Inhibition of nitrifiers by grass root extracts.
Extracts of the roots of Hyparrhenia filipéndula and other grasses were tested against soil enrichment cultures of NH3-and NO2 oxidizers, by measuring nitrite production or consumption during short incubations. The roots of H. filipéndula contained a water-soluble heat-labile dialysable substance which inhibited nitrifying bacteria. Water extracts of these roots quickly lost their inhibitory activity when stored in a refrigerator under aseptic conditions; alcohol extracts contained inhibitors which appeared to be heat stable. Other common grass species of the Rhodesian Highveld also contained a heat-labile, water-soluble inhibitor. The water, in which roots of Eragrostis curvula roots had been growing for three weeks, contained a heat-labile inhibitor. Results do not oppose earlier suggestions that changes in availability of mineral N in soil are due to inhibition of nitrification by grass root exudates.