Pyrogenic carbon stocks are controlled by hydro-edaphic conditions in different forest types of the Northern Brazilian Amazon

Abstract

Pyrogenic carbon (PyC) is considered one of the most stable components found in soil, corresponding to an expressive amount of refractory organic matter. Despite PyC significantly affect soil properties and processes, it has not been considered in global carbon cycle models due to uncertainties related to its quantification. To provide a greater understanding of PyC importance and its role in the Amazon carbon cycle, we estimate PyC soil stocks dispersed in the vertical soil column of different forest types (ombrophilous, deciduous and semideciduous) which occur in a hydro-edaphic gradient in the Maracá Island, northern Brazilian Amazon. Soil profiles measuring one meter in depth were sampled at 30 permanent plots arranged along the eastern portion of Maracá Island. The soil samples were fractioned in 10 subsamples corresponding to each 10 cm layer of the 1 m column and all of them were analyzed by the thermal method. We observed that PyC contributed to a mean of 1.92 % of the TOC (total organic carbon) present in the samples, and the PyC/TOC ratios have also evidenced a tendency to increase with depth. For absolute values, deciduous forests occurring in seasonal flooding soils had the highest stock of PyC (1.27 Mg PyC ha−1) when compared with ombrophilous (non-flooding) and semideciduous (predominance of deep rock fragments) forests. Our results indicate that atmospheric deposition was the major source of PyC entrances into the soil and that hydro-edaphic features control the spatial and vertical distribution of PyC in the study area. These findings are key points for reducing uncertainties regarding PyC stocks and improves our understanding on the carbon stocks variability in the Amazonian soil.