Variation in the inorganic and organic N contents and in the resistance of organic N to step-wise acid hydrolysis (four steps of increasing hydrolytic intensity) was analysed in burned soils sampled (0-5 and 5-10 cm depth) at different times after high severity forest wildfires, as well as in one soil heated in the laboratory at 150, 210 or 350 degreesC. Ammonium content increased in the 0-5 and 5-10 cm soil layers immediately after burning and after heating at 150, 210 or 350 degreesC. The positive effect of burning on NH4(+)-N persisted for several months and was particularly intense I year after burning. Two, 5 and 10 years after wildfire, NH4(+)-N in burned soils was similar or slightly higher than in the corresponding unburned soils. Nitrite and nitrate were very low or undetectable in most burned and unburned samples analysed. Organic N strongly decreased in one of the soils immediately after wildfire and in the soil heated at 350 degreesC; conversely, in the other recently burned soil and in samples heated at 150 and 210 degreesC, organic N was slightly higher or similar to that in the corresponding unburned soils. During the 2 years after wildfire, organic N changed little and in soils sampled 5 and 10 years after burning the organic N contents were close to those of the unburned control soils. Independent of the variation in organic N content, the lability of organic N dramatically changed after burning or heating, this change remaining for at least 2 years after wildfire. The amount of N solubilised by acid hydrolysis strongly decreased in both 0-5 and 5-10 cm soil layers after wildfire and in soils heated at 210 or 350 degreesC. This decrease was mainly due to the loss of labile N (solubilised after hydrolysis with 1 and 3 N HCl), while the hydrolysable N obtained with 6 N HCl changed little or even increased. On the contrary, the amount and/or percentage of N resistant to acid hydrolysis (residual N) increased in recently burned soils and in soils heated at 210 or 350 degreesC (except in the 5-10 cm layer of one soil). In the months following burning, differences in the lability of organic N between the burned and unburned soils decreased, especially in the 0-5 cm soil layer, although 2 years after wildfire the effect of burning on the organic N compounds was still detectable. In soils sampled 5 or 10 years after wildfire, the amounts and percentages of the residual N and of different fractions of hydrolysable N were similar to those of the unburned soils. The relationship between the reduction in hydrolysable N and the alteration of N mineralisation (previously studied) was also analysed. (C) 2003 Elsevier B.V. All rights reserved.
