Nitrification in a zeoponic substrate

Clinoptilolite is a zeolite mineral with high cation exchange capacity used in zeoponic substrates that have been proposed as a solid medium for growing plants or as a fertilizer material. The kinetics of nitrification has not been measured for NH4 + saturated zeoponic substrate. Experiments were conducted to evaluate the production of NO2 – and NO3 –, and nitrifier populations in zeoponic substrates. Small columns were filled with zeoponic substrate inoculated with a commercial inoculum or soil enrichment culture of nitrifying bacteria. In addition to column studies, a growth chamber study was conducted to evaluate the kinetics of nitrification in zeoponic substrates used to grow radishes (Raphanus sativus L.). The zeoponic substrate provided a readily available source of NH4 +, and nitrifying bacteria were active in the substrate. Ammonium oxidation rates in column studies ranged from 5 to 10 (mu)g N g^–1 substrate h^–1, and NO2 – oxidation rates were 2 to 9.5 (mu)g N g^–1 substrate h^–1. Rates determined from the growth chamber study were approximately 1.2 (mu)g N g^–1 substrate h^–1. Quantities of NH4 + oxidized to NO2 – and NO3 – in inoculated zeoponic substrate were in excess of plant up-take. Acidification as a result of NH4 + oxidation resulted in a pH decline, and the zeoponic substrate showed limited buffering capacity.