δ15 N natural abundance in permafrost soil indicates impact of fire on nitrogen cycle

The impact of fire on the nitrogen (N) cycle of natural ecosystems is arguable. Here we report and interpret an observation from boreal ecosystems in the Lena River basin, Sakha Republic (Yakutia), Russian Federation. Different types of permafrost soil (0–30 cm depth) were sampled along transects (60–150 m length) from the forest edge towards the centre of four separate thermokarst depressions under grassland. The average values of δ15N were remarkably similar within three transects, but differed systematically between them. Three findings point towards fire being the cause of the observed pattern. First, the spatial extent of systematic differences in soil δ15N coincides with the extent of typical fire scars in the region. Second, soil enrichment in 15N is larger in the proximity of settlements, where fire is generally more frequent than in more remote places. Third, there is a significant positive correlation between δ15N values and the ratio of black C to total N. These findings point towards fire having a marked impact on soil δ15N and, accordingly, on the N cycle of this cold and dry ecosystem. Copyright © 2011 John Wiley & Sons, Ltd.