Scielo RSS <![CDATA[Journal of soil science and plant nutrition]]> vol. 10 num. 3 lang. es <![CDATA[SciELO Logo]]> <![CDATA[<strong>ARBUSCULAR MYCORRHIZAE IN AGRICULTURAL AND FOREST ECOSYSTEMS IN CHILE</strong>]]> Arbuscular mycorrhizal (AM) association plays a key role in the sustainability of terrestrial plant ecosystems, in particular those presenting limitations for the establishment and subsequent growth of plants. In Chile, more than 50% of arable soils are originated from volcanic ashes, showing in general several constraints to crop production, such as low pH, high exchangeable aluminum content and low levels of available P. Under these conditions, the management of AM fungal propagules using adequate cultural management practices emerges as a successful alternative in order to maximize the positive effects of AM symbiosis on plant growth in these types of soil. This review presents the results of several years of research about the effect of different agronomic and forest management practices on the density and functionality of the native fungal populations in volcanic soils from Southern Chile, and their subsequent effect on the improvement of soil characteristics. These investigations have contributed to a better understanding of the role played by AM symbiosis in such soils and provide guidance on the most appropriate alternatives to increase its presence and functionality. <![CDATA[MICROBIAL OILS AND FATTY ACIDS: EFFECT OF CARBON SOURCE ON DOCOSAHEXAENOIC ACID (C22:6 N-3, DHA) PRODUCTION BY THRAUSTOCHYTRID STRAINS]]> Thraustochytrids are marine protists found throughout the world in estuarine and marine habitats. These microorganisms have attracted interest, because their lipids contain different long chain polyunsaturated fatty acids (PUFAs). Those able to produce docosahexaenoic acid (C22:6 n-3, DHA) are the most studied because of the physiological importance of this PUFA in human beings. Their heterotrophic cultivation offers several challenges since biomass, lipid content, and fatty acid profile are dependent on growth conditions. In this work the effects of C source and its concentration on DHA production by different thraustochytrid strains are reviewed. Results obtained by different investigators on the use of alternative and low cost nutrient sources for production of DHA by thraustochytrids are also presented. <![CDATA[<strong>Al <sup>3+</sup> - Ca<sup>2+</sup> INTERACTION IN PLANTS GROWING IN ACID SOILS</strong>: <strong>AL-PHYTOTOXICITY RESPONSE TO CALCAREOUS AMENDMENTS</strong>]]> High aluminum (Al) concentrations as Al3+ represent an important growth and yield limiting factor for crops in acid soils (pH ≤5.5). The most recognized effect of Al-toxicity in plants is observed in roots. However, damages in the upper parts (including stem, leaves and fruits) may also be present. In addition, Al-toxicity triggers an increase in reactive oxygen species (ROS), causing oxidative stress that can damage the roots and chloroplasts, decreasing normal functioning of photo synthetic parameters. Al-toxicity may also increase or inhibit antioxidant activities, which are responsible to scavenge ROS. As result of the negative effects of toxic Al, root metabolic processes, such as water and nutrient absorption, are disturbed with a concomitant decrease in calcium (Ca) uptake. Ca plays a fundamental role in the amelioration of pH and Al-toxicity through Al-Ca interactions improving physiological and biochemical processes in plants. Ca is a useful amendment for correcting these negative effects on crops growing in acid soils. This is an agronomic practice with alternatives, such as limestone or gypsum. There is little information about the interaction between amendments and Al-toxicity in physiological and biochemical processes in crops. Thus, the main objective of this review is to understand the interactions between Al3+ and Ca amendments and their effects on the physiology and biochemical responses in crops growing in acid soils. <![CDATA[<strong>BIOLOGICAL ASPECTS INVOLVED IN THE DEGRADATION OF ORGANIC POLLUTANTS</strong>]]> Worldwide use of pesticide has increased dramatically during the last two decades. As a consequence, pesticide residues and their transformation products are frequently found in groundwater and surface waters. This review summarizes information about polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), some chlorophenols; it mainly empathizes on pesticides, their incorporation into the environment, microorganisms involved in their degradation and, some physico-chemical aspects of pesticides behavior in soils. Details about residues of pesticides in groundwater and superficial water found in some foreign countries and in Chile are reported, as well. The fungal degradation of organic pollutants (xenobiotics) is considered as an effective method to remove these pollutants from the environment by a