Effect of vegetation on soil C, N, P and other minerals in Oxisols at the forest-savanna transition zone of central Africa

The forest-savanna transition zone, which evolves as a result of past climate change, is widely distributed in central Africa. Because nutrient-poor soils (Oxisols) are widely distributed in this area, it is necessary to understand the characteristics of soil nutrients in relation to the vegetation. We collected 52 soil samples from five pits each for two different vegetation types (forest and savanna) in this area and evaluated the effect of vegetation type on soil physicochemical properties [pH, soil texture, cation-exchange capacity, bulk density, crystalline and non-crystalline aluminum (Al) and iron (Fe)] and nutrient status [carbon (C), nitrogen (N), phosphorus (P) and other soil minerals]. We also evaluated the fractionated P. Whereas most physicochemical properties were similar between the two vegetation types throughout the soil profile (0-80cm depth), clay content, bulk density and soil pH clearly differed between the vegetations at the surface layer (0-10cm). At 80cm soil depth, soil C, N and P were 87.9, 7.7 and 3.7Mgha(-1), respectively, in forest, and 98.6, 7.1 and 3.1Mgha(-1), respectively, in savanna. Although there was no clear difference between the amounts of soil C, N and P, the upper-soil (0-40cm)C:N ratio was clearly lower in forest (11.0-12.0) compared with savanna (13.0-15.7), because the main plant species in the forest can fix N effectively. We also found a smaller ratio of sodium hydroxide (NaOH)-extractable inorganic P to total soil P in forest compared with savanna. Because the content of crystalline and non-crystalline Al and Fe in forest soil was similar to that of savanna, the different soil C:N ratio would cause different availability of P between the vegetation types, although the mechanism is unclear. These results indicate that savanna vegetation is N-limited and forest vegetation is N-saturated (and possibly P-limited) in this zone. We also found that, at 20cm soil depth, total soil potassium (K) in forest was 1590kgha(-1), which was 930kgha(-1) less than that in savanna (2520kgha(-1); P<0.05), although a similar difference was not measured for Na, Ca, and magnesium (Mg). Because we observed lower soil pH in forest, not only plant K uptake but also K leaching loss would contribute to lower soil K in forest.