Isotopic signatures of biotic and abiotic N2O production and consumption in the water column of meromictic, ferruginous Lake La Cruz (Spain)
Abstract
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 (similar to 8-10 m), N2O concentrations above atmospheric equilibrium were associated with comparatively low delta N-15-N2O, high delta N-15-NH4+, and high N2O N-15-site-preference (SP) values (up to similar to 29 parts per thousand), suggesting N2O production by nitrification. N2O concentrations were highest (23-33 nM) near the depth of oxygen depletion (similar to 11-14.5 m), likely due to production by nitrifier denitrification and/or denitrification, as indicated by decreasing SP values (as low as 12 parts per thousand). Further below (similar to 14.5-17 m), N2O consumption was indicated by increasing SP values and a delta O-18-vs.-delta N-15 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 N-15-labeled or unlabeled substrate, NO2- was reduced by Fe(2+ )to N2O, even at low nitrite concentrations (5 mu M 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 delta O-18 vs.-delta N-15 relationship of similar to 1 : 1, as experimentally determined, could help to detect it. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000553512Publication status
publishedExternal links
Journal / series
Limnology and OceanographyVolume
Pages / Article No.
Publisher
WileyMore
Show all metadata