Nitrous oxide cycling and isotopic overprinting in lake basins with contrasting mixing regimes

Nitrous oxide (N 2 O) is a potent greenhouse gas and ozone-destroying substance, and lakes contribute variably to its atmospheric budget. We investigated the seasonal variability of N 2 O cycling in two contrasting basins of Lake Lugano, Switzerland: the meromictic North Basin (NB) and the holomictic South Basin (SB). Over one full seasonal cycle, we measured N 2 O concentration and site-specific isotopomer compositions to assess spatio-temporal dynamics. Both basins showed near-neutral N 2 O exchange with the atmosphere, without clear seasonal patterns of N 2 O emission versus uptake (average air–water exchange flux of 0.04 ± 0.3 µmol m −2 d −1 ). Keeling-plot analysis indicated a nitrification-driven N 2 O source in the oxic water column in both basins, as inferred from its position-specific isotopic nitrogen composition. In the anoxic water column in the North Basin, N 2 O was persistently depleted, whereas in the deep South Basin N 2 O consumption was observed only during late-stage stratification. Net N 2 O consumption in the hypolimnia of both basins was accompanied by isotopic shifts toward lighter δ 15 N bulk and δ 15 N β values, contrary to trends observed in pure cultures. These inverse apparent isotope effects suggest concurrent N 2 O production during net consumption. Using a simplified box-model (i.e., isotope mass balance) approach, we estimated that 5–17% of the consumed N 2 O needed to be replaced by simultaneous N 2 O production in order to match the observed in situ δ 15 N bulk -N 2 O data. Our findings highlight the complexity of N 2 O cycling in lake systems and reveal that isotopic overprinting from simultaneous N 2 O production and consumption can obscure traditional isotope-based interpretations. At the same time, they underscore the value of stable isotopes in disentangling N 2 O production and consumption mechanisms.