Prescribed burning is a major control over element cycles in Tallgrass prairie (Eastern Kansas, USA). In this paper we report potential effects on fire on non-symbiotic nitrogen fixation. Fire resulted in additions of available P in ash, which may stimulate nitrogen fixation by terrestrial cyanobacteria. Cyanobacterial nitrogenase activity and biomass responded positively to additions of ash or P in laboratory assays using soil. Further assays in soils showed that cyanobacteria responded to changes in available N: available P ratio (aN:P) across a range of concentrations. Nitrogen fixation rate could be related empirically to a N:P via a log-linear relationship. Extrapolation of laboratory results to the field yielded a maximal estimate of 21 kg N ha-1y-1. Results support arguments from the marine and terrestrial literature that P availability is central to regulation of ecosystem N budgets. Key words: Cyanobacteria, fire, acetyline reduction, ash