Inorganic and organic N pools in soils burned or heated: immediate alterations and evolution after forest wildfires

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 °C. Ammonium content increased in the 0–5 and 5–10 cm soil layers immediately after burning and after heating at 150, 210 or 350 °C. The positive effect of burning on NH4+-N persisted for several months and was particularly intense 1 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 °C; conversely, in the other recently burned soil and in samples heated at 150 and 210 °C, 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 °C. 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 °C (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.