How to highlight slash-and-burn agriculture in ancient soils? A modern baseline of agrarian fire imprint in the Guatemalan lowlands using charcoal particle analysis

This paper deals with the identification of past slash-and-burn agriculture in soils. Slash-and-burn is inferred to have been used by ancient people since the emergence of agriculture but its detection in archaeological and paleoenvironmental records often remains ambiguous, mainly because we lack knowledge about how agrarian fires are recorded in soils. We study charcoal assemblages from profiles excavated in four contemporary farm plots in the tropical forest of northern Guatemala, in order to provide a baseline of the slash-and-burn signature. By combining analyses of charcoal >2 mm (pedoanthracology) with infra-millimetric charcoal 100–400 µm, we compare charcoal concentration, distribution of size classes, morphotypes and taxon spectra between plots and between layers in each profile. Our results show that 1) different practices (length of cultivation cycle, duration of use, number of fires) leave different charcoal assemblages; 2) soil structure and porosity seem to greatly impact the preservation of the slash-and-burn signal by modifying the degree of infiltration of infra-millimetric charcoal; 3) infra-millimetric particles identified as Asteraceae achenes are homogenously abundant in the profiles, suggesting that achenes have a different behavior in soils compared to the other particles, and that they might be more impacted by aerial transport. Our study shows that the combination of pedoanthracology with analysis of infra-millimetric soil charcoal has the potential to track changing slash-and-burn regimes, and thus to clarify spatial–temporal patterns of agriculture intensification and extensification in people history.