Potential rates of ammonium oxidation, nitrite oxidation, nitrate reduction and denitrification in the young barley rhizosphere

Potential activities (enzyme contents) of ammonium (NH4+) oxidizing, nitrite (NO2−) oxidizing, nitrate (NO3−) reducing and denitrifying bacteria were measured in bulk and rhizosphere soil obtained from young barley plants in the field. The activities as well as pools of inorganic N (NH4+, NO2− and NO3−) were followed for 3 weeks in the beginning of the growth season (May–June). During the 3 weeks two separate periods of rain gave dramatic changes in soil water content. A rainfall in the beginning of the sampling period, resulting in a short-term wetting of the soil, stimulated the potential nitrification in the rhizosphere. On the other hand, potential denitrification only responded positively to the second, longer wetting of the soil. The potential activities were not affected by changes in the pools of inorganic N. As judged from the potential activities (enzyme contents), both groups of nitrifiers, NO3− reducer, and denitrifiers seemed most abundant in the rhizosphere, where the activities were from 1.5 to 2.5 times higher than in the bulk soil. NH4+ oxidizers and NO2− oxidizers showed a distinct and similar trend throughout the sampling period, which was also the case for NO3− reducers and denitrifiers. The results indicate that potential activities (enzyme contents) in the four bacterial groups were less dependent on their inorganic N substrates in the bulk and rhizosphere soils, but showed distinct and different responses to duration of soil wetting after rainfall.