Soil N biochemical diversity and numerical taxonomy as tools in the pedogenetic study of a fossil profile

Distribution and diversity of the soil biochemical N fractions were studied in a fossil profile, a Podzol (IIA1 and IIBhs horizons) buried under a Cambisol (IA1 and IB horizons), to elucidate the genesis and evolution of the profile. The buried IIA1 horizon showed the lowest N biochemical diversity, which suggested a strong change in the pedogenetic conditions, probably not related to climate since another nearby Podzol was not affected. Soil numerical classification showed that the Cambisol is atypical, its two horizons being similar to the underlying podzolic IIBhs horizon in its physical and chemical characteristics. Based on biochemical N fractions, the IA1 and IB horizons were grouped with horizons from Cambisols and with podzolic horizons, respectively. The distribution of the organic N forms in the fossil Podzol was similar to that of a nearby recent Podzol in the Bhs horizon but not in the A1 horizon. The organic N of the fossil IIA1 horizon showed characteristics, such as extremely low content of α-amino acid N and high content of non-hydrolyzable N, similar to those of burned soils from the same region. Moreover, the presence of charcoal and the very low total N content in the IIA1 horizon suggest that a wildfire could have affected the A1 horizon of the original Podzol and the surrounding podzolic area, promoting soil erosion and thus the fossilization of the studied Podzol by podzolic colluvial sediments eroded from the upper part of the slope. The similarity of the biochemical N fractions distribution for the upper IA1 horizon with that of corresponding horizons from Cambisols suggests that currently pedogenetic processes are promoting the formation of a Cambisol in the podzolic sediments that fossilized the Podzol.

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