Monitoring land degradation in the semiarid tropics using an inferential approach: the Kabomo basin case study, Nigeria

Abstract

Lack of baseline data on soil fertility status for most semiarid areas to a very large extent hampers the success of land degradation monitoring. However, this can be overcome by adopting an inferential approach which presupposes that soils of an area of uniform geologic, geomorphic and climatic characteristics differ mainly because of the uses to which they are put. On this basis, soil conditions of a long-standing vegetation community can be used to assess the extent of soil changes resulting from land-use practices, provided that both the vegetation community and the land-use plots are located in an area having the uniform environmental parameters noted above. This approach was adopted in the Kabomo area of Nigeria, a typical semiarid tropical ecosystem, to monitor the extent of soil degradation resulting from 20 years (1972–92) of arable farming, livestock ranching and legume grass farming. Using a systematic sampling procedure, topsoil (0–15 cm) and subsoil (20–30 cm) samples were collected from both the long-standing vegetation community plot (over 80 years old) which was chosen to serve as the control, and the three land-use plots. The collected samples were then analysed for texture, bulk density, water content, water stable aggregates greater than 0·50 mm, organic matter, pH, cation exchange capacity (CEC), total nitrogen, available phosphorus and exchangeable bases. The mean value of every property for each plot was then divided by the mean value of the same property for the control plot in order to assess the extent of change (i.e. extent of degradation) in the property. The Student's t-test was then used to assess the significance of such a change. The results obtained revealed that, in general, the mean values of the various soil properties, with the exception of sand and bulk density, have declined by between 3 per cent and 72 per cent, and in most cases the declines are statistically significant. Sand and bulk density, on the other hand, show some increases of between 6 per cent and 78 per cent, though only the increase in respect of the sand fractions are statistically significant. Similarly, it was observed that the extent of degradation was much greater under the arable land and least under the livestock ranching plot. The potential causes of these trends were discussed, while suggestions were offered on how best to utilize this approach in carrying out effective monitoring of land resources in the semiarid tropics. © 1997 John Wiley & Sons, Ltd.