Ancient human agricultural practices can promote activities of contemporary non-human soil ecosystem engineers : a case study in coastal savannas of French Guiana

Some South American lowland environments bear impressive legacies of pre-Columbian engineering activities: vestiges of agricultural raised fields that have persisted since their abandonment centuries or millennia ago. We aimed to test the hypothesis that ancient raised fields were "re-engineered" by non-human soil organisms, leading to their maintenance against erosion. In a raised-field landscape in a seasonally flooded coastal savanna of French Guiana, we characterized the distribution of soil macro-invertebrates (ants, termites, earthworms) and plant roots between ancient raised fields (in this site, circular mounds) and inter-mound areas and between dry and wet seasons, and quantified the influence of these organisms on soil physical properties and texture. Social insect colonies were highly concentrated in mound soils; their density and species richness were maintained across seasons. Biomass of plant roots was higher in mounds than in inter-mound areas. Adult earthworms were inactive in deep soil layers during the dry season, becoming active at the surface of mounds during the wet season. Combined engineering activities of these organisms in the soil of ancient raised fields led to the accumulation of stable macroaggregates and pores, which should reduce the redistribution of fine soil particles between mounds and inter-mounds caused by erosion. Since their abandonment, and perhaps before, raised fields have attracted a diverse and abundant community of soil engineers that enhance the stability of mound soils, allowing their maintenance against erosion