Influence of the hydrodynamic regime on nitrogen transformation processes in lacustrine sediments

Vertical and temporal distribution of nitrogen transformation rates in two monomictic lake sediments. Rate process measurements were quantified from ¹⁵N-stable isotope addition experiments for Lakes Baldegg and Sarnen located in central Switzerland. Rates were sampled from 0-15 cm depth, and measured on a bi-monthly basis (March 2021-March 2022). Rates were determined from ¹⁵N labeled substrate additions: (Exp1) ¹⁵N-NO₃^-; (Exp2) ¹⁵N-NH₄⁺ + ¹⁴N-NO₂^-; (Exp3) ¹³C/¹⁵N-lypholized algae biomass (98% ¹³C- and ¹⁵N-labelled Spirulina platensis, Sigma-Aldrich). From the headspace, the production of ¹⁴N¹⁵N and ¹⁵N¹⁵N (N₂ gas) was determined by gas-chromatography isotope ratio mass spectrometry.