SOIL-NITROGEN AVAILABILITY AND NITRIFICATION IN MEDITERRANEAN SHRUBLANDS OF VARYING FIRE HISTORY AND SUCCESSIONAL STAGE

The short-term effect of a single fire, and the long-term effect of re cent fire history and successional stage on total and mineral N concen tration, net nitrogen mineralization, and nitrification were evaluated in soils from a steep semi-arid shrubland chronosequence in southeast Spain. A single fire significantly increased soil mineral N availabil ity and net nitrification. Increasing fire frequency in the last few d ecades was associated with a sharp decrease in surface soil organic ma tter and total N concentrations and pools, and with changes in the lon g-term N dynamic patterns. The surface-soil extractable NH4+:NO3- rati o increased throughout the chronosequence. All net mineralized N in la boratory incubations from all sites was converted to nitrate, suggesti ng that allelochemic inhibition of net nitrification is probably not i mportant in this system. Net nitrification in samples during incubatio n increased through the sere. The maximum rate of net nitrification (k (max)) increased through the first three stages of the sere. A linear relationship was found between total soil N and N mineralization, and both k(max) and net nitrification for the first three stages of the se re, suggesting that total N and ammonification are likely to be the co ntrol mechanisms of nitrification within the sere. The oldest site exh ibited the lowest specific k(max) and the highest potential soil respi ration rate suggesting that a lower N quality and increasing competiti on for ammonium might also limit nitrification at least in the long-un burned garrigue site.