Humans are fundamentally changing the nitrogen (N) cycle. Simple measures to detect aquatic N₂ fluxes are needed. We measured dissolved N₂:Ar patterns in twelve experimental pond mesocosms representing a N:P input gradient ranging from ~2 to 110 (molar). Low N:P mesocosms had N₂:Ar saturation ratio values consistently below equilibrium (net N fixation), and high N:P mesocosms had N₂:Ar values above equilibrium (net denitrification). Both planktonic N fixation estimated from isotope mixing models and N₂ fluxes measured by sediment core incubations confirmed a shift from net N fixation to net denitrification as N:P ratios increased. Photic zone N₂:Ar saturation ratios were negatively correlated with increasing measured N fixation while saturation ratios from benthic waters were positively correlated with increasing sediment N₂-N flux. Field measurements of N₂:Ar ratios in excess of equilibrium in the hypolimnion of 8 off-channel lakes of the Lower Mississippi River (LMR) confirmed that these backwater habitats are significant denitrification habitatsduring summer disconnection from the river. However, we also observed evidence for net N fixation with depleted N₂:Ar ratios in the epilimnion of all lakes. Our mesocosm results combined with our field study demonstrate that N₂:Ar ratios can be used to identify hotspots or hot moments of net denitrification and N fixation. Delineating the direction and magnitude of N₂ flux in space and time at larger scales may improve understanding of global changes in aquatic N cycling.

Primary Presenter: Jason Taylor, USDA-ARS NSL (jason.taylor@usda.gov)

Authors:

Isabelle Andersen, Baylor University, Dept. of Biology  (isabelle_andersen1@baylor.edu)

Alex Hoke, Baylor University, Dept. of Biology  (alex_hoke1@baylor.edu)

Cliff Ochs, University of Mississippi, Dept. of Biological Sciences  (byochs@olemiss.edu)

Patrick Kelly, Wisconsin Department of Natural Resources  (patrick.kelly@wisconsin.gov)

Thad Scott, Baylor University, Dept. of Biology  (thad_scott@baylor.edu)