Scielo RSS <![CDATA[Journal of soil science and plant nutrition]]> vol. 15 num. 2 lang. es <![CDATA[SciELO Logo]]> <![CDATA[<b>Future challenges and perspectives for applying microbial biotechnology in sustainable agriculture based on a better understanding of plant-microbiome interactions</b>]]> An intensive agricultural production is necessary to satisfy food requirements for the growing world population. However, its realization is associated with the mass consumption of non-renewable natural resources and with the emission of greenhouse gases causing climate changes. The research challenge is to meet sustainable environmental and economical issues without compromising yields. In this context, exploiting the agro-ecosystem services ofsoil microbial communities appears as a promising effective approach. This chapter reviews the research efforts aimed atimproving a sustainable and healthy agricultural production through the appropriate management of soil microorganisms.First, the plant-associated microbiome is briefly described. Then, the current research technologies for formulation and application of inocula based on specific beneficial plant-associated microbesare summarized. Finally, the perspectives and opportunities to manage naturally existing microbial populations, including those non-culturable, are analyzed. This analysis concerns: (i) a description of the already available, culture-independent, molecular techniques addressed at increasing our understanding of root-microbiome interactions; (ii) how to improve the ability of soil microbes for alleviating the negative impacts of stress factors on crop productivity; and (iii) whether plants can structure their root-associated microbial communities and, leading on from this, whether the rhizosphere can be engineered (biased) to encourage beneficial organisms, while prevent presence of pathogens. <![CDATA[<b>Enzymes of importance to rhizosphere processes</b>]]> All processes and functions taking place in the rhizosphere are dominated by the activities of plant roots, rhizosphere microorganisms and root-microorganism interactions, and enzymes are recognized as main actors of all activities occurring in rhizosphere environments. Rhizosphere enzymes have, in general, a higher activity than those operating in bulk soil, as the rhizosphere soil is richer in organic C substrates. Enzymes, produced and released by both roots and microorganisms concur to altering the availability of nutrients in the rhizosphere, being implied in the hydrolysis of C-substrates and organic forms of nutrients such as N, P and S. The production and activity of rhizosphere enzymes is controlled by several factors, in turn depending on soil-plant-microorganism interactions. In general, higher activity of rhizosphere enzymes can be interpreted as a greater functional diversity of the microbial community. An interesting aspect is their involvement in the possible removal of both inorganic and organic pollutants from the terrestrial food chain. The lack of satisfying methodologies for measuring the location and activity of rhizosphere enzymes has often hampered a clear knowledge of their properties and functions. Sophisticated technologies, now available, will be helpful to reveal the origins, locations and activities of enzymes in rhizosphere. The main scope of the present paper is to cover briefly general and specific concepts about rhizosphere enzymes and their role in soil processes. Examples chosen among those published recently, supporting and confirming properties, features, and functions of rhizosphere enzymes will be illustrated. <![CDATA[<b>Ecophysiological role of <i>Embothrium coccineum</i>, a Proteaceae species bearing cluster roots, at increasing Phosphorus availability in its rhizosphere</b>]]> Native forests in southern South America are constantly subjected to natural disasters such as volcanic eruptions. Soil affected by volcanic ash contain large amounts of total P but low P availability, as this element is strongly adsorbed to soil colloids (i.e. allophane). This lack of available P is one of the main limitations to plant growth. In this context, it is necessary an in-deep study of plant species that have developed some root physiological strategies for P acquisition. An example of this is the formation of cluster roots by Proteaceae species. Recently, information has been reported that aids to our understanding of the functioning of Proteaceae species growing in volcanic soils. The aim of this review is to discuss the ecophysiological role of Proteaceae species growing in young volcanic soils, with a special emphasis on Embothrium coccineum, a pioneer species of extremely disturbed environments. In summary, we reveal here that E. coccineum has several features that make it suitable for recovering degraded soils in south-central Chile. Some of these characteristics include its ability to survive and successfully establish in poor soils due to its specialized roots adaptation and its ability to shed its leaves under stressful conditions. According to recent evidence, E. coccineum has relatively low