Nitrification and denitrification in soil from: A clear-cut Norway spruce (Picea abies) stand

Laboratory measurements of nitrification and denitrification were made on samples of the humus layer from a Norway spruce (Picea abies L.) experimental site that was clear-cut 2y before this study. For 30y before the clear-cutting, the stand had been repeatedly fertilized with nitrogen and/or limed. In addition to the nitrogen-fertilized and/or limed experimental plots, there was also a clear-cut control plot and a forested reference plot that was not clear-cut. Aerobic incubation experiments were used to study net production of (NO2 + NO3)-N, and an aerobic soil-suspension technique was used to study the nature and pH-dependency of nitrification. The numbers of nitrifiers were determined by a MPN method. Nitrification, which was active only in clear-cut plots, was acid-sensitive (no production at pH 4) and autotrophic, as it was inhibited by 2.5 Pa of acetylene. When the amount of NH4-N was not limiting, as in soil suspension, the pH controlled nitrification: in a pH gradient from 4.2 to 6.2, higher pH values resulted in higher production of (NO2 + NO3)-N. The soils from the clear-cut plots had an abundant community of nitrifiers, whereas in the forested reference soil, less than 10 NH4-oxidizers cm(-3) Of soil were observed. Denitrification occurred in soils from all the clear-cut plots, and the main product was N-2. Denitrification started in the forested reference soil only after the addition of NO3-N in the laboratory, and the main product was N2O. In the soils from the clear-cut plots, denitrification was limited first by a lack of NO3-N and second by pH. (C) 1998 Elsevier Science Ltd. All rights reserved.