Effects of two soil reclamation techniques on the distribution of the organic N compounds in a 15N labelled burnt soil

The evolution of the soil organic-N forms and their bio-availability was studied in a 15N labelled and burnt soil (BLS) after two successive reclamation steps under greenhouse conditions: a 3-month growing period of Lolium, without (BLS-L) or with poultry manure addition (4 and 8 Mg ha− 1: BLS + PM4-L and BLS + PM8-L), followed by a 12-month growing phase of pine seedlings (BLS-P, BLS + PM4-P and BLS + PM8-P). The results were compared with those obtained for the homologous labelled unburnt soil (LS, LS-L and LS-P) to evaluate the efficacy of these reclamation techniques in the mitigation of the drastic post-fire changes exhibited by the major biologically available N pool in terrestrial ecosystems: the soil organic N. The significant and steady decrease of the 15N enrichment observed in the unburnt soil during the successive plant growth cycles (LS > LS-L > LS-P) contrasts with the lack of significant changes, in both the content of total organic 15N and the atom % 15N in excess, among the treatments with the burnt soil (BLS ≥ BLS-L ≥ BLS-P). These results showed that: a) in LS, N mineralization proceeds faster for the recently incorporated N (15N enriched) than for the native N, supplying the growing vegetation with inorganic N more 15N enriched than the bulk soil N; and b) in BLS, soil combustion has reduced the usually higher biological availability of the recently added N to levels similar to those of the endogenous N.The re-vegetation with Lolium and Pinus and the addition of poultry manure mitigated the high differences observed in the size of the amino acid and the organic derived NH4+–N pools due to the combustion process, which are usual between burnt and unburnt soils. Conversely, these burnt soil reclamation techniques (re-vegetation and poultry manure addition), even jointly used, were unable to reduce the huge differences observed between the burnt and the unburnt soils for the other N fractions considered (amides, amino sugars, hydrolysable unidentified-N, hydrolysable organic N and un-hydrolysable N) that accounted for more than 80% of the soil organic N. Consequently, it seems that without the introduction of N2-fixing microorganisms or plants in the burnt soils the recovery of the natural soil organic N composition will take place slowly.