Deforestation and cultivation effects on characteristics of oxisols in the highlands of Madagascar

Soils of tropical forests are often inherently nutrient poor, although the extents of extremely infertile tropical forest soils are not as large as previous estimates may suggest. This paper presents findings from a study of change in soil quality in relation to deforestation and land use change in the highlands of Madagascar. A synthesis of some of the available research results related to soil characteristics of tropical forest, and their response to disturbance and conversion (i.e. deforestation) is made. The study was conducted in an area in the eastern highlands of Madagascar. The predominant soil types in the eastern highlands of Madagascar are Oxisols, which are acid and have a high content of low activity clays. The chemical characteristics of forest soils were found to be highly variable, with soil organic carbon (SOC) and total nitrogen (TN) contents ranging from 22.8 to 120.8 and 2.2 to 8.8 g kg− 1, respectively. Conversion of forest to cropland (tavy) reduced SOC contents by 23.8 g kg− 1 in the first year after clearing and by 11.3 g kg− 1 year− 1 on average in the first 3 years of cultivation. Mixed fallow systems recovered SOC at rates of about 6.5 g kg− 1 year− 1. Available phosphorus (P) and exchangeable base cations (Ca, Mg and K) increased after clearing as a result of biomass burning while cation exchange capacity is largely determined by SOC content and follows similar trends as SOC after clearing. The long term trend was, however, in the direction of significant decreases in available P while the sum of base cations showed little change relative to natural forest soils.