西アフリカ, ニジェールの固定砂丘地帯における地形・土壌環境と土地荒廃

抄録

A geomorphological and pedological survey carried out in a fixed dune area of Niger, West Africa, revealed that (1) landforms consist of 8 major landform units such as plateau, hill-foot sand dune “sand epron”, pediplain, slope from pediplain to riverain plain, riverain plain, valley, sand dune, and inselberg and (2) each landform unit is characterized by particular soil types, divided into sandy and clayey soils.
Sandy soils are derived from sand cover, and have an eluvial surface horizon and reddish brown subsurface ones. The degree of rubefaction in the soil profile is stronger on a hill-foot sand dune than on a pediplain. When the sand cover is thick, the rubefaction is stronger even on a pediplain. The sandy soil on riverain plain is eluviated almost completely, and becomes white sandy soil composed of quarts sand. Cultivated fields of pearl millet, the main crop in this region, are spread on the sandy soil area, especially at lower hill-foot sand dunes, pediplains with thick sand cover, riverain plains, and other sand dunes. The clayey soil develops on the plateau, the surface of which is covered by tiger bush. The soil fertility is quite low at any soils.
Bare lands occur on plateaux without sand cover, upper hill-foot sand dune with thick sand cover, pediplains with thin sand cover, and slopes along temporal water way. The following four factors are critical for the land degradation : (1) thickness of sandy layer, (2) topographical position, (3) gradient of slope, and (4) surface crustability of soil.
Even sandy soils form surface crust easily on the structureless surface horizon due, to some fine materials for crusting. The circular bare lands around termitaria are characterized by a harder surface crust due to fine materials translocated from a deeper horizon. The surface crust must inhibit infiltration of rain, which results in reduction of available water in the soil, and in dominant surface run-off. White sandy soils on riverain plain do not form the surface crust due to lack of fine materials while clayey soils on the plateau always form continuous surface crust.
The “desertification” in the study area is a phenomenon related with depletion of sand cover. Therefore, in order to prevent “desertification”, how to manage sand cover must be studied seriously.