Availability of 15 N from pioneer herbaceous plants to pine seedlings in 17 reclaimed burnt soils 18 19

A pot experiment was used to assess the N uptake by pine seedlings 24 during two years on a burnt soil added with 15 N-labelled ryegrass, obtained from 25 a 15 N-enriched sample of this soil after the fire. Nitrogen concentration in needles, 26 stems and roots of seedlings decreased significantly from the first to the second 27 1.12% of the added ryegrass-15 N was recovered: two thirds in the soil organic N 30 pool and one third in the pine seedlings. Therefore, the post fire pulse of 31 inorganic N, which was successfully kept in the burnt soil-plant system through 32 its uptake by the pioneer species, is available for trees at the medium term. Pine 33 seedlings assimilated 16.4% and 16.9% of the added ryegrass-15 N in the first and 34 the second year, respectively. This result contrasts with the usual yearly decrease 35 of added N uptake by plants; a possible explanation is the transient increase of 36 available N in burnt soils that would have modified the mineralization pattern