Arbuscular mycorrhizae in the rehabilitation of fragile degraded tropical lands

The aim of this work was to establish a vegetative cover with an introduced grass on an infertile substrate which contains little mycorrhizal inoculum. A field experiment was carried out in La Gran Sabana, Venezuela, in an area that was disturbed in 1991 and in which no spontaneous recolonization by plant species occurred. Five treatments were set up in which an introduced grass. Brachiaria decumbens, was sown. The treatments were: non-inoculated control (NI); inoculated with a concentrated mix of arbuscular mycorrhizal fungi (AMF) (385 spores g^–1 inoculum) at 1750kgha^–1 (I); fertilized with triple superphosphate, 100kgha^–1 (P); inoculated with AMF and simultaneously fertilized with triple superphosphate (I+P); another control treatment, to which previously sterilized AMF inoculum was added (S). In all cases B. decumbens was seeded at 30kgha^–1. A soil microorganism inoculum free of mycorrhizae was added to all the treatments. Five months after sowing the grass, above and below ground biomass, % arbuscular mycorrhizal (AM) colonization, root length and nutrient uptake were measured. The results showed an increase in plant cover, biomass and uptake of nutrients in the I+P treatment in comparison with all the other treatments. The rehabilitation of degraded lands in La Gran Sabana does not seem possible solely with the application of chemical fertilizers. It was evident that mycorrhizae are required to achieve rehabilitation, given that the I+P treatment led to significantly better results than those achieved with treatment P. The importance of mycorrhizae in the restoration of these lands is supported by the finding that, of the native plants which re-established in the different treatments, 81% were mycorrhizal. Received: 23 February 1997