Chemical, physical, and biochemical soil properties and plant roots as affected by native and exotic plants in Neotropical arid zones

At 'Cerro Saroche' National Park, one of the few protected arid zones in northern South America (Venezuela), two exotic plants, Kalanchoe daigremontiana and Stapelia gigantea, have become established. Our goal was to examine what type of relationship existed between these exotics and some biotic (enzymes activities and microbes, fine root mass, and the associated nutrient content) and abiotic (physical-chemical characteristics) soil properties. Soil samples were collected during the dry season from sites at which both exotics have become established and from places inhabited only by native plants. K. daigremontiana grew successfully on soil with greater clay contents for the best supply of Ca. S. gigantea grew in the same soil conditions than the resident plants, but the dense and shallow fine root mass system of this species probably allowed it to exploit a greater upper soil volume that the native plants coexisting with it. Urease activity was highest at the K. daigremontiana site and was related to the high soil organic carbon. Both alkaline phosphatase and fluorescein diacetate hydrolysis were not affected by the presence of exotic plants. These are the first data on the interactions between invasive plants and soil properties in tropical arid lands.