foliar nutrient resorption leaving at least half of the nutrients in its senescent leaves; this, in turn, promotes nutrient cycling via mineralization of its leaves. Finally, we conclude that the cluster roots of E. coccineum promote P solubilization and mineralization in the rhizosphere soil allowing for increased P availability for the plant itself and potentially also for neighbouring species. <![CDATA[<b>Soil carbon controlled by plant, microorganism and mineralogy interactions</b>]]> Rhizosphere, a thin area of soil surrounding roots receiving carbon (C) exudation from plants, represents a site of intense competition for available C and nutrient between surface-reactive particles and soil microorganisms. This competition can reduce the amount of available C to a critical level, it becomes limiting for microbial growth and soil organic matter decomposition. On the other hand, acceleration or retardation of decomposition of soil organic C caused by root activity is termed rhizosphere priming effect (RPE). This effect has been increasingly recognized to play a crucial role on native C destabilization as is influenced by fresh C availability, microbial activity and soil mineralogy such as crystallinity of clay minerals and Al-, Fe-oxides. Combining microbial ecology and soil mineral interactions, we can understand how soil characteristics and climate change can influence below ground competition and finally RPE. In this review, we focus on the competition for available C in soil, limiting our analyses to the interaction at rhizospheric space, where most processes between microorganisms and mineral phase occurs. <![CDATA[<b>Organic amendments as sustainable tool to recovery fertility in intensive agricultural systems</b>]]> Intensive agriculture is a farming system characterised by a large use of inputs, causing a large pressure on the environment. As peculiar and efficient example of intensive agriculture cultivation under plastic tunnels provides several advantages for farmers due to improvement of microclimatic conditions coupled with a relatively low investment costs. In the Mediterranean Basin such cultivation systems reach about 200,000 ha mainly in Spain, Turkey, Italy, and Morocco. As downside, intensive agriculture negatively affects soil fertility principally because of a loss in soil organic matter. Sustainable practices providing organic amendments could be a useful tool to maintain or increase organic matter content in agricultural soils, preserving and improving soil fertility. An improved knowledge of management factors affecting soil quality is crucial to plan farming systems that effectively maintain soil fertility. Therefore, this review focuses on the potential value of organic amendments in the recovery of soil fertility, in particular in sites under plastic cover intensive farming system. Following a brief overview of the effects of intensive agriculture on soil, the review describes various organic amendments used in agriculture and their benefits on soil fertility, to conclude with the need, in the future researches, to identify organic amendments able to maximize a recovery of soil fertility. <![CDATA[<b>The impact of grassland management on biogeochemical cycles involving carbon, nitrogen and phosphorus</b>]]> Grassland introduction into intensively managed agricultural landscapes may enhance soil organic matter (SOM) content and ecosystem services. However, the magnitude of this effect depends on grassland management practices, and their influence the soil system. The aim of this paper is to highlight these impacts and their consequences for SOM dynamics and element cycling. We focused in particular on the effect of different grassland management practices in terms of grazing regime, fertilization, and species choice. While carbon, nitrogen and phosphorus cycles are more strongly coupled under grassland as compared to permanent cropping, uncoupling of elemental cycles may occur through management intensification. Grazing regime, fertilization and species choice affect elemental coupling and SOM turnover via organic matter input and rhizosphere activity to different extent, thereby resulting in contrasting SOM storage. Grazing may be more beneficial for SOM contents compared to mowing up to a certain animal density depending on soil type and pedoclimatic context. SOM storage may be increased in some cases through specific fertilizer additions, whereas in others no change was observed. Species choice, e.g. high diversity or introduction of legumes, influence element budgets and soil nutrient availability through plant physiological constraints as well as intra- or interspecific interactions. The effect of different plant species mixtures on soil parameters has rarely been elucidated. We conclude that the impact of grassland management practices on SOM of different soil types and the resulting ecosystem services, such as C and nutrient storage need further research in contrasting pedoclimatic contexts. More studies on the controls of belowground biogeochemical cycling of elements are necessary in order to fully