Nutrient storage in termite (Macrotermes bellicosus) mounds and the implications for nutrient dynamics in a tropical savanna Ultisol

Abstract

The role of mounds of the fungus-growing termite Macrotermes bellicosus (Smeathman) in nutrient recycling in a highly weathered and nutrient-depleted tropical red earth (Ultisol) of the Nigerian savanna was examined by measuring stored amounts of selected nutrients and estimating their rates of turnover via the mounds. A study plot (4 ha) with a representative termite population density (1.5 mounds ha−1) and size (3.7 ± 0.4 m in height, 2.4 ± 0.2 m in basal diameter) of M. bellicosus mounds was selected. The mounds were found to contain soil mass of 9249 ± 2371 kg ha−1, composed of 7502 ± 1934 kg ha−1 of mound wall and 1747 ± 440 kg ha−1 of nest body. Significant nutrient enrichment, compared to the neighboring topmost soil (Ap1 horizon: 0–16 cm), was observed in the nest body for total nitrogen (N) and exchangeable calcium (Ca), magnesium (Mg) and potassium (K), and in the mound wall for exchangeable K only. In contrast, available (Bray-1) phosphorus (P) content was found to be lower in both the mound wall and the nest body than in the adjacent topmost soil horizon. Consequently, the mounds formed by M. bellicosus contained 1.71 ± 0.62 kg ha−1 of total N, 0.004 ± 0.003 kg ha−1 of available P, 3.23 ± 0.81 kg ha−1 of exchangeable Ca, 1.11 ± 0.22 kg ha−1 of exchangeable Mg and 0.79 ± 0.21 kg ha−1 of exchangeable K. However, with the exception of exchangeable K (1.2%), these nutrients amounted to less than 0.5% of those found in the topmost soil horizon. The soil nutrient turnover rate via M. bellicosus mounds was indeed limited, being estimated at 1.72 kg ha−1 for organic carbon (C), 0.15 kg ha−1 for total N, 0.0004 kg ha−1 for available P, 0.15 kg ha−1 for exchangeable Ca, 0.05 kg ha−1 for exchangeable Mg, and 0.06 kg ha−1 for exchangeable K per annum. These findings suggest that the mounds of M. bellicosus, while being enriched with some nutrients to create hot spots of soil nutrients in the vicinity of the mounds, are not a significant reservoir of soil nutrients and are therefore of minor importance for nutrient cycling at the ecosystem scale in the tropical savanna.