Enhanced benthic nitrous oxide and ammonium production after natural oxygenation of long‐term anoxic sediments

Abstract

Coastal and shelf sediments are central in the global nitrogen (N) cycle as important sites for the removal of fixed N. However, this ecosystem service can be hampered by ongoing deoxygenation in many coastal areas. Natural reoxygenation could reinstate anoxic sediments as sites where fixed N is removed efficiently. To investigate this further, we studied benthic N cycling in previously long-term anoxic sediments, following a large intrusion of oxygenated water to the Baltic Sea. During three campaigns in 2016-2018, we measured in situ sediment-water fluxes of ammonium (NH þ 4 ), nitrate (NO À 3 ), oxygen (O 2 ), dissolved inorganic carbon, and NO À 3 reduction processes using benthic chamber landers. Sediment microprofiles of O 2 , nitrous oxide (N 2 O), and hydrogen sulfide were measured in sediment cores. At a permanently oxic station, denitrification to N 2 was the main NO À 3 reduction process. Benthic N 2 O production appeared to be linked to nitrification, although no net N 2 O fluxes from the sediment were detected. At newly oxygenated sites, dissimilatory NO À 3 reduction to NH þ 4 comprised almost half of the total NO À 3 reduction. At these stations, the removal of fixed N was inefficient due to high effluxes of NH þ 4 . Sedimentary N 2 O production was associated with incomplete denitrification, accounting for 41-88% of the total denitrification rate. Microprofiling revealed algae aggregates as potential hotspots of seafloor N 2 O production. Our results show that transient oxygenation of euxinic systems initiates benthic NO À 3 reduction, but may not lead to efficient sedimentary removal of fixed N. Instead, recycling of N compounds is promoted, which may accelerate the return to anoxia.

The fate of fixed nitrogen in Santa Barbara Basin sediments during seasonal anoxia

“…Nevertheless, the previous estimate includes large uncertainties and the rates calculated from stable isotope mass balance represent signals integrated over multiple seasons (Sigman et al, 2003), whereas our measurements represent snapshots obtained in one season of one year when the bottom water NO3was not depleted. N2 production rates at seasons more depleted in NO3concentrations in the bottom water compared to our study might more closely resemble rates estimated by Sigman et al N2 production rates in this study were higher than most of those reported in other studies using in-situ incubations with benthic flux chambers (Bonaglia et al, 2017;De Brabandere et al, 2015;Hall et al, 2017;van Helmond et al, 2020;Hylén et al, 2022). Elevated rates in the SBB are likely a result of the high organic matter content of sediment (4.1 -6.8% total organic carbon; Table 1), supporting high microbial respiration rates, and little (max.…”

“…These rates were up to an order of magnitude higher than those measured using shipboard whole-core incubations (3.5 ± 1.0 μmol m -2 d -1 ) with samples from a similar depth (544 m) in the anoxic part of the Soledad Basin (Townsend-Small et al, 2014). A recent study using in-situ chamber incubations with 15 NO3in the Eastern Gotland Basin reported rates (~15 -68 μmol m -2 d -1 ) similar to or higher than the rates we measured in the SBB (Hylén et al, 2022). Because the physicochemical context of the Soledad Basin is more similar to the SBB than the Eastern Gotland Basin, we expected the N2O production rates in the Soledad Basin to be close to those in the SBB.…”

Biogeochemical functioning of the Baltic Sea

“…Denitrification dominates in Baltic Sea sediments (van Helmond et al 2020;Hietanen and Kuparinen, 2008;Jäntti et al, 2011), although DNRA may be more active under certain conditions (Bonaglia et al, 2017;Jäntti and Hietanen, 2012). Anaerobic ammonium oxidation (anammox), leading to the loss of fixed N as N 2 gas, has been found to be insignificant in Baltic Proper sediments (Hylén et al, 2022). Microbial denitrification rates have been shown experimentally to be accelerated by the presence of bioturbating meiofauna, especially of the phylum Nematoda (Bonaglia et al, 2014).…”

Particle-associated denitrification is the primary source of N2O in oxic coastal waters

“…Numerous studies have explored the processes and rates of N cycling in the eutrophied coastal oceans, providing a valuable baseline understanding of the intense N dynamics in this critical system 6,9,10 . However, the pathways and production rates of nitrous oxide (N 2 O), a potent greenhouse gas and ozone-depleting substance, which represents an important climate feedback of the marine N cycle to anthropogenic perturbation, are much less studied compared to other N cycling processes, and have been only recently investigated in a limited number of coastal sites such as the Chesapeake Bay 11 , the Baltic Sea 12,13 , and the inner shelf of the East China Sea and South China Sea 14,15 . This lack is probably due to the technical challenge of simultaneously disentangling the complex N 2 O production processes and quantifying the rates in highly dynamic coastal systems.…”