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The presence of trees in grasslands frequently results in resource islands of higher soil quality. Therefore, some native trees are often preserved or agroforestry species are interplanted when land is cleared for pasture. We evaluated soil quality changes associated with the conversion of a native thorn forest (caatinga) into silvo-pastoral systems in semiarid NE Brazil. Soil nutrients, organic matter, microbial biomass and soil enzymes under native caatinga, the canopy of two preserved native (Ziziphus joazeiro and Spondias tuberosa) and one introduced tree species (Prosopis juliflora), and under a planted pasture of Cenchrus ciliaris were compared. The two preserved tree species maintained high nutrient and organic matter contents and high biological activity levels not only relative to the grass but also relative to the native caatinga. The non-deciduous Z. joazeiro was more effective than the deciduous S. tuberosa, and enriched surface soils by 100 mg P kg(-1). The complete replacement of natural caatinga with C. ciliaris pasture or a C. ciliaris-P. juliflora silvopastoral system had, after 14 years, decreased microbial biomass C and N contents, and ss-glucosidase activity, but did not affect soil nutrient or organic matter status. The biological activity under the trees of the tree-grass association was greater than under grass alone, although P. juliflora did not maintain higher nutrient or organic matter levels. The search for indigenous alternatives to the main agroforestry species may provide viable alternatives for improved land management and the conservation of biodiversity. Microbiological and biochemical parameters responded more readily to changes in land management than chemical fertility indices. They are important indices for the impact of vegetation changes and its associated land quality changes.