Isotopic signatures of biotic and abiotic N2Oproduction and consumption in the water column of meromictic, ferruginous Lake La Cruz (Spain)

Lakes can be important sources of the potent greenhouse gas nitrous oxide (N2O) to the atmosphere, but to what extent abiotic processes may contribute to lacustrine N2O production remains uncertain. We assessed pathways of N2O production and reduction in the water column of meromictic and iron‐rich Lake La Cruz, Spain, including chemodenitrification‐induced N2O formation via the reaction of reactive nitrogen (N) (e.g., NO2−) with ferrous iron (Fe[II]). In the oxic waters (~8–10 m), N2O concentrations above atmospheric equilibrium were associated with comparatively low δ15N‐N2O, high δ15N‐NH4+, and high N2O 15N‐site‐preference (SP) values (up to ~29‰), suggesting N2O production by nitrification. N2O concentrations were highest (23–33 nM) near the depth of oxygen depletion (~11–14.5 m), likely due to production by nitrifier denitrification and/or denitrification, as indicated by decreasing SP values (as low as 12‰). Further below (~14.5–17 m), N2O consumption was indicated by increasing SP values and a δ18O‐vs.‐δ15N relationship (1.8–2.9) typical for stand‐alone N2O reduction. The coupled N‐vs.‐O isotope signatures thus highlight the spatial, redox‐dependent separation of incomplete and complete denitrification. In incubations with sterile‐filtered lake water and 15N‐labeled or unlabeled substrate, NO2−was reduced by Fe2+to N2O, even at low nitrite concentrations (5 μM NO2−). In the water column, the spatial separation of NO2−and Fe(II) during our samplings appears to preclude elevated rates of chemodenitrification, but during periods of overlapping NO2−and Fe(II) in Lake La Cruz, and potentially in other lakes, its distinct N2O δ18O‐vs.‐δ15N relationship of ~1 : 1, as experimentally determined, could help to detect it.