The chemolithotrophic ammonium-oxidizing community in a nitrogen-saturated acid forest soil in relation to ph-dependent nitrifying activity

Abstract

The relationship between pH and nitrification in different layers of a nitrogen-saturated acid Douglas fir forest soil was studied. Nitrification potentials (on basis of dry weight) of overground degrading needles, litter and fermentation layers were much higher than those of the humus and upper mineral layers. In all layers, nitrate production was probably due to chemolithotrophic bacteria as it was inhibited by acetylene. The litter layer contained relatively high numbers (10 5 g dry soil −1) of acid-sensitive ammonium-oxidizing bacteria, whereas the numbers in the other layers were just above or lower than the detection limit (10 3 g dry soil −1) of the MPN method used. Measurements of nitrate production in soil suspensions indicated that a pH increase (pH 6 vs pH 4) stimulated ammonium oxidation in the litter layer but not or to a lesser extent in the fermentation and humus layers. It is argued that both acid-sensitive and acid-tolerant ammonium-oxidizing bacteria are contributing to nitrification in the litter layer. In the fermentation and humus layers acid-tolerant or even acidophilic bacteria are thought to be responsible for ammonium oxidation.