Statistical Modeling to Predict N2O Production Within the Hyporheic Zone by Coupling Denitrifying Microbial Community Abundance to Geochemical and Hydrological Parameters

Abstract:

The hyporheic zone (HZ) of streams may be a significant source of nitrous oxide (N₂O). However, the biogeochemical processes controlling N₂O emissions remain poorly constrained due to difficulties in obtaining high-resolution chemical, physical, and biological data from streams. Our research elucidates specific controls on N₂O production within the HZ by coupling the distribution of denitrifying microbial communities to flow dynamics (i.e. hydraulics and streambed morphology) and biogeochemical processes. We conducted a large-scale flume experiment that allowed us to constrain streambed morphology, flow rate, organic carbon loading, grain size distribution, and exogenous nitrate loading while enabling regular monitoring of dissolved oxygen, pH, alkalinity, nitrogen species, and elemental concentrations in the HZ. We also employed real-time PCR (qPCR) to quantify the distribution of denitrifying functional genes (nirS and nosZ, nitrite reductase and nitrous oxide reductase genes, respectively) in HZ sediment cores as a measure of denitrifying microorganism abundance. A steady increase in N₂O was observed after 8 hours of residence time with a peak in concentration (9.5 µg-N/L) recorded at hour 18. Abundance of nosZ increased an order of magnitude between hours 8 and 18 (2.6x10⁶ to 2.1x10⁷ gene copy #/g dry sediment). nirS abundance remained within the same order of magnitude between hours 8 and 18 (1.7x10⁷ to 3.8x10⁷). Linear and nonlinear mixed-effects models were used to investigate N₂O production in the HZ as a function of total nitrogen, nirS, nosZ, residence time, and dissolved oxygen. N₂O production was localized at redox-controlled hotspots within the subsurface and concentrations were strongly correlated with the availability of nitrogen when an interaction with nosZ abundance was considered. On-going analysis will provide predictions of N₂O production and support for conditions under which the HZ could be a significant contributor of N₂O emissions. These results are also being used to parameterize a reactive transport model for predicting N₂O production from stream sediments with different bedform morphologies, flow rates, and reactant concentrations.