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 (N 2 O) to the atmosphere, but to what extent abiotic processes may contribute to lacustrine N 2 O production remains uncertain. We assessed pathways of N 2 O production and reduction in the water column of meromictic and iron-rich Lake La Cruz, Spain, including chemodenitrification-induced N 2 O formation via the reaction of reactive nitrogen (N) (e.g., NO À2 ) with ferrous iron (Fe [II]). In the oxic waters ($8-10 m), N 2 O concentrations above atmospheric equilibrium were associated with comparatively low δ 15 N-N 2 O, high δ 15 N-NH þ 4 , and high N 2 O 15 N-site-preference (SP) values (up to $29‰), suggesting N 2 O production by nitrification. N 2 O 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), N 2 O consumption was indicated by increasing SP values and a δ 18 O-vs.-δ 15 N relationship (1.8-2.9) typical for stand-alone N 2 O 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 15 N-labeled or unlabeled substrate, NO À 2 was reduced by Fe 2+ to N 2 O, even at low nitrite concentrations (5 μM NO À 2 ). In the water column, the spatial separation of NO À 2 and Fe(II) during our samplings appears to preclude elevated rates of chemodenitrification, but during periods of overlapping NO À 2 and Fe(II) in Lake La Cruz, and potentially in other lakes, its distinct N 2 O δ 18 Ovs.-δ 15 N relationship of $1 : 1, as experimentally determined, could help to detect it.Nitrous oxide (N 2 O) is a potent greenhouse gas and an important ozone-layer-depleting substance in the atmosphere (Wang et al. 1976;Ravishankara et al. 2009). Terrestrial ecosystems including inland waters are relevant sources of both natural and anthropogenic N 2 O, but with a high uncertainty in their contribution to global N 2 O emissions (Maavara et al. 2019). N 2 O production in these ecosystems has been attributed primarily to microbially mediated nitrogen (N)-transforming processes (Tian et al. 2020). The relative contribution of abiotic N 2 O production mechanisms is still a matter of debate, particularly in lakes.The main microbial N 2 O-producing processes are nitrification, nitrifier denitrification, and denitrification. During the first step of nitrification, where ammonia-oxidizing bacteria (AOB) and/or ammonia-oxidizing archaea (AOA) oxidize