Oscillations in soil bacterial redox reactions

• We report carbon and ammonium spatial oscillations in soils.
• We associate these with activator–inhibitor type microbial ecosystem dynamics in soils.
• A competitive heterotroph-fermenter system exhibits boom-and-bust oscillations.
• The oscillations are relaxational in conditions of starvation.
• We expect that spatial diffusion will cause travelling waves which explain the observations.

Spatial oscillations in soil contaminant concentration profiles are sometimes observed, but rarely commented on, or are attributed to noisy data. In this paper we consider a possible mechanism for the occurrence of oscillatory reactant profiles within contaminant plumes. The bioremediative reactions which occur are effected by bacteria, whose rôle is normally conceived of as being passive. Here we argue that competition, for example between heterotrophic and fermentative bacteria, can occur in the form of an activator–inhibitor system, thus promoting oscillations. We describe a simple model for the competition between two such microbial populations, and we show that in normal oligotrophic groundwater conditions, oscillatory behaviour is easily obtained. When such competition occurs in a dispersive porous medium, travelling waves can be generated, which provide a possible explanation for the observed soil column oscillations.