ARBUSCULAR MYCORRHIZAE IN THE REHABILITATION OF FRAGILE DEGRADED TROPICAL LANDS

The aim of this work was to establish a vegetative cover with an intro duced grass on an infertile substrate which contains little mycorrhiza l inoculum. A field experiment was carried out in La Gran Sabana, Vene zuela, in an area that was disturbed in 1991 and in which no spontaneo us recolonization by plant species occurred. Five treatments were set up in which an introduced grass. Brachiaria decumbens, was sown. The t reatments were: non-inoculated control (NI); inoculated with a concent rated mix of arbuscular mycorrhizal fungi (AMF) (385 spores g(-1) inoc ulum) at 1750 kg ha(-1) (I); fertilized with triple superphosphate, 10 0 kg ha(-1) (P); inoculated with AMF and simultaneously fertilized wit h triple superphosphate (I+P); another control treatment, to which pre viously sterilized AMF inoculum was added (S). In all cases B. decumbe ns was seeded at 30 kg ha(-1). A soil microorganism inoculum free of m ycorrhizae was added to all the treatments. Five months after sowing t he grass, above and below ground biomass, % arbuscular mycorrhizal (AM ) colonization, root length and nutrient uptake were measured. The res ults showed an increase in plant cover, biomass and uptake of nutrient s in the I+P treatment in comparison with all the other treatments. Th e rehabilitation of degraded lands in La Gran Sabana does not seem pos sible solely with the application of chemical fertilizers. It was evid ent that mycorrhizae are required to achieve rehabilitation, given tha t the I+P treatment led to significantly better results than those ach ieved with treatment P. The importance of mycorrhizae in the restorati on of these lands is supported by the finding that, of the native plan ts which re-established in the different treatments, 81% were mycorrhi zal.