Researchers examine N transformations in Columbia River hyporheic zone.
The hyporheic zone (HZ) is the subsurface zone where river water and groundwater exchange and mix. Researchers used multiple laboratory columns containing sediments from the Columbia River HZ to examine how dynamic changes in groundwater and surface water mixing affect microbial communities and their biogeochemical function with respect to nitrogen (N) transformations. Flow rates and directions were systematically varied to evaluate the impacts of flow dynamics on N biogeochemistry and microbial community composition and function. Experimental results were incorporated into numerical models to help interpret dominant processes and their effects.
Variable discharges from upstream dams cause frequent changes in direction and magnitude of flow in the HZ, making it challenging to predict pathways and concentrations of N. The results of this study show that water flow direction and source, and their periodic oscillations, have a profound impact on the pathways and rates of N transformation in HZ sediments. Microbial communities showed the ability to adapt to flow conditions over periods of several days, but microbial function remained stable under short-term (sub-daily) changes in flow conditions. These results demonstrate the importance of considering recent hydrologic conditions (historical contingencies) in predictions of HZ biogeochemical function.
The HZ is an active biogeochemical region where chemicals and nutrients carried by groundwater and surface water mix and stimulate microbial activities. Strong chemical gradients develop, and promote the rapid transformation of carbon, N, and other elements.
Inorganic N is commonly present in groundwater and surface water, but at elevated concentrations it is considered a contaminant. N cycling also plays a critical role in healthy ecosystem functioning, and is known to be influenced by hydrologic exchange between groundwater and river water. For these reason, researchers sought to better understand how N transforms in HZs that experience significant daily, monthly, and seasonal variations in hydrologic flow conditions.
Researchers created five laboratory columns using sediment samples collected from the HZ in the Columbia River, downstream from the Priest Rapids Dam. This particular HZ is considered hydrologically dynamic, which can make it challenging to predict changes in the microbial communities and biogeochemical processes that affect N.
In the study, researchers suggest that it’s essential to investigate the spatial and temporal variations in N transformations under variable fluid flow. They subjected each of five columns to various flow rates and effluent and pore water conditions to simulate possible scenarios in the HZ.
The results imply that variations in the mixing zone greatly affect both microbial function and the biogeochemical processes responsible for transforming N. In fact, water flow direction and sources, and their periodic oscillations, have a profound impact on the pathways and rates of N transformation in HZ sediments. As N pathways changed both over time and in different spatial locations, so did the interactions between N and other elements and the composition and function of the microbes in the system.
For these reasons, researchers say, caution should be applied when interpreting results of correlation analysis on HZ systems, particularly when dynamic changes in hydro-biogeochemical conditions occur with different timing and frequency.
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This research is supported by the U.S. Department of Energy, Office of Science, Biological and Environmental Research as part of the Subsurface Biogeochemical Research (SBR) Program through PNNL’s SBR Science Focus Area (SFA) project. F. Xu, Y. Liu, A. Yan, and C. Liu also acknowledge the supports from Research Funds from National Natural Science Foundation of China and China Postdoctoral Science Foundation.
Yuanyuan Liu, et al. “Effect of Water Chemistry and Hydrodynamics on Nitrogen Transformation Activity and Microbial Community Functional Potential in Hyporheic Zone Sediment Columns.” Environ. Sci. Technol. 2017. 51(9):4877-4886. DOI: 10.1021/acs.est.6b05018
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