适用的量化沉积物-水N通量的新方法来满足这一迫切需要。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)量化热点和热点时刻对该系统中氮去除的重要性，3)帮助准确地确定一年中从水库放水可以提高系统氮去除效率的时间，从而减少下游氮的输送和随后对下游生态系统的影响。最后，这个项目将通过在跨学科背景下支持研究生和本科生的专业发展来促进教学、培训和学习。

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.