process which is currently known as bioremediation. Therefore, the degradation of pesticides by soil microorganisms with particular attention to white-rot fungi is also addressed. Finally, a simple and effective system (biobed) to minimize environmental contamination from pesticide manipulation, especially when filling the spraying equipment, a typical point source of contamination, is presented here. <![CDATA[<strong>MOBILITY AND BIOAVAILABILITY OF HEAVY METALS AND METALLOIDS IN SOIL ENVIRONMENTS</strong>]]> In soil environments, sorption/desorption reactions as well as chemical complexation with inorganic and organic ligands and redox reactions, both biotic and abiotic, are of great importance in controlling their bioavailability, leaching and toxicity. These reactions are affected by many factors such as pH, nature of the sorbents, presence and concentration of organic and inorganic ligands, including humic and fulvic acid, root exudates, microbial metabolites and nutrients. In this review, we highlight the impact of physical, chemical, and biological interfacial interactions on bioavailability and mobility of metals and metalloids in soil. Special attention is devoted to: i) the sorption/desorption processes of metals and metalloids on/from soil components and soils; ii) their precipitation and reduction-oxidation reactions in solution and onto surfaces of soil components; iii) their chemical speciation, fractionation and bioavailability. <![CDATA[MECHANISMS AND PRACTICAL CONSIDERATIONS INVOLVED IN PLANT GROWTH PROMOTION BY RHIZOBACTERIA]]> Rhizobacteria are capable of stimulating plant growth through a variety of mechanisms that include improvement of plant nutrition, production and regulation of phytohormones, and suppression of disease causing organisms. While considerable research has demonstrated their potential utility, the successful application of plant growth promoting rhizobacteria (PGPR) in the field has been limited by a lack of knowledge of ecological factors that determine their survival and activity in the plant rhizosphere. To be effective, PGPR must maintain a critical population density of active cells. Inoculation with PGPR strains can temporarily enhance the population size, but inoculants often have poor survival and compete with indigenous bacteria for available growth substrates. PGPR often have more than one mechanism for enhancing plant growth and experimental evidence suggests that the plant growth stimulation is the net result of multiple mechanisms of action that may be activated simultaneously. The aim of this review is to describe PGPR modes of action and discuss practical considerations for PGPR use in agriculture. <![CDATA[<strong>BACTERIAL DEGRADATION AND BIOREMEDIATION OF CHLORINATED HERBICIDES AND BIPHENYLS</strong>]]> Chlorinated herbicides (e.g. s-triazines) and polychlorobiphenyls (PCBs) are persistent organic pollutants (POPs) that are widely distributed in the environment. s-Triazine herbicides are used in agriculture and forestry in diverse regions of the world. PCBs were produced worldwide for industrial applications, and an important amount of these compounds have been released into the environment. PCBs and s-triazines are toxic compounds that could act as endocrine disrupters and cause cancer. Therefore, environmental pollution with s-triazines and PCBs is of increasing concern. Bioremediation is an attractive technology for the decontamination of polluted sites. Microorganisms play a main role in the removal of POPs from the environment. Diverse bacteria able to degrade s-triazines and PCBs have been characterized. Bacterial degradation of s-triazine herbicides involves hydrolytic reactions catalyzed by amidohydrolases encoded by the atz genes. Anaerobic and aerobic bacteria are capable of biotransforming PCBs. Higher chlorinated PCBs are subjected to reductive dehalogenation by anaerobic microorganisms. Lower chlorinated biphenyls are oxidized by aerobic bacteria. Genome analyses of PCB-degrading bacteria have increased the knowledge of their metabolic capabilities and their adaptation to stressful conditions. For the removal of s-triazines and PCBs from the environment, efficient bioremediation processes have to be established. In this report, bacterial degradation of s-triazines and PCBs is described and novel strategies to improve bioremediation of these POPs are discussed. <![CDATA[ROLE OF ENZYMES IN THE REMEDIATION OF POLLUTED ENVIRONMENTS]]> Environmental pollution is growing more and more due to the indiscriminate and frequently deliberate release of hazardous, harmful substances. Research efforts have been devoted to develop new, low-cost, low-technology, eco-friendly treatments capable of reducing and even eliminating pollution in the atmosphere, the hydrosphere and soil environments. Among biological agents, enzymes have a great potentiality to effectively transform and