Determining the Variability of Electromagnetic Signals As Affected By Land Use in a Tropical Savanna.

In addition to the variable nature of tropical soils, tropical natural savanna soils are subjected to indiscriminate land use. This presents a substantial problem in soil sampling and characterization. Consequently, evaluating the variability of soil patterns and properties at the plot-scale is of great value in understanding the magnitude of resource depletion due to land use. Hitherto, traditional methods have been employed to understand soil variability, however, these methods are invasive, labour, cost and time intensive and do not adequately assess within field variability. Using geophysical techniques which are non-invasive, time and cost efficient, we determined the potential of electromagnetic induction (EMI) technique to assess the spatial variability of soil properties in a changing land use gradient from pristine to anthropogenic conditions. EMI-based apparent electrical conductivity (ECa) measurements were obtained at two (2) different depths (shallow=0-0.75m and deep=0.75-1.5m). Geostatistics were used to examine the underlying spatial structure of the data as a function of land use and soil depth. Our findings showed that ECa shallow (ECa_s) was generally more variable than ECa deep (ECa_d) across different land uses. Natural land uses of forest (ECa_d range= 23.9-27.7 mS/m) and grasslands (ECa_d range=66.1-67.1 mS/m) had a higher range of ECa_d values than anthropogenic land uses. Anthropogenic land uses, however, had higher ECa_s values (agriculture ECa_s range = 5.0-11.3 mS/m, quarry ECa_s range= 15.0-30.5 mS/m, residential ECa_s range = 31.0-80.1 mS/m) than natural land uses. The spatial structure of all land uses at both depths was described by spherical semivariogram models. The subsequent kriged maps using the semi-variogram models showed greater variability in the anthropogenic than natural land uses suggesting that deleterious human activities which decrease soil cover and increase soil compaction degrades soil quality in tropical savannas.