understand and manage belowground processes via aboveground plant communities. <![CDATA[<b>Nutrient cycling in the mycorrhizosphere</b>]]> Optimizing the turnover and recycling of nutrients, a fundamental issue for the sustainability and productivity of agro-ecosystems is depending on the functionality of a framework of plant-soil interactions where microbial populations are involved. Both mutualistic symbionts and saprophytic microorganisms living at the root-soil interfaces, the rhizosphere, or in the plant-associated soil, are recognized as essential drivers of nutrient cycling, availability and capture. Among the mutualistic symbionts, arbuscular mycorrhizal (AM) fungi are one of the most influential groups of soil biota because after establishing the AM symbiosis with most plant species they enhance plant nutrient uptake properties. Saprophytic microorganisms are recognized for their abilities to propel nitrogen (N) fixation and/or phosphorus (P) mobilization, two fundamental processes for sustain plant productivity. Mycorrhiza establishment changes the biological and physical-chemical properties of the rhizosphere, developing the so-called mycorrhizosphere. Particularly relevant is the mycorrhizosphere of legume plants since it also involves the symbiosis with N2-fixing nodulating rhizobial bacteria. In this overview of mycorrhizosphere interactions related to nutrient cycling, after describing the protagonist microorganisms, the mechanisms responsible for nutrient acquisition by AM-plants are first analyzed. Then, the processes involved in mycorrhizosphere establishment and functions are described. Finally, the achievements derived from managing selected AM fungi and beneficial bacteria interactions (mycorrhizosphere tailoring) are discussed. The use of 15N and 32P to elucidate the contribution of the mycorrhizosphere components to plant nutrient acquisition is detailed. <![CDATA[<b>Availability of Mn, Zn and Fe in the rhizosphere</b>]]> This review paper critically assesses the literature on soil-microbe-plant interactions influencing availability of micronutrients in the rhizosphere. The emphasis is placed on Zn and Mn, but Fe is also covered to some extent. Micronutrient availability in the rhizosphere is controlled by soil and plant properties, and interactions of roots with microorganisms and the surrounding soil. Plants exude a variety of organic compounds (carboxylate anions, phenolics, carbohydrates, amino acids, enzymes, etc.) and inorganic ions (protons, phosphate, etc.) to change chemistry and biology of the rhizosphere and increase micronutrient availability. Increased availability may result from solubilization and mobilization by short-chain organic acid anions, amino acids and other low-molecular-weight organic compounds. Acidification of the rhizosphere soil increases mobilization of micronutrients (eg. for Zn, 100-fold increase in solubility for each unit of pH decrease). For diffusion-supplied micronutrients, the uptake rate is governed by the soil nutrient supply. Fertilisation with micronutrients (more so in case of Zn than Fe) can be effective in increasing the concentration of micronutrients at the soil-root interface. In addition, micronutrient-efficient crops and genotypes can increase an available nutrient fraction and hence increase micronutrient uptake. Our understanding of the physiological processes governing exudation and the soil-plant-microbe interactions in the rhizosphere is currently inadequate, especially in terms of spatial and temporal variability in root exudation as well as the fate and effectiveness of organic and inorganic compounds in increasing availability of soil micronutrients and undesirable trace elements. The interactions between microorganisms and plants at the soil-root interface are particularly important as well obscure. <![CDATA[<b>Rhizosphere effect on pesticide degradation in biobeds under different hydraulic loads</b>]]> Interactions between microorganisms and root exudates in a biobed system with vegetal (grass) cover could enhance pesticide degradation. Otherwise, a high water load may generate high concentrations of pesticides in lixiviates. We studied the effect of the vegetal cover on the degradation of a mixture of atrazine (ATZ), chlorpyrifos (CHL) and iprodione (IPR) (35 mg L-1 each) in a biobed system operated with two different hydraulic loads (0.6 and 1.2 L of tap water per day). The concentration of the pesticides and their main metabolites were measured in the lixiviates during 60 days, as well as in the biomixtures at the end of the study. Dehydrogenase activity in the biomixtures and organic acid exudation from the vegetal cover were also analysed. The vegetal cover diminished the lixiviation of pesticides and their metabolites mainly at the lower hydraulic load used. The degradation of the pesticides was high (>95%) and increased in biobeds with vegetal cover and low hydraulic load. Degradation metabolites of CHL and IPR were formed during pesticide degradation; however they were degraded in the biobed and were not detected in lixiviates at the end of the study. In