Influence of tree species and ground vegetation on nitrification in an acid forest soil

Soil N transformations were studied at Ironhill, near Liphook, UK as part of a forest fumigation experiment. Nitrification potential was measured in a humoferric podzol soil, of pH 3 (in 0.01 M CaCl2). An initial experiment into nitrogen mineralisation potential indicated that nitrification was linked strongly to the species of coniferous tree growing in the soil. Transfer of soil solution between soils had no influence on mineralisation potential and allelopathic effects of the trees were not demonstrated. The initial finding was attributed subsequently to the type of ground vegetation and its management. Attempts to reproduce soil conditions, which promoted nitrification, were partially successful. Soil, from the Ironhill site, was incubated with various nitrogenous substrates and other nutrients and sources of carbon to test whether heterotrophs were responsible for nitrification. Organic N (which was ammonified) promoted nitrification, but the addition of ammonium was inhibitory unless supplied with a readily available carbon source such as acetate. Nitrification potential was unaffected when soils were incubated with an inhibitor of autotrophic nitrification. The results of these experiments supported strongly the hypothesis that heterotrophic organisms were responsible for nitrification in this soil.