Factors influencing nitrification and denitrification variability in a natural and fire-disturbed Mediterranean shrubland

The main factors influencing the variability of nitrification and denitrification enzyme activity (DEA), in soil of a Mediterranean shrubland, were investigated in an undisturbed area and in plots treated with experimental fires of two different intensities. Soil was sampled 4 times during I year after burning, in periods characterised by different environmental conditions. In the control, net nitrification ranged from 0.95 to 1.32 mug NO(3)(-)-N g(-1) day(-1) and did not show significant average differences among sampling events, probably because water content and NH(4)(+)-N availability were both limiting the process at any time. Overall, nitrification seemed associated with microsites where NH(4)(+)-N production was higher. This pattern was not evident in the burned plots, where, moreover, a partial reduction of activity immediately after burning was observed. DEA showed higher variability among sampling events, as compared with nitrification, with a peak in winter. It also presented a close spatial relationship with microsites of nitrification activity. In the burned sites, water content, organic C and NO(3)(-)-N content concurred to explain DEA variability together with NH(4)(+)-N availability and nitrification activity. Plots treated with intense fire showed the lowest values of DEA, especially in the period more favourable to this activity.