Enumeration and inoculation of nitrifying bacteria in soilless potting media

Abstract

Autotrophic nitrifying organisms were enumerated in soilless potting media using the most probable number (MPN) technique. Populations of NH4+and NO2- oxidizing organisms varied widely between two soilless media-Metro-Mix 220 and 350. Estimates for NH4+ oxidizing organisms ranged from 0.7 to 7.8 x 105 organisms/cm3, while NO2- oxidizers ranged from 1.3 to 9.5 x 105 organisms/cm3. Population numbers were similar to those typically reported in soils. There was a significant effect of medium type, NH4+ N: NO3- N fertilizer ratio, and planting on MPN counts of both groups of organisms, with significant interaction between several of the factors. Estimates of NH4+ oxidizers were not linearly correlated with NH4+ oxidizing activity, implying low counting efficiency, heterotrophic nitrification, or rate-limiting substrate NH4+ level. In a separate study, a soilless potting medium was inoculated with pure cultures of either Nitrosomonas europaea or Nitrobacter agilis. Rates of NH4+ and NO2- oxidation increased, respectively, as inoculum volume increased. Inoculation with nitrifying bacteria may help in the overall management of N in the rhizosphere and be feasible alternatives for the prevention of either NH4+ or NO2- phytotoxicity with fertilizers containing urea or NH4+.