Nitrogen dynamics at undisturbed and burned Mediterranean shrublands of Salento Peninsula, Southern Italy

Fire is a major disturbance in shrubland ecosystems of the Mediterranean basin, with high potential to alter ecosystem nitrogen (N) stocks and N cycling. However, postfire effects on gross rates of soil N turnover (ammonification, nitrification, microbial immobilization, denitrification) have rarely been investigated. We determined gross rates of N turnover including nitrous oxide fluxes and dinitrogen emissions in the mineral soil of unburned and burned shrublands of Southern Italy 6 months after a natural fire. In soil of burned plots, both gross ammonification and gross nitrification were significantly higher than in soil of unburned plots (2.2 +/- 0.3 versus 0.6 +/- 0.1 mg N kg(-1) sdw day(-1) for ammonification and 1.1 +/- 0.1 versus 0.5 +/- 0.1 mg N kg(-1) sdw day(-1) for nitrification). Microbial immobilization, in particular of nitrate, could not compensate for the increase in inorganic N production, therefore soil nitrate concentrations were considerably higher at the burned plots. Soil microbial biomass carbon and nitrogen concentrations were significantly lower in soils of burned plots than in soils of unburned plots. Dinitrogen was the dominant end product of denitrification and emitted at higher rates from the unburned plots than from the burned plots (0.094 +/- 0.003 versus 0.004 +/- 0.002 mg N kg(-1) sdw day(-1), while there was no net nitrous oxide flux (burned plots) or slight net nitrous oxide uptake (control plots). These results show that postfire patterns of gross N turnover in soil can exhibit a significant reduction of both microbial N retention and N gas losses via denitrification.