Direct O2 control on the partitioning between denitrification and dissimilatory nitrate reduction to ammonium in lake sediments

Lacustrine sediments are important sites of fixed-nitrogen (N) elimination through the reduction of nitrate to N-2 by denitrifying bacteria, and they are thus critical for the mitigation of anthropogenic loading of fixed N in lakes. In contrast, dissimilatory nitrate reduction to ammonium (DNRA) retains bioavailable N within the system, promoting internal eutrophication. Both processes are thought to occur under oxygen-depleted conditions, but the exact O-2 concentration thresholds particularly of DNRA inhibition are uncertain. In O-2 manipulation laboratory experiments with dilute sediment slurries and (NO3-)-N-15 additions at low-to sub-micromolar O-2 levels, we investigated how, and to what extent, oxygen controls the balance between DNRA and denitrification in lake sediments. In all O-2-amended treatments, oxygen significantly inhibited both denitrification and DNRA compared to anoxic controls, but even at relatively high O-2 concentrations (>= 70 mu mol L-1), nitrate reduction by both denitrification and DNRA was observed, suggesting a relatively high O-2 tolerance. Nevertheless, differential O-2 control and inhibition effects were observed for denitrification versus DNRA in the sediment slurries. Below 1 mu mol L-1 O-2, denitrification was favoured over DNRA, while DNRA was systematically more important than denitrification at higher O-2 levels. Our results thus demonstrate that O-2 is an important regulator of the partitioning between N loss and N recycling in sediments. In natural environments, where O-2 concentrations change in near-bottom waters on an annual scale (e.g., overturning lakes with seasonal anoxia), a marked seasonality with regards to internal N eutrophication versus efficient benthic fixed-N elimination can be expected.