Beneficial Soil Microorganisms, an Ecological Alternative for Soil Fertility Management

Abstract

The new challenges that will be faced by agriculture in the twenty-first century impose the adoption of strategies able to increase food production without further increase the area of arable land and with low environmental impact. Soil microorganisms are a major component of the natural fertility of soils. They can promote plant growth, increase crop productivity and contribute significantly to the mineral nutrition of crop plants. This review examines the up-to-date knowledge about the potential and existing uses of beneficial microbes as biofertilizers and gives an outline of their modes of action. Plant growth promoting microorganisms (PGPM) influence plant nutrition and growth through various mechanisms including nitrogen fixation, breakdown of organic matter, solubilization of sparingly soluble minerals, release of chelating compounds and biologically active substances such as phytohormones, vitamins and enzymes, and increase of the root system efficiency in nutrient uptake. Non symbiotic soil- or endophytic bacteria belonging to the genera Azospirillum, Azotobacter, Acetobacter, Gluconacetobacter, Azoarcus, Bacillus, Paenibacillus, Burkholderia, Herbaspirillum, Clostridium, Klebsiella, Enterobacter, Citrobacter, and Pseudomonas are able to fix atmospheric N₂ and have been found to be responsible for supply of biologically fixed nitrogen to crop plants. They may also improve plant growth through production of bioactive metabolites and indirect mechanisms, such as suppression of phytopathogens or induction of resistance to pathogens in plants. Numerous species of soil and rhizosphere microorganisms may solubilize insoluble mineral phosphates, mainly through acidification and production of organic acids, and thus mobilize the enormous reserves of phosphorus (P) that are stored in most soils and are otherwise unavailable to plants. Generally, fungi exhibit greater P-solubilizing ability than bacteria. Members of the genera Aspergillus, Penicillium, and Trichoderma are particularly efficient P-solubilizers. Among bacteria, good results have been obtained with Bacillus spp. and Pseudomonas spp., especially in combination with P-solubilizing fungi and arbuscular mycorrhizal fungi (AMF). Under field conditions, the combined use of P-solubilizing microorganisms with mineral fertilizers such as rock-phosphate has often given successful results. Also, the biological activities of microorganisms in the rhizosphere can mediate the solubility, and hence the availability at root surface of micro-nutrients, of which most soils are defective due to the fast depletion resulting from intensive farming. While other elements are also involved, Fe, Mn and Zn deficiencies have the greatest impact on the yields and quality of agricultural produce. PGPM and AMF have the capability to alter soil pH and modify the equilibrium of many chemical and biochemical reactions, such as precipitation/dissolution, adsorption/desorption, complexation/dissociation, and oxidation/reduction of metal cations and thus regulate the plant uptake. Beside enhancing plant nutrition under limited or deficient conditions, they may also reduce detrimental effects of excess of micronutrients, which may occur in acid or polluted soils. In conclusion, in order to reduce the environmental and economic costs of the massive use of synthetic fertilizers and to obtain safer food, the use of PGPM as biofertilizers appears to be a concrete perspective. All the evidence summarized in this review clearly shows that beneficial soil microorganisms, either alone or in combination with mineral or organic fertilizers, may be utilized to increase crop productivity and maintain the fertility of soils without threatening the environment. These multifunctional agents are a renewable resource with low environmental impact. Therefore, beneficial soil microbes should be further studied and exploited for the development of sustainable agriculture.