Postfire nitrogen balance of Mediterranean shrublands: Direct combustion losses versus gaseous and leaching losses from the postfire soil mineral nitrogen flush

Fire is a major factor controlling global carbon (C) and nitrogen (N) cycling. While direct C and N losses caused by combustion have been comparably well established, important knowledge gaps remain on postfire N losses. Here, we quantified both direct C and N combustion losses as well as postfire gaseous losses (N2O, NO and N-2) and N leaching after a high-intensity experimental fire in an old shrubland in central Spain. Combustion losses of C and N were 9.4Mg C/ha and 129kg N/ha, respectively, representing 66% and 58% of initial aboveground vegetation and litter stocks. Moreover, fire strongly increased soil mineral N concentrations by several magnitudes to a maximum of 44kg N/ha 2months after the fire, with N largely originating from dead soil microbes. Postfire soil emissions increased from 5.4 to 10.1kg N ha(-1)year(-1) for N-2, from 1.1 to 1.9kg N ha(-1)year(-1) for NO and from 0.05 to 0.2kg N ha(-1)year(-1) for N2O. Maximal leaching losses occurred 2months after peak soil mineral N concentrations, but remained with 0.1kg N ha(-1)year(-1) of minor importance for the postfire N mass balance. N-15 stable isotope labelling revealed that 33% of the mineral N produced by fire was incorporated in stable soil N pools, while the remainder was lost. Overall, our work reveals significant postfire N losses dominated by emissions of N-2 that need to be considered when assessing fire effects on ecosystem N cycling and mass balance. We propose indirect N gas emissions factors for the first postfire year, equalling to 7.7% (N-2-N), 2.7% (NO-N) and 5.0% (N2O-N) of the direct fire combustion losses of the respective N gas species.