detoxify polluting substances because they have been recognized to be able to transform pollutants at a detectable rate and are potentially suitable to restore polluted environments. This brief review will examine some classes of pollutants and enzymes capable of transforming them effectively into innocuous products. Particular attention will be devoted to pollutants with a high polluting potential such as polyphenols, nitriles, PAHs, cyanides and heavy metals. The enzymatic processes developed and implemented in some of these detoxification treatments will be examined in details. The main advantages as well as the main drawbacks that are still present in the extensive application of enzymes in the in situ restoration of polluted environments will be discussed. <![CDATA[EFFECTIVENESS OF THE APPLICATION OF ARBUSCULAR MYCORRHIZA FUNGI AND ORGANIC AMENDMENTS TO IMPROVE SOIL QUALITY AND PLANT PERFORMANCE UNDER STRESS CONDITIONS]]> Plant growth is limited in arid and/or contaminated sites due to the adverse conditions coming from heavy metal (HM) contamination and/or water stress. Moreover, soils from these areas are generally characterised by poor soil structure, low water-holding capacity, lack of organic matter and nutrient deficiency. In order to carry out a successful re-afforestation, it is necessary to improve soil quality and the ability of plants species to resist this harsh environment. The symbiosis with arbuscular mycorrhizal (AM) fungi has been proposed as one of the mechanisms of plant heavy metal tolerance and water stress avoidance. On the other hand, addition of organic amendments to the soil can reverse degradation of soil properties. Agro-waste residues such as dry olive cake (DOC) and sugar beet waste (SB) supplemented with rock phosphate (RP) can be used as organic amendments after fermentation by Aspergillus niger. The application of A. niger- treated DOC and/or SB to semi-arid soils and/or HM-contaminated soils increased aggregate stability, soil enzymatic activities, water soluble C and water soluble carbohydrates as well as nutrient availability, especially P. AM inoculation, using adapted endophytes, was more efficient with respect to increasing plant nutrition and growth as well as plant tolerance to drought or HM-stress conditions. The combined treatments involving mycorrhiza fungi inoculation and addition of the amendments into the soil can be proposed as a successful revegetation strategy for plant performance in P-deficient soils under semiarid Mediterranean conditions. The beneficial effectiveness of this symbiosis with suitable AM fungi in A. m'ger-treated agro-waste residue-amended soil can also be regarded as a successful biotechnological tool for reclamation of HM-contaminated soils. <![CDATA[<strong>FLAXSEED AS A SOURCE OF FUNCTIONAL INGREDIENTS</strong>]]> Scientific evidence supports flaxseed consumption; however, a large sector of the population is still unaware of the benefits associated to its consumption and its possible applications as functional food ingredient in foodstuffs. Flaxseed is mainly known by its high alpha-linolenic acid content, but it is also a lignan source, soluble fibre and protein, compounds which are biologically active in the prevention of some non-transmissible chronic diseases. Southern Chile has comparative advantages for the cultivation of this crop. Together with its full processing, this crop could strengthen regional industry. The purpose of the present review is to highlight the nutritional properties of flaxseed. <![CDATA[<strong>THE ROLE OF COAL COMBUSTION PRODUCTS IN MANAGING THE BIOAVAILABILITY OF NUTRIENTS AND HEAVY METALS IN SOILS</strong>]]> Coal provides the primary source of energy for many countries including Asia, South America, North America and Europe, and large quantities of waste products are produced during the combustion of coal in coal-fired power stations. This review paper gives an overview of the various types of coal combustion products (CCPs) and their utilization in various sectors, with a particular emphasis on the role of CCPs in managing the bioavailability of nutrients and heavy metals in relation to enhancing soil chemical fertility and mitigating metal contaminated soils. Currently, CCPs are used extensively in agricultural and construction industries. In agricultural industries, they are used primarily as an amendment to improve the physical and chemical properties of soil, as a source of liming material to ameliorate soil acidity and as a nutrient source to supply calcium and sulphur. In the construction industry, they are used mainly as a source of concrete, roofing material and road surface sealing. Increasingly, CCPs are used in the remediation of contaminated environments including control of acid mine drainage, mitigating phosphorus leaching in farm lands and immobilization of toxic metals in mine sites and agricultural soils.