Soil ecosystem engineers such as termites and ants are able to modify physico-chemically the surrounding environment through the production of biogenic structures, thus affecting the availability of trophic and spatial resources for other organisms. The aim of this study was to assess the concentrations of C.,g, NH4+ and NO3- in the biogenic structures produced by a soil-feeding termite (Spinitermes sp.; Termitinae) and a fungus-grower ant species (Atta laevigata; Myrmicinae; tribe Attini) and the surrounding soil in the Colombian "Llanos". We tested the hypothesis that higher concentrations occur in the biogenic structures compared to control soil and that deposition of new building material at the top of the biogenic structures also increases nutrient concentrations. Sampling was conducted along a transect, at regular intervals proportional to the size of the biogenic structure. Average C,9 and NH4+ concentrations were significantly higher in termite mounds than in ant nests and the control soil, whereas NO3- concentrations were similar. For both types of biogenic structures, the highest difference in nutrient concentrations was found between the top Centre of the biogenic structure and the control soil, but significant differences were only reported for Spinitermes mounds and the control soil. No significant effect of land use on nutrient concentrations in the biogenic structures produced by either species was observed. We conclude that the activities of soil ecosystem engineers contribute to the variability of nutrient concentrations through the formation of biogenic structures.