Land quality changes following the conversion of the natural vegetation into silvo-pastoral systems in semi-arid NE Brazil

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 na tive caatinga, the canopy of two preserved native (Ziziphus joazeiro and Sp ondias tuberosa) and one introduced tree species (Prosopis juliflora), and under a planted pasture of Cenchrus ciliaris were compared. The two preserv ed tree species maintained high nutrient and organic matter contents and hi gh biological activity levels not only relative to the grass but also relat ive to the native caatinga. The non-deciduous Z. joazeiro was more effectiv e 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, de creased microbial biomass C and N contents, and ss-glucosidase activity, bu t did not affect soil nutrient or organic matter status. The biological act ivity under the trees of the tree-grass association was greater than under grass alone, although P. juliflora did not maintain higher nutrient or orga nic matter levels. The search for indigenous alternatives to the main agrof orestry species may provide viable alternatives for improved land managemen t and the conservation of biodiversity. Microbiological and biochemical par ameters 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.