Arbuscular Mycorrhizae and PGPR Applications in Tropical Savannas

Abstract

In Venezuela, most of agricultural soils are acid with low fertility. In these soils available forms of P are scarce and not readily available. Most of P is inorganic associated with iron and aluminum phosphates (45%), and is available by solubilization. To improve production and soil fertility it is recommended to fertilize with rock phosphate, organic amendments as cover crops and use of biofertilizers. Microorganisms related to P nutrition were studied in an acid Ultisol, located in central-northern Venezuela. 25% of native plants were mycorrhized and had phosphate solubilizing bacteria (PSB) in their rhizospheres, highlighting Burkholderia sp. solubilizing iron, aluminum and calcium phosphates and rock phosphate. Native Glomeromycota fungi (NGF) were reproduced in trap pots to obtain a consortia inoculum. Greenhouse tests were carried out with Zea mays L. co-inoculated with NGF in consortium and/or Burkholderia sp. Rock phosphate was added in recommended doses. Cover crops were also applied into the soil as organic matter similar to suggested agroecological management. Inoculation of NGF and PSB improved maize biomass when fertilized with rock phosphate. In all cases of BSP-AM interaction, the percent of active mycorrhizal root colonization with succinate dehydrogenase activity was higher, suggesting cooperation of this bacteria in Glomeromycota colonization and functionality, and a possible role as mycorrhiza-helper bacteria. Fertilization indicated that rock phosphate was adequate. Its use together with beneficial microorganisms appears as an alternative to improve crop production in low fertility soils.