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 15N‐labelled ryegrass, obtained from a 15N‐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‐15N 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‐15N 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 15N‐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. Copyright © 2008 John Wiley & Sons, Ltd.