Takashi Kosaki

Effects of 3-year cultivation on the soil nutrient status in a tropical forest and savanna of Central Africa, as determined by the microbial responses to substrate addition

ABSTRACT The forest–savanna transition zone is widely distributed on nutrient-poor Oxisols in Central Africa, and a population explosion has led to the rapid cultivation of these vegetation types in this zone. To reveal and compare the effects of short-term (3 years) cultivation on the soil nutrient status of the forest and savanna vegetation in this area, we evaluated microbial nutrient limitation and availability by conducting hourly measurements of soil microbial respiration after the addition of glucose in combination with nitrogen (N) and/or phosphorus (P) to soils that were collected from a forest site (FOR), a savanna site (SAV), as well as cropland for 3 years derived from a forest (Crop-F) and a savanna (Crop-S), in eastern Cameroon. The N addition had little effect on the pattern of microbial respiration rate for the FOR and Crop-F sites, indicating N rich for microbes. In contrast, N addition resulted in the increases in maximal respiration rates after the exponential increase for the SAV and Crop-S sites, indicating microbial N limitation, and cultivation accelerated the soil N depletion. Furthermore, we observed that P addition resulted in the increase in the maximal respiration rates, indicating microbial P limitation for all sites, except for FOR site. Since the cultivation significantly affected the microbial properties only in the forest ecosystem, such as the increase in the microbial specific growth rate and the decreased microbial C:N and C:P ratios, these changes would induce the P limitation for Crop-F. These results indicate that (1) the FOR site was a N-rich ecosystem for soil microbes, and 3 years of cultivation in the Crop-F site did not alter the high soil N status but induced microbial P limitation, with the changes in the microbial properties, and that (2) the SAV site was N and P limited for soil microbes, and 3 years of cultivation clearly decreased the soil N availability.

Soil Fertility Status in Equatorial Africa: A Comparison of the Great Rift Valley Regions and Central/Western Africa

Soil fertility was measured in contrasting regions of sub-Saharan Africa, i.e., Tanzania, Rwanda, western D. R. Congo, Cameroon, Nigeria, Burkina Faso, Ivory Coast, and others, with special reference to geological and climatic conditions. The general properties of the soils in each region of equatorial Africa could be summarized as follows: when comparing the soils in central and western regions of equatorial Africa, the soils in Tanzania are affected more or less by the Great Rift Valley movement, including volcanic activity, and are considered relatively fertile. The base level is generally high, and soils are therefore not intensely acidic, and soil texture is intermediate. The soil organic matter (SOM) level is moderately high, partially affected by volcanic activity and relatively high elevation. Clay mineral composition also suggests that the soil in this region is somewhat less weathered and possibly supplies more mineral nutrients than soils in the other regions. A similar, but more definitive, advantage of volcanic soil could be found for the soils in the volcanic regions of the highlands of Rwanda and eastern D. R. Congo. The soils in this region are characterized by high base levels, high cation exchange capacity (CEC) values (which are influenced by the presence of both SOM and 2:1 clay minerals), intermediate to clayey soil texture, and relatively high SOM levels; the latter two are affected by parent materials and cool temperatures. In contrast to the Great Rift Valley regions, a large part of Cameroon is situated on the Cameroonian plateau, which is composed of Precambrian basement rocks under humid climates. The soils in this region are characterized by a strong acidic nature, high levels of exchangeable Al, fewer base components, moderately low SOM level, and clayey soil texture, dominated by inactive kaolin minerals. The soils in the western regions of equatorial Africa, such as the Nigeria/Benin and Burkina Faso/Ivory Coast/Liberia regions, are commonly characterized by the presence of sandy soils. The sand content usually exceeds 70 %, while clay content is less than 20 %. As a result, the base reserve is typically low, and the SOM level is less than 10–15 g C kg−1 soil.