Charcoal and biological activity in formiguer soils of Catalonia (Spain): Application of a micromorphological approach

The "formiguers" (earth mounds to burn woody plant residues) constituted a common practice of fertilization of soils in many regions of Spain in the past. The result of combustion is the formation of char, charcoal, soot, graphitic carbon and black carbon, which increased soil fertility. We applied a micromorphological approach to study the relationship between charcoal and features due to biological activity on some soils of Catalonia where "formiguers" were traditionally applied. Five sites with different land uses were selected in the north and south of Catalonia, and soil thin sections were obtained from undisturbed samples. The micromorphological character-ization and the quantification of the total porosity, biopores, charcoal and biospheroids due to earthworms were determined. Samples from horticultural fields showed higher total porosity (15%) than those from other fields (9%). A similar pattern appeared in relation to charcoal abundance, 1.3% in horticultural plots compared to near null presence in other plots. The biopore percentage was approximately twice (11.2%) in the north sites with respect to the south sites, and biospheroids were absent in the latter while their mean abundance was 1% in the former. Our results suggest that the presence of charcoal fragments have significantly influenced bioporosity in the studied soils. The micromorphological techniques proved useful to study of the distribution and abundance of charcoal and biospheroids in formiguer soils in Catalonia.