Nitrate reduction coupled with pyrite oxidation in the surface sediments of a sulfide‐rich ecosystem

Most studies of denitrification have focused on organic carbon as an electron donor, but reduced sulfur can also support denitrification. Few studies have reported nitrate (NO 3 − ) reduction coupled with pyrite oxidation and its stoichiometry in surface sediments, especially without experimental pyrite addition. In this study, we evaluated NO 3 − reduction coupled with sulfur oxidation by long‐term incubation of surface sediments from a sulfide‐rich ecosystem in Akita Prefecture, Japan. The surface sediments were sampled from a mud pool and a riverbed. Fresh sediments and water were incubated under anoxic conditions (and one oxic condition) at 20°C. NO 3 − addition increased the SO 4 2− concentration and decreased the NO 3 − concentration. SO 4 2− production (∆SO 4 2− ) was strongly and linearly correlated with NO 3 − consumption (∆NO 3 − ) during the incubation period ( R 2  = 0.983, P  < 0.01, and n  = 8), and the slope of the regression (∆NO 3 − /∆SO 4 2− ) and the stoichiometry indicated sulfur‐driven NO 3 − reduction by indigenous autotrophic denitrifying bacteria. Framboidal pyrite and marcasite (both FeS 2 ) were present in the sediments and functioned as the electron donors for autotrophic denitrification. Both ∆NO 3 − and ∆SO 4 2− were higher in the riverbed sediment than in the mud pool sediment, likely because of the higher amount of easily oxidizable S (pyrite) in the riverbed sediment. Consistently low ammonium (NH 4 + ) concentrations indicated that NO 3 − reduction by dissimilatory NO 3 − reduction to NH 4 + was small but could not be disregarded. Our results demonstrate that sulfide‐rich ecosystems with easily oxidizable metal‐bound sulfides such as FeS 2 near the ground surface may act as denitrification hot spots.