The effect of the soil-feeding termite Cubitermes niokoloensis on soil microbial activity in a semi-arid savanna in West Africa

Soil-feeding termite mounds are an important, highly specific soil microbial compartment in semi-arid savannas. The aim of this study was to determine the role of these mounds in organic matter dynamics, taking the soil nitrogen cycle as representative of microbial mediated activity. Measures of microbial activity (soil respiration) and aspects of the nitrogen cycle (arginine ammonification, nitrification and denitrification) were made in the mound of Cubitermes niokoloensis, a representative species of the soil-feeding guild in dry savanna. Simultaneous measurements were made of mineral nitrogen concentration, total bacterial count (using AODC) and denitrifying and nitrifying bacterial densities. All measurements were made in two different compartments of the mound, the internal and external walls. Although organic matter was richer and there was a significant higher density of bacteria in the internal wall of the mound (by a factor of 2), the metabolic activity in this compartment was not significantly different from the surrounding savanna soil. This was not true for the external wall of the mound, suggesting that there was a difference in oxygen availability between these two compartments. Compared to the reference savanna soil, the mound was a hot spot of mineral nitrogen (100-times NH4+ and 50-times NO3-) representing 25% of the total nitrogen content of the mound, compared with only 2% in the savanna soil. This high level of mineral nitrogen was associated with a higher density of denitrifying bacteria and increased denitrification and ammonification potentials (3 and 4 times, respectively) in the mound compartments compared with the reference soil. However, the specific activity of the denitrifying bacteria was lower in the internal wall of the mound than in the reference soil. Also, no potential nitrification was observed in the mound, demonstrating a serious perturbation of the nitrogen cycle induced by the soil-feeding termite C. niokoloensis in the sandy savanna soil, with, as a consequence, an enrichment in organic matter and nutrients for plants. This study reinforces the view of termites as soil engineers in semi-arid savannas, modifying their environment in both of its biotic and abiotic components.