Investigating a Cryptic Iron-Nitrogen Cycle in Lake Okeechobee Sediments`

The overall objective was to elucidate the effect of iron (Fe) on nitrogen (N) diagenesis in Lake Okeechobee. Somewhat counterintuitively, sediment ammonium (NH+4) inventories decreased during algal growth as dissolved organic nitrogen (DON) inventories increased. Whole core incubations were staged for denitrification experiments using isotopic N tracer. Core incubations showed the percentage of sediment N removal increase between summer (25 ± 21 %) and winter (39 ± 13 %). The amendment of Fe2+ enhanced this seasonal effect likely via dissimilatory nitrate reduction to ammonium (DNRA). The isotopic signature of N2 flux also suggested an additional, sedimentary, N2 source via Fe coupled anaerobic oxidation of ammonium (feammox). Sediment slurry incubations supported the occurrence of both DNRA and feammox, showing first that nitrate (NO3−) was converted to NH4+ via DNRA, which contributed 23-26% of overall NO3− reduction.
Fe amendment in slurries similarly stimulated the feammox process. However, aged Fe minerals accumulated linearly with N bound to Fe (Fe-N) in a subseasonal sediment time series, suggesting Fe-organic matter aggregation may lower the sediment NH4+ equilibrium concentration and benthic flux.