Chemical variations in the biostructures produced by soil ecosystem engineers. Examples from the neotropical savannas

“Ecosystem engineers” are those organisms capable to modify physically the environment by producing “biogenic” structures (BS). Termites, earthworms, ants and other large macroinvertebrates produce BS with varying properties. In this study, our objective was to quantify total Corg (g per kg dry soil), and concentrations (μg per g dry soil) in different parts of the BS produced by termites and ants to test the hypothesis that higher concentrations are found where new building material is deposited, i.e. at the top of the BS. The study was carried out in a natural savanna (NS), an introduced grass-legume pasture (IP) and a gallery forest (GF) at the Carimagua Research Station in the Eastern Plains of Colombia. Progressive sampling distances across the BS were used, i.e. from the top to the base of the BS by using proportional distances, i.e. 20–100% for large BS and 50–100% for the smallest BS, and these were compared with two types of control soil, 120% or 150% in the case of large and small BS, respectively, and soil sampled 1 m away from the BS. All the BS analysed had, in general, higher concentrations of nutrients than the control soil. There were differences in the variables measured in the BS according to the organism that produced them. The lowest values of Corg were observed in the BS (surface dumps) deposited by fungus-growing ants (Trachymyrmex sp. in the NS, and Atta laevigata in the GF), while the highest concentrations were found in the BS produced by termites in the GF, where a high N concentration was also observed. Nutrient concentrations were higher in general in the BS than in the control soil in all cases. However, other BS seemed not to have any influence in the surrounding soil. We concluded that the activity of soil ecosystem engineers increased the spatial variability of chemical parameters measured in this study. The ecological significance of these differences is discussed.