Dissimilatory nitrate reduction to ammonium coupled to Fe(II) oxidation in sediments of a periodically hypoxic estuary

Estuarine sediments are critical for the remediation of large amounts of anthropogenic nitrogen (N) loading via production of N₂ from nitrate by denitrification. However, nitrate is also recycled within sediments by dissimilatory nitrate reduction to ammonium (DNRA). Understanding the factors that influence the balance between denitrification and DNRA is thus crucial to constraining coastal N budgets. A potentially important factor is the availability of different electron donors (organic carbon, reduced iron and sulfur). Both denitrification and DNRA may be linked to ferrous iron oxidation, however the contribution of Fe(II)-fueled nitrate reduction in natural environments is practically unknown. This study investigated how nitrate-dependent Fe²⁺ oxidation affects the partitioning between nitrate reduction pathways using ¹⁵N-tracing methods in sediments along the salinity gradient of the periodically hypoxic Yarra River estuary, Australia. Increased dissolved Fe²⁺ availability resulted in significant enhancement of DNRA rates from around 10–20% total nitrate reduction in control incubations to over 40% in those with additional Fe²⁺, at several sites. Increases in DNRA at some locations were accompanied by reductions in denitrification. Significant correlations were observed between Fe²⁺ oxidation and DNRA rates, with reaction ratios corresponding to the stoichiometry of Fe²⁺-dependent DNRA. Our results provide experimental evidence for a direct coupling of DNRA to Fe²⁺ oxidation across an estuarine gradient, suggesting that Fe²⁺ availability may exert substantial control on the balance between retention and removal of bioavailable N. Thus, DNRA linked to Fe²⁺ oxidation may be of general importance to environments with Fe-rich sediments