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Evidence for Microbial Mediated Nitrate Cycling Within Floodplain Sediments During Groundwater Fluctuations

Conceptual nitrogen cycle within the Rifle aquifer based on measurements made in the present study, and recent molecular based studies. The figure shows the switch in metabolic processes at the capillary fringe, and also the likely positioning of anaerobic chemolithoautotrophs in the naturally reduced zones of the aquifer. This aquifer is underlain by an impermeable Wasatch layer indicated in the figure.

Schematic of the biogeochemical pathways that cycle nitrogen at the capillary fringe of the Rifle floodplain in Colorado.

[Reprinted under a Creative Commons Attribution 4.0 International License (CC BY 4.0) from Bouskill, N. J., et al. "Evidence for Microbial Mediated NO3− Cycling Within Floodplain Sediments During Groundwater Fluctuations." Frontiers in Earth Sciences 7, 189 (2019). DOI:10.3389/feart.2019.00189.‌]

The Science

Alluvial sediments subject to the seasonal rise and fall of groundwater are regions of outsized biogeochemical activity relative to their spatial extent in many floodplain environments. This study documents significant changes in the nitrogen cycle under fluctuating hydrological conditions.

The Impact

Researchers used natural abundance stable isotopes to document pathways and mechanisms leading to the accumulation and dissipation of nitrate under aerobic and anaerobic conditions in floodplain sediments at a Rifle, Colo., field site. Their findings significantly improve the understanding of global nitrogen cycling.

Summary

A team of researchers from Lawrence Berkeley National Laboratory and Stanford University characterized subsurface nitrogen biogeochemistry at the Rifle field site where snowmelt-driven fluctuations in water table depth change the saturation profile of vadose zone sediments and hence their redox status. The team collected depth-resolved water samples over a year. They analyzed porewater nitrogen concentrations, nitrous oxide and nitrogen gas, and the natural abundance stable isotopes of nitrate (δ15NNO3 and (δ18ONO3) to determine the role that abiotic and biological mechanisms play in the fate of nitrate. The study concludes that biological nitrogen cycling in Rifle sediments is predominantly attributable to temporally uncoupled nitrification-denitrification reactions. As the water table rises, these reactions occur sequentially as aerobic conditions that favor nitrification and the accumulation of nitrate give way to anaerobic conditions, which favor denitrification rather than anaerobic ammonium oxidation.

Principal Investigator

Nicholas Bouskill
Lawrence Berkeley National Laboratory
[email protected]

Program Manager

Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This work was performed as part of Lawrence Berkeley National Laboratory’s Sustainable Systems Scientific Focus Area funded by the Office of Biological and Environmental Research within the U.S. Department of Energy’s Office of Science under contract DE-AC02-05CH11231.

References

Bouskill, N. J., et al. "Evidence for Microbial Mediated NO3− Cycling Within Floodplain Sediments During Groundwater Fluctuations." Frontiers in Earth Sciences 7 189  (2019). https://doi.org/10.3389/feart.2019.00189.