Availability of 15N from pioneer herbaceous plants to pine seedlings in reclaimed burnt soils.

A pot experiment was used to assess N uptake by pine seedlings during 2 years on a burnt soil to which was added (15)N-labelled ryegrass, obtained from a (15)N-enriched sample of this soil after a fire. The nitrogen concentration in needles, stems and roots of seedlings decreased significantly from the first to the second growing period (from 2.55, 1.30 and 2.19% to 1.19, 0.47 and 1.00%, respectively), with needles accounting for 53-58% of the pine-N. At the end of the experiment, 98.87 +/- 1.12% of the added ryegrass-(15)N was recovered: two-thirds in the soil organic N pool and one-third in the pine seedlings. Therefore, the post-fire pulse of inorganic-N, which was successfully kept in the burnt soil-plant system through its uptake by the pioneer species, is available for trees in the medium term. Pine seedlings assimilated 16.4% and 16.9% of the added ryegrass-(15)N in the first and second year, respectively. This result contrasts with the usual yearly decrease of added N uptake by plants; a possible explanation is the transient increase of available N in burnt soils that would have modified the mineralization pattern of the (15)N-labelled phytomass. The pine-N derived from the ryegrass-N decreased from 4.05% in the first year to 2.53% in the second one, with 3.10% being the 2-year weighed average. In addition to the direct contribution of ryegrass to pine-N nutrition reflected by these figures, the rapid post-fire establishment of a herbaceous cover on the burnt soil also provides important indirect benefits for tree nutrition by reducing organic- and inorganic-N losses.