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

Abstract

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 ( ), nitrate ( ), oxygen (O 2 ), dissolved inorganic carbon, and 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 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 reduction to comprised almost half of the total reduction. At these stations, the removal of fixed N was inefficient due to high effluxes of . 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 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.