Flow regulates biological NO₃¯ and N₂O production in a turbid sub-tropical stream

Abstract

Streams play a critical role in attenuating the excess reactive nitrogen generated from human activities. These systems can consequently also emit significant amounts of N₂O, a potent greenhouse gas. Models and manipulative experiments now suggest that hydrology regulates the balance between nitrogen removal and N₂O production. We aimed to empirically test this hypothesis by measuring changes in the concentration and isotopic composition of NO₃¯ (δ¹⁸O, δ¹⁵N) and N₂O (δ¹⁸O, δ¹⁵N, site preference) in hyporheic sediments and surface water of a 30 m reach over eight days of falling stream discharge (2.7 to 1.8 m³ s¯¹). The stream was persistently heterotrophic (productivity/respiration: 0.005 - 0.2), while changes in conductivity, δ¹⁸O-H₂O, and ²²²Rn indicated that hyporheic mixing decreased and net groundwater inputs increased as discharge declined. The shallow groundwater had high inorganic N concentrations (2 – 10 mg 1¯¹), but increased in groundwater inputs could not fully explain the concurrent increases in NO₃¯ (1 – 3 mg N 1¯¹) and N₂O (700 to 1000% saturation) in the surface water. Biologically, rather than solely hydrologically, regulated stream nitrogen export was confirmed by changes in N₂O and NO₃¯ isotopic composition. However, isotope patterns indicated that nitrification, not denitrification, increased surface water NO₃¯ and N₂O concentrations as hyporheic exchange decreased. These findings empirically demonstrate how flow dynamics regulate biological NO₃¯ production as well as transport, with implications for predicting aquatic N₂O emissions.