Direct O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; control on the partitioning between denitrification and dissimilatory nitrate reduction to ammonium in lake sediments

Adeline N Y Cojean,Fabio Lepori,Jakob Zopfi,Moritz F Lehmann,Claudia Frey,Alan Gerster

doi:10.5194/bg-16-4705-2019

Abstract

Abstract. Lacustrine sediments are important sites of fixed-nitrogen (N) elimination through the reduction of nitrate to N2 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 O2 concentration thresholds particularly of DNRA inhibition are uncertain. In O2 manipulation laboratory experiments with dilute sediment slurries and 15NO3- additions at low- to sub-micromolar O2 levels, we investigated how, and to what extent, oxygen controls the balance between DNRA and denitrification in lake sediments. In all O2-amended treatments, oxygen significantly inhibited both denitrification and DNRA compared to anoxic controls, but even at relatively high O2 concentrations (≥70 µmol L−1), nitrate reduction by both denitrification and DNRA was observed, suggesting a relatively high O2 tolerance. Nevertheless, differential O2 control and inhibition effects were observed for denitrification versus DNRA in the sediment slurries. Below 1 µmol L−1 O2, denitrification was favoured over DNRA, while DNRA was systematically more important than denitrification at higher O2 levels. Our results thus demonstrate that O2 is an important regulator of the partitioning between N loss and N recycling in sediments. In natural environments, where O2 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.

Highlights

Over the last decades, intensive human activities have dramatically affected the nitrogen (N) cycle in aquatic systems through elevated inputs of reactive N
In O2 manipulation laboratory experiments with dilute sediment slurries and 15NO−3 additions at lowto sub-micromolar O2 levels, we investigated how, and to what extent, oxygen controls the balance between dissimilatory nitrate reduction to ammonium (DNRA)
External N loading can lead to excessive algal blooms in the upper water column, and the subsequent decomposition of the sinking algal biomass is often associated with O2 depletion in the deeper water column and possibly seasonal or permanent anoxia (e.g. Blees et al, 2014; Lehmann et al, 2004, 2015)

Summary

Introduction

Intensive human activities have dramatically affected the nitrogen (N) cycle in aquatic systems through elevated inputs of reactive (biologically available) N. External N loading can lead to excessive algal blooms in the upper water column, and the subsequent decomposition of the sinking algal biomass is often associated with O2 depletion in the deeper water column and possibly seasonal or permanent anoxia Under oxygen-depleted conditions, both in the water column and in sediments, anaerobic N transformation processes such as denitrification, anammox and/or dissimilatory nitrate reduction to ammonium (DNRA) become important. While denitrification and anammox can mitigate excessive N loading (eutrophication) by converting reactive nitrogen (NO−3 , NO−2 , NH+4 ) to N2, which subsequently returns to the atmosphere, DNRA retains a bioavailable form of nitrogen within the ecosystem, fostering internal eutrophication of lakes (Tiedje, 1988)

Results

Discussion

Conclusion