In-situ N2:Ar ratios describe the balance between nitrogen fixation and denitrification in shallow eutrophic experimental lakes

Authors: Taylor, Jason; ANDERSEN, ISABELLE - Baylor University; HOKE, ALEXA - Baylor University; KELLY, PATRICK - Rhodes College; SCOTT, JEFFERSON - Baylor University

Submitted to: Biogeochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/23/2023
Publication Date: 7/17/2023
Citation: Taylor, J.M., Andersen, I.M., Hoke, A.K., Kelly, P.T., Scott, J.T. 2023. In-situ N2:Ar ratios describe the balance between nitrogen fixation and denitrification in shallow eutrophic experimental lakes. Biogeochemistry. 166:283-301. https://doi.org/10.1007/s10533-023-01063-6.
DOI: https://doi.org/10.1007/s10533-023-01063-6

Interpretive Summary: Excess nitrogen and phosphorus from urban, industrial and agricultural sources can impact water quality by initiating harmful algal blooms (HABS), hypoxia, and overall habitat degradation in freshwater and marine ecosystems. Scientists need better tools for quantifying processes that influence the role of nitrogen in water quality degradation. Dissolved nitrogen gas import (nitrogen fixation) and export (denitrification) from water bodies are key components of understanding the role of nitrogen in water quality. We used twelve experimental pond systems that were dosed with varying degrees of nitrogen to quantify nitrogen fixation and denitrification rates while simultaneously measuring nitrogen gas concentrations from water samples. We found that when systems were fixing nitrogen gas concentrations were below expected values and when systems were removing nitrogen via denitrification, gas concentrations were above expectations. Our results confirm that simple measures of nitrogen gas concentrations can be used by scientists to track dominant nitrogen cycling processes and provide important info to water quality managers when developing strategies for reducing nutrient pollution impacts to lakes, rivers and coastal ecosystems.

Technical Abstract: Quantifying net N2 flux, the balance between nitrogen (N) fixation and denitrification, may help solve the decades-long debate about the significance of N relative to phosphorus (P) limitation in pond, lake, and reservoir ecosystems. Dissolved N2:Ar ratios can provide information about the role of these two processes in lakes but have not been experimentally verified. We measured dissolved N2:Ar patterns in twelve experimental pond mesocosms representing a N:P input gradient ranging from ~2 to 110 (molar). Mesocosms with the lowest N:P ratios had N2:Ar saturation ratio values consistently below equilibrium (i.e., net N fixation), whereas mesocosms with high N:P ratios had N2:Ar values consistently above equilibrium (i.e., net denitrification, demonstrating a shift from N2 import to export as pond N:P ratios increased. Both planktonic N-fixation estimated from isotope mixing models and N2 fluxes measured by sediment core incubations confirmed a shift from net N fixation to net denitrification as N:P ratios increased. N2:Ar saturation ratio values in the photic zone were negatively correlated with increasing measured N fixation while saturation ratios from benthic waters were positively correlated with increasing sediment N2-N flux. Our water column N2:Ar saturation ratio measurements combined with quantitative measures of N fixation and benthic N2 flux confirm that grab samples for dissolved gasses can provide critical information regarding the relative contribution of net N fixation and denitrification to balancing N availability relative to P within shallow lakes at broad scales