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Nitrogen distribution was studied, by successive 1M (H1) and 3M HCl (H2) hydrolyses, on a natural soil before (NS) and after 15

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The evolution of the soil organic-N forms and their bio-availability was studied in a 15 N 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 15 N enrichment observed in the unburnt soil during the successive plant growth cycles (LS N LS-L N LS-P) contrasts with the lack of significant changes, in both the content of total organic 15 N and the atom % 15 N 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 ( 15 N enriched) than for the native N, supplying the growing vegetation with inorganic N more 15 N 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 NH 4 + -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 N 2 -fixing microorganisms or plants in the burnt soils the recovery of the natural soil organic N composition will take place slowly.

The effects of poultry manure (PM), used for the reclamation of a 15 N-labeled burnt soil, on N nutrition of pine seedlings were evaluated during one year in a pot experiment. Six treatments were used: 15 N-labeled soil (LS), 15 N-labeled burnt soil (BLS), and BLS+PM at doses equivalent to 1, 2, 4, and 8 t ha -1 of dry PM (PM1, PM2, PM4, and PM8, respectively). Either in the whole tree or the different organs, N concentration (1) decreased (p ≤ 0.05) in the order LS > BLS, BLS+PM1, BLS+PM2, BLS+PM4 > BLS+PM8 and (2) was negatively correlated with phytomass production (p < 0.05 to p < 0.01). The two highest amounts of assimilated N per kg dry soil were found in LS and BLS (130-134 mg) and the lowest in BLS+PM8 (87 mg), the other treatments being in an intermediate range (108-115 mg). Irrespective of the soil treatment, 56%-66% of the pine-N were accounted for by needles, 29%-32% by roots, and 8%-12% by stems, the differences among organs being always significant (p < 0.05). The percentage of pine-N derived from PM (%PNDFM) increased steadily with PM dose, from 1.7% in BLS+PM1 to 13.3% in BLS+PM8, reaching values for the two highest PM doses within the range found for 15 N mineral fertilizers in forests. From 8.2% to 16.9% of the PM-N was assimilated by the pines. Although differences among treatments were not significant, the two highest values were found in BLS+PM4 and BLS+PM8. Therefore, the 15 N data showed clearly that there is a positive medium-term effect of PM on pine N nutrition during the reafforestation phase of burnt-forest reclamation. The lower total N uptake by pine seedlings in the PM treatments was explained in terms of lower levels of soil available N due to its exportation with the phytomass of the preceding ryegrass culture, used for the early protection phase in the burnt soil-reclamation procedure.