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

Quantifying net N-2 flux, the balance between nitrogen (N) fixation and denitrification, may help inform the decades-long debate about the significance of N relative to phosphorus (P) limitation in pond, lake, and reservoir ecosystems. Dissolved N-2:Ar ratios can provide information about the role of these two processes in lakes but have not been experimentally verified. We measured dissolved N-2:Ar patterns in twelve experimental pond mesocosms representing a N:P gradient ranging from 7 to 835 (molar) that was driven by large differences in NO3-N availability. Mesocosms with NO3-N < 0.1 mg L-1 had N-2:Ar saturation ratio values consistently below equilibrium (i.e., net N fixation), whereas mesocosms with NO3-N > 1 mg L-1 had N-2:Ar values consistently above equilibrium (i.e., net denitrification, demonstrating a shift from N-2 import to export as pond N availability increased). Both planktonic N fixation estimated from isotope mixing models and N-2 fluxes measured by sediment core incubations confirmed a shift from net N fixation to net denitrification as NO3-N concentrations increased. N-2: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 N-2-N flux. N-2:Ar saturation ratios were consistently < 1 in mesocosms with low N availability and high contributions of fixed N. Mesocosms with high N loading and low N fixation had high saturation ratios indicative of denitrification in benthic samples but dissolved gases collected from the photic zone were less consistent. Our water column N-2:Ar saturation ratio measurements combined with quantitative measures of N fixation and benthic N-2 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 within shallow lakes at broad scales.