Daniele Tonina, University of Idaho Kevin Feris and Shawn Benner, Boise State University
The hyporheic zone is the band of saturated sediment surrounding the stream, where stream waters mix with pore-water. This zone plays an important role in the nitrogen cycle, which has been radically altered by anthropogenic food and energy production. Furthermore, this zone may be a significant source of the potent greenhouse gas nitrous oxide (N2O), potentially emitting for up to 0.7 Tg y-1, equivalent to 10% of global anthropogenic N2O emissions. While the degree of hyporheic exchange is strongly influenced by stream flow and streambed topography, the relationship between those physical processes and the resulting microbially-mediated geochemical reactions leading to N2O generation and release remains poorly understood. The goal of this interdisciplinary research is to understand, quantify, and parameterize the influence of streambed hydraulics and morphology on the emission of N2O from the hyporheic zone. The generation of N2O via denitrification primarily occurs within the streambed sediments where the catalyzing microbial community is present. Therefore, the mass transport of oxygen, nitrates, ammonium and N2O by hyporheic flow strongly influences reaction rates, residence times, and subsequent N2O production. By extension, stream flow and channel morphology presumably control, and may be effective predictors of, N2O generation rates. However, a number of important in-situ processes remain poorly understood. For example, the amount of reactive nitrogen converted to N2O versus N2 is quite small (0-6%) but highly uncertain, due in part to an incomplete understanding of the genetic composition of the dominant microbial community and the hydrologic factors affecting their distribution and activity. Additionally, controls on the contribution of in-situ reactive nitrogen generation vs. that delivered by the stream are not easily determined in natural systems. Indeed, the inherent spatiotemporal complexity of natural systems limits the strength of traditional observational approaches and precludes explicit expression of these interactions in predictive mathematical models. To overcome limitations of traditional observational field approaches, this research will use a series of manipulative large-scale flume experiments. Large-scale flume experiments will provide unprecedented control while maintaining intrinsically important variables such as a realistic microbial community, water composition, stream flow, and channel structure.
This research will develop new understanding of the fundamental interaction among surface-subsurface water exchange, nitrogen transformation and N2O emissions from the hyporheic zone and ultimately from streams. It will explain the role of the hyporheic zone as a biochemical transformation zone by coupling hyporheic hydraulics to biochemical reactions, through numerical-analytical models and flume experiments. Results from this work will act as building blocks for modeling N2O emissions from streams at the watershed scale. Thus, this first stage of controlled manipulative experimentation is essential in advancing our knowledge of water resources at the large scale.
