SHORT-TERM EFFECTS OF A WILDFIRE ON THE NITROGEN STATUS AND ITS MINERALIZATION KINETICS IN AN ATLANTIC FOREST SOIL

A Humic Cambisol developed over granite under Pinus pinaster Sol. loca ted in the Atlantic climate zone, which had been affected by a high in tensity wildfire, was studied 1 month after burning. The soil had a ve ry rich organic matter A horizon, 30 cm deep. The effects of the fire on the N status and N mineralization capacity were estimated comparing the surface (0-5 cm) and subsurface (5-1 0 cm) layers from the burnt soil with the corresponding layers from the same unburnt soil. N miner alization kinetics were determined by aerobic incubation at 28-degrees -C for 11 weeks. The fire increased the total N content in the surface layer but not in the subsurface. Total inorganic N, which was mainly in the form of NH4+-N, increased after the burning in both layers, whe reas NO3--N content, which was very low, only increased in the subsurf ace layer. The fire increased the N mineralization capacity, but did n ot modify organic N mineralization behaviour. Ammonification largely p redominated over nitrification in both the unburnt and the burnt soils . N mineralization kinetics followed the first order equation N(m) = N 0(1 - e(-Kt)) but the fire affected the kinetic parameters. The potent ially-mineralizable N decreased and the kinetic constant increased in the burnt samples showing that the wildfire reduced the mineralizable organic-N reserves and increased the mineralization rate, thus predict ing a rapid depletion of the labile organic N. The temporary ability o f the burnt soil to supply available N is recommended to be used to gr ow an early crop to avoid physical soil degradation.