Sulfate reduction and a molybdate-induced soluble nitrogen flush in sediments during incubation

It has been known that molybdate inhibits sulfate-reducing bacteria (SRB). We observed a significant increase in soluble N from a salt marsh sediment sample after a molybdate-treated (MT) incubation when it was compared with the control. This observation led to an investigation into the source of this N flush from the sediment. The amount of the extractable N released during a MT incubation at 35°C increased with time up to 7 d reaching a plateau. The extractable N was mainly in organic forms during the first week of incubation followed by a quick conversion to ammonium N within 3 d. The pattern and forms of the increased extractable N during the MT incubation suggest an organic N source in the system induced by MT incubation. The net increase in extractable N during the MT incubation, i.e. the increased extractable N after correction for the control, is positively correlated with the sulfate reduction rate of the sample. We propose that the net increase in extractable N induced by the MT is a quantitative indicator of the SRB biomass N and its ratio to the net chloroform-fumigation–extraction N is an indicator of the SRB biomass N-to-total microbial biomass N ratio in a sediment.