Termites play a significant role in soil-forming processes of the tropics. The influence of termites on pedogenesis as affected by the toposequence, however, has rarely been explored. We investigated the soil physicochemical and morphological characteristics of epigeal mounds constructed by Macrotermes bellicosus (Smethman) compared with those of surrounding pedons along a toposequence (bottom, fringe and upland sites) of an inland valley in central Nigeria. The physicochemical and morphological properties of the mound soils varied according to structural units but were generally different from those of the adjacent pedons. The differences included finer texture, higher electrical conductivity, total N, exchangeable bases (Ca, Mg and K) and effective cation exchange capacity and lower C ⁄ N ratio and exchange acidity in the mound than the pedon at each toposequence position. This tendency to modify the soil properties was more prominent in the nest body where the termites actually live, that is, in the hives, royal cell and base-plate, than in the soils below the nest and the other mound parts, that is, the external wall, internal wall and pillars. We found this trend to a greater or lesser degree at all toposequence positions. Our findings suggest that: (1) M. bellicosus can manipulate the mound soils according to functional applications of structure units or environmental requirements for its livelihood, regardless of local soils; (2) M. bellicosus makes ecological patches (hot spots) at all toposequence positions in the same measure; (3) the influence of M. bellicosus on the pedogenesis is reduced in the lowlands compared with the uplands because the number and volume of the mounds were substantially lower in the bottom and fringe sites compared with the upland site.

Many species of termite (Isoptera) build their nests inside mounds because a mound has direct and positive feedback effects on the termite colonies through the maintenance of humidity and protection of the population from enemies, e.g. ants (Jouquet et al. 2006, Korb 2003, Noirot & Darlington 2000). Soil manipulation by termites (Isoptera) for mound construction is of particular interest for many researchers in terms of pedogenesis of the tropics (Lavelle et al. 1992, Lobry de Bruyn & Conacher 1990). The termites select soil particles according to ecological requirements such as water availability (Jouquet et al. 2002, 2007) and improve soil structural stability by means of application of clay particles and saliva/excreta (Fall et al. 2001, Jouquet et al. 2004). The nest-building activity of the termites inevitably causes regional translocation of soils (Bagine 1984, Holt & Lepage 2000) and distinctive patches in local ecosystems, which contributes to ecological diversity (Lavelle et al. 1992). This is the reason why termites are regarded as an ecological engineer (Jouquet et al. 2006). Soil-particle selection by the termites, however, has not been fully explored in relation to diverse ecologies and landscapes in Africa.

In this paper, we review the soil fertility characteristics and the nature of material in the West African lowlands in comparison with paddy soils in tropical Asia to examine their potential for rice cultivation. Soil samples collected from major lowland ecosystems, i.e., inland valleys (185 locations) and flood plains (62 locations), in 13 countries (Benin, Burkina Faso, Cameroon, Cote d'Ivoire, Ghana, Guinea, Niger, Nigeria, Liberia, Mali, Senegal, Sierra Leone, and Togo) generally show low values of pH, total C and N, available (Bary-2) P, exchangeable Ca and Mg, effective cation exchange capacity (ECEC), and clay content. These properties of the 87 topsoil samples selected from 247 collected samples are well associated with mineralogical composition. The clay and primary minerals predominantly consist of kaolinite and quartz, respectively, which suggests that the lowland soils in the region have low nutrient-holding capacity and a limited potential for inherent nutrient supply. In general, soil pH, available P, exchangeable bases (Ca, Mg, K, and Na), and ECEC decrease while total C, total N and exchange acidity (Al and H) increase with increasing rainfall. This tendency is mostly explained by the enhanced biomass production and soil weathering sequence governed by the climate. In terms of rice production, the lowland soils in West Africa have lower values of general fertility parameters and poorer mineralogical characteristics compared to paddy soils in tropical Asia, which includes Bangladesh, Cambodia, India, Indonesia, Malaysia, Myanmar, Philippines, Sri Lanka, and Thailand. In addition, deficit levels of S and Zn for rice production are widely observed in the lowland soils in West Africa. These findings suggest that soil fertility characteristics show substantially less potential for rice production in West Africa than in tropical Asia.Discipline: Soils, fertilizers and plant nutrition Additional key words: agro-ecological zones, flood plains, inland valleys, mineralogical composition, sulfur and zinc deficiencyThe present paper is a joint contribution from the projects 'Development of Sustainable Rice Farming Systems in Low Activity Clay

This study investigated the influence of moundbuilding termites on soil particle dynamics on the land surface and in soil-forming processes by examining the amount of soil particles in mound structures of Macrotermes bellicosus in a highly weathered Ultisol of tropical savanna. Soil particle turnover via the mounds was estimated using particle stock data and soil turnover data from previous studies. A 4-ha study plot with six mounds of relatively uniform shape and size was investigated. Soil mass constituting the mounds was 6,166 ± 1,581 kg mound À1 within which the mound wall and nest body accounted for 5,002 ± 1,289 and 1,164 ± 293 kg, respectively. The mound wall contained a significantly larger amount of clay (252 ± 9.97 g kg À1 ) balanced with a lower sand content (676 ± 26.5 g kg À1 ) than in the adjacent surface (Ap1) horizon, (46.4 ± 12.8 g clay kg À1 ; 866 ± 83.2 g sand kg À1 ); the nest body had much higher clay content (559 ± 51.0 g kg À1 ) but less sand (285 ± 79.2 g kg À1 ) than the mound wall. As a result, the mounds of M. bellicosus accumulated clay of 2,874 ± 781 kg ha À1 (corresponding to 2.52% of clay stock in the Ap1 horizon) along with an estimated clay turnover rate of 169 kg ha À1 year À1 . These findings suggest a positive feedback effect from termite mound-building activity on soil particle dynamics in tropical savanna ecosystems: M. bellicosus preferentially use subsoil material for mound construction, resulting in relocation of illuvial clay in the subsoil to the land surface where clay eluviation from the surface soil and its illuviation in the subsoil are major soil-forming processes.