The effects of secondary forest clearance and subsequent land-use on erosion losses and soil properties in the Blue Mountains of Jamaica

As pressure on land and forest resources increases, there is a growing need to assess and improve the sustainability of slash-and-burn agriculture in tropical uplands. On steep-hillslopes (24-32 degrees) in the forest buffer zone of the Blue Mountains of Jamaica, following clearance of secondary forest the relative impact on surface runoff, soil erosion and soil properties of three land-use treatments: maintained weed-free without cultivation (bare); cultivated with herbaceous crops (agriculture); and cultivated with herbaceous crops and intercropped with Calliandra calothyrsus contour hedges (agroforestry) was assessed over a 5-year period and compared with an uncleared secondary forest control (forest). The forest provided good protection against surface runoff (which was consistently less than 0.2% of rainfall) and soil erosion losses (less than 500 kg ha(-1) per year); agriculture caused a seven-fold increase in surface runoff and 21-fold increase in soil erosion. However, agroforestry was effective in conservation of water (45% reduction in runoff compared with agriculture) and soil (erosion reduced by 35%). The clearance of the secondary forest led to large changes in most measured soil properties. Over 5 years concentrations of organic matter declined by 31%, total N by 38%, exchangeable K by 47%, Ca by 43% and Mg by 56%; over the same period bulk density increased by 48%. Of the soil properties more subject to year-to-year fluctuations, after 5 years the concentration of available P was 36% less in cleared plots than in forest, exchangeable Na was 50% less and moisture content 43% less. Only total P concentration and pH were unaffected. Five years after forest clearance there was no good evidence that these changes had stabilised and very little difference in soil properties was found amongst the three land-use treatments. However, within the agroforestry plots, exchangeable K, Na and sand concentrations became higher under the hedgerows than between them (by 14, 9 and 8%, respectively), whilst clay concentration became higher between them (by 9%). The results indicate that this low-input, contour-tree-hedgerow technology is effective at soil and water conservation through the sieve-barrier effect and increased water infiltration (respectively) and has the potential to enhance the sustainability of this land-use system at a plot scale.