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 (N) generated from human activities. Consequently, streams can also emit significant amounts of the potent greenhouse gas N₂O. Models and manipulative experiments now suggest that hydrology regulates the balance between N removal and N₂O production, but validating this hypothesis under field conditions has been difficult. We aimed to redress this knowledge gap by measuring changes in the concentration and isotopic composition of NO₃¯ (δ¹⁸O-NO₃¯, δ¹⁵N-NO₃¯) and N₂O (δ¹⁸O-N₂O, δ¹⁵N-N₂O, ¹⁵N-N₂O site preference) in the sediments and surface water of a 30 m stream reach as discharge dropped from 2.7 to 1.8 m3 s−1. Over the eight-day measurement period the changes in conductivity, δ¹⁸O-H₂O, and ²²²Rn indicated that hyporheic mixing decreased and net groundwater inputs increased as discharge declined. This coincided with increases in surface water NO₃¯ (1–3 mg N 1¯¹) and N₂O (700–1000% saturation) that were beyond what could be explained by increased groundwater N inputs. Instead, both N₂O and NO₃¯ isotopic composition indicated that concentration increases were caused by increasing within-stream production (nitrification), rather than decreased reduction (denitrification), as hyporheic exchange decreased. This highlights the importance of oxidising processes in regulating N cycling even under strongly heterotrophic conditions (productivity/respiration: 0.005–0.2). Together these findings provide a first empirical confirmation that relatively short term (daily-weekly) stream flow dynamics directly regulate biological cycling of both NO₃¯ and N₂O.