FIG 6.

Plot of dissolved nitrogen versus dissolved argon concentrations at sites sampled for metagenomics analysis. Dissolved Ar and N₂ are expressed in milliliters of the respective gas at standard temperature (273.15 K) and pressure (101.325 kPa) per kg of water. Bold lines represents gas concentrations in water which are in equilibrium with the atmosphere at the given temperature. Arrows indicate competing processes that can alter gas concentrations, and gray dashed lines indicate excess air in groundwater relative to atmosphere (with upper and lower lines representing addition of unfractionated excess air relative to equilibrium concentrations at 10 and 15°C). The black horizontal arrow depicts additional excess N₂ inferred to be from biological processes (denitrification or anammox). Reconstructed N₂ data (in equilibrium with inert atmospheric gases), based on groundwater recharge temperatures and excess air concentrations derived from dissolved Ne and Ar data (shown as black circles). Recharge temperature is the temperature of recharging water at the time it enters the groundwater system. Excess air is dissolved air in excess of the equilibrium soluble amount at the given recharge temperature (thought to originate from processes such as bubble entrapment occurring during recharge and subsequent dissolution under increased hydrostatic pressure). The difference between these and the measured N₂ data (numbered blue circles and their shift along the x axis relative to the corresponding numbered black circles) indicates the amount of N₂ in excess, formed via denitrification and/or anammox at each site. Error bars show the combined statistical standard uncertainty from all processes and calculations contributing to the measurement uncertainty, expressed as 1 standard deviation. Groundwater from wells SR1-2, BW8, BW19, and RF2-3 is characterized as oxic, while groundwater from E1 and N3 is dysoxic-suboxic (Table S1).