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.
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 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: 1) established a new method for estimating chemical fluxes between sediments and the water column with a particular focus on nitrogen (N) cycling, and 2) characterized especially active times and places for N2 and N2O production in the sediments of a small reservoir. The spatial characterization of N transformations was focused on a region of the reservoir known as the internal shoreline that is expected to provide ideal conditions for microbial N removal. The temporal characterization of N transformations was focused on an annual dam spill in Lacamas Lake. To establish new methods, the program integrated 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). Established mass-balance approaches for quantifying dinitrogen and nitrous oxide production were complimented with more novel hypolimnion gas accumulation and flux gradient approaches. By developing and employing a flux gradient approach, we succeeded in resolving in situ N fluxes on scales of weeks and tens of meters, thereby resolving the "hot moments" and "hot spots" of rapid N2 flux and and nitrous oxide production. Measurements using this new method confirmed that the internal shoreline was a zone of particularly rapid transformation, so special effort was made to characterize N dynamics in this region. The novel flux estimation techniques will be applied in future research (NSF DEB1355211 to Harrison and Henderson) to better understand this and other systems and the cycling of other chemical compounds between the water column and sediments (e.g. phosphorus, sulfur, and iron). 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 addressed this pressing need by developing a broadly applicable new method for quantifying sediment-water N fluxes. Results also 1) lend insight into the fundamental hydrologic and biogeochemical controls on N cycling within a reservoir system, and 2) suggest the importance of hot spots and hot moments for N removal in this system. Finally, this project has promoted teaching, training, and learning by supporting the professional development of graduate and undergraduate students in an interdisciplinary context.
