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 l−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.