『Abstract
Conventional understanding of the nitrogen cycle in marine sediments
has changed in recent years with the discovery of an alternative
pathway for ammonia oxidation via the reduction of manganese oxides
(during anoxic nitrification). In anoxic sediments, the potential
for manganese oxides to serve as oxidant for nitrification may
be considerable yet previous work on manganese-rich sediments
has suggested anoxic nitrification may not be significant. In
this study, the potential for anoxic nitrification in a range
of sediment types was investigated. Laboratory incubation of sediment
from three sites on the Humber Estuary, a microbially diverse
environment, showed anoxic accumulation of nitrate, nitrite and
dinitrogen gas, with and without the addition of synthetic manganese
oxides. Incubation experiments confirmed anoxic nitrification
as microbially mediated, with heat-killed controls yielding negative
results. The anoxic nitrification reaction significantly depleted
ammonia concentrations, and occurred simultaneously with manganese-,
iron- and sulphate reduction, and methanogenesis. Taken in conjunction
with other studies, results suggest anoxic nitrification may not
only be dependent on total manganese concentrations but on manganese
dynamics. Anoxic nitrification may be explained as a non-steady
state reaction, dependent on the recent stability of a sediment
system. Physical perturbation of sediments may cause the redistribution
and/or introduction of manganese oxides and promote anoxic nitrification.
The significance and persistence of anoxic nitrification is likely
to depend on the frequency and magnitude of sediment perturbation,
which explains why the reaction varies so widely across studied
sites, and why it may not occur in some manganese-rich sediment.
Keywords: Manganese oxides; Nitrogen cycle; Ammonia; Diagenesis;
Humber Estuary, UK』
1. Introduction
2. Methodology
2.1. Sediment sampling
2.2. Incubation experiments
2.3. Synthetic MnO2
2.4. Pore-water analysis
2.5. Gas analysis
3. Results and discussion
3.1. Spurn Head sediment
3.1.1. Biogeochemical cycles
3.1.2. Anoxic nitrification
3.2. Skeffling sediment
3.2.1. Biogeochemical cycles
3.2.2. Anoxic Nitrification
3.3. Welwick sediment
3.3.1. Biogeochemical cycles
3.3.2. Anoxic nitrification
4. Conclusions
4.1. The anoxic nitrification reaction
4.2. Implications
Acknowledgement
References