Hyperalkaline surface environments can occur naturally or because of contamination by hydroxide-rich wastes. The high pH produced in these areas has the potential to lead to highly specialised microbial communities and unusual biogeochemical processes. This paper reports an investigation into the geochemical processes that are occurring in a buried, saturated, organic–rich soil layer at pH 12.3. The soil has been trapped beneath calcite precipitate (tufa) that is accumulating where highly alkaline leachate from a lime kiln waste tip is emerging to atmosphere. A population of anaerobic alkaliphilic bacteria dominated by a single, unidentified species within the Comamonadaceae family of β-proteobacteria has established itself near the top of the soil layer. This bacterial population appears to be capable of nitrate reduction using electron donors derived from the soil organic matter. Below the zone of nitrate reduction a significant proportion of the 0.5N HCl extractable iron (a proxy for microbial available iron) is in the Fe(II) oxidation state indicating there is increasing anoxia with depth and suggesting that microbial iron reduction is occurring. Supplemental materials are available for this article. Go to the publisher's free online edition of Geomicrobiology Journal to view the free supplemental file.