Uso de topsoil como fonte de inóculo de microrganismos simbiontes para leguminosas florestais usadas na recuperação de áreas degradadas na Caatinga

Abstract

The piçarra consists of a subsoil material composed mainly of silt, sand and gravel used as embankment in pump jack bases, access roads and other industrial structures used in the onshore oil exploration in the state of Rio Grande do Norte, Brazil. The mining of this material occurs in small areas, but causes great impact in the landscape due to the removal of the native vegetation and of the soil upper layers. At the end of the exploration, the piçarra mines and other decommissioned oil exploration locations must be recovered, which involves the planting of pioneer species capable of establishing in the piçarra. Several species of the Leguminosae family have the capacity to adapt to degraded soils without organic matter. However, this adaptation depends on the establishment of symbioses with nitrogen-fixing bacteria (rhizobia) and arbuscular mycorrhizal fungi (AMF). The first chapter of this study aimed to characterize the piçarra and the topsoil of adjacent areas with native vegetation of the Caatinga biome in relation to the presence of rhizobia and spores of AMF. The study areas consisted of two decommissioned mines of piçarra (J1 and J2), and a waste deposit and a pump jack base, both covered with piçarra. Soils were sampled in the 0 to 10 cm layer and characterized for fertility, texture and presence of rhizobia and AMF. The AMF spores were rescued using the wet-sieving technique, counted and identified. The density of rhizobia was determined by the most probable number (MPN) obtained from inoculation of bait plants of Mimosa tenuiflora (Mart.) Benth. (jurema-preta) and Mimosa caesalpiniifolia Benth. (sabiá). The fertility and organic matter contents of the piçarra plots were lower than those of native vegetation. The textural class of the collected samples was predominantly sandy. The highest MPN of rhizobia was obtained in the J2 deposit and in native vegetation adjacent to it, especially in the rainy season. The number of AMF spores was higher in native vegetation areas especially in the dry season. The second chapter aimed to evaluate the efficiency of the use of topsoil, from native vegetation area, as an inoculant of AMF and rhizobia for M. caesalpiniifolia Benth species. An experiment was outlined with the following treatments: (1) inoculation of plants with 10 g of topsoil from a native area nearby J1; (2) idem T1 with topsoil of a native area nearby J2; (3) rhizobium inoculant and (4) rhizobia + AMF produced in the laboratory; (5) absolute control (TA); (6) TA with autoclaved substrate; (7) nitrogenous control (TN) and (8) TN + AMF. The double inoculation with rhizobia and AMF promoted the highest plant growth, nodule mass and mycorrhizal colonization, but statistically similar to treatments with topsoil. The results demonstrated the ability of topsoil to provide propagules of rhizobia and AMF efficient in promoting the growth of the tested species. Further studies should be carried out to confirm this potential with other native Caatinga species as well as to evaluate the performance of topsoil as an inoculant under field conditions.