SPATIAL-DISTRIBUTION OF NITRIFICATION AND DENITRIFICATION IN AN ACID FOREST SOIL

The spatial distribution of nitrification and denitrification has been studied in a forest located in central France. The potential for nitrification in the A1 soil horizon showed a high variability at a 1 m scale and was even absent at locations where a thick upper litter layer was present. At these locations, the highest nitrification ability was found in the first 2 cm of the litter, demonstrating that recent stages may be colonized by nitrifiers. These findings suggested that the process may be governed by O2 diffusion. In the litter, the potential for nitrification was found to be equally distributed between the Abies alba needles and the aggregates of organic matter particles mainly resulting from faunal activity. At the same scale the denitrification process was never absent and showed little variability. Consequently, the distribution of this process has been studied at the scale of 0.2 mm x 0.5 mm aggregates. All the aggregates appeared to be fully colonized by soil bacteria. However, some aggregates did not exhibit any denitrifying ability when they were individually incubated in rich or minimal medium. This suggests that a limit of the spatial distribution of denitrifying organisms was reached at a 0.5 mm scale, which is the size of currently encountered aggregates in this forest soil.