Isotopic signatures of biotic and abiotic <scp> N ₂ O </scp> production 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 atmosphere, but what extent abiotic processes may contribute lacustrine N2O production remains uncertain. We assessed pathways and reduction in water column meromictic iron-rich Lake La Cruz, Spain, including chemodenitrification-induced formation via reaction reactive nitrogen (N) (e.g., NO 2 − $$ {\mathrm{NO}}_2^{-} ) with ferrous iron (Fe[II]). In oxic waters (~8–10 m), concentrations above atmospheric equilibrium were associated comparatively low δ15N-N2O, high δ 15 N - NH 4 + {\updelta}^{15}\mathrm{N}\hbox{-} {\mathrm{NH}}_4^{+} , 15N-site-preference (SP) values (up ~29‰), suggesting by nitrification. highest (23–33 nM) near depth oxygen depletion (~11–14.5 likely due nitrifier denitrification and/or denitrification, as indicated decreasing SP (as 12‰). Further below (~14.5–17 consumption was increasing a δ18O-vs.-δ15N relationship (1.8–2.9) typical for stand-alone reduction. The coupled N-vs.-O isotope signatures thus highlight spatial, redox-dependent separation incomplete complete denitrification. incubations sterile-filtered lake 15N-labeled or unlabeled substrate, reduced Fe2+ N2O, even at nitrite (5 μM ). column, spatial Fe(II) during our samplings appears preclude elevated rates chemodenitrification, periods overlapping potentially other lakes, its distinct ~1 : 1, experimentally determined, could help detect it.