general, an increase in organic acid exudation by vegetal cover was observed caused by chemical stress after pesticide application. The increase was similar at both hydraulic loads. Efficient colonisation of wheat straw by fungi was observed by confocal microscopy. <![CDATA[<b>Biogeochemical processes at soil-root interface</b>]]> Rhizosphere is a microsite where interactions among roots, microorganisms, soil constituents (minerals and organic matter), and soil solution take place. Biomolecules produced by plants and microorganisms and soil organic substances are involved in many biogeochemical processes at soil-root interface such as: a) weathering of clay minerals and release of Al and Fe, b) formation of nanoprecipitates and organo-mineral complexes, c) sorption/desorption of cations and anions on/from soil colloids and d) bioavailability of nutrients and pollutants. Many exudates form strong complexes with Fe and Al ions, retard or inhibit their hydrolytic reactions and promote the formation of noncrystalline or short-range ordered Al and Fe nanoprecipitates. The so-called iron plaques, present on many wetland plant roots, are Fe(III)-oxyhydroxides (mainly ferrihydrite). These precipitates may interact with biopolymers (proteins, polysaccharides, DNA, RNA and so on), phyllosilicates, soil organic substances as well as microorganisms forming organo-mineral complexes. Root exudates play a vital role on the sorption/desorption of nutrients and pollutants at soil-root interface. The processes, which affect the sorption of cations and anions on sorbents present in the rhizosphere are particularly complex, being sorption of cations quite different from that of anions. Root exudates usually inhibit the sorption of anions, but may promote or inhibit the sorption of cations. They may desorb, at least partially, nutrients and pollutants previously sorbed on soil components, promoting their bioavailability for plants and microorganisms. Finally, some plants release chelating organic ligands able to complex metals (e.g. Al) which become less toxic. Many factors control these processes, for example; pH, nature and concentration of the biomolecules present in the rhizosphere, nature of sorbent and sorbate, reaction time. <![CDATA[<b>Biotic interactions in the rhizosphere in relation to plant and soil nutrient dynamics</b>]]> The rhizosphere is the interface between roots and the soil where nutrient absorption for plant growth in agroecosystems is facilitated. An abundant and diverse rhizosphere biome is involved in biogeochemical processes, including bacteria, fungi and soil fauna, driving soil C, N and P dynamics. Plant carbon photosynthates allocated to the root and rhizosphere are priming microbial activities important for plant nutrition such as organic matter decomposition, P solubilization, N fixation, mycorrhizal nutrient transport and biocontrol of root pests. While substantial information is available on the role of individual groups of the rhizosphere microbiome in biogeochemical processes, less attention has been given to the interactions between different beneficial rhizosphere microorganisms. Also, interactions between soil fauna and rhizosphere microorganisms still remain relatively unexplored. In order to improve our knowledge on the role of the rhizosphere in C, N and P biogeochemical processes a more holistic and functional approach is required. In this review, state of the art information on the role of biotic interactions in the rhizosphere on C, N and P biogeochemical processes relevant for plant nutrition in agroecosystems is presented. <![CDATA[<b>Phosphorus disequilibrium in the tripartite plant-ectomycorrhiza-plant growth promoting rhizobacterial association</b>]]> Plant roots and rhizospheres are colonized by an extensive and diverse microbial community. These microbes may form mutualistic, commensal, and/or pathogenic relationships and influence agricultural and forest productivity. Symbiotic ectomycorrhizal (EcM) fungi colonize the roots of many tree species, and the literature on these associations extensively describes their influence on plant nutrient relations and response to environmental stress. Similarly, soil bacteria ubiquitously colonize roots and rhizospheres and many of these bacteria may also play roles in influencing tree productivity. In particular, plant growth promoting rhizobacteria (PGPR) positively affect plant growth by altering nutrient availability in soils and inducing changes in plant hormone balance, plant stress resistance, and immunity pathways. In nature, EcM fungi and soil PGPR co-exist and the interaction and composition of this multi-tiered rhizosphere community aids in the acquisition of nutrient resources from soils as well as host plant response to environmental stress. The assembly of EcM communities is influenced by tree species and environmental conditions, and the tree and EcM species further influence PGPR community structure. Functionally, these symbiotic associations exhibit unique expression profiles and ecophysiological activities within the tripartite association. EcM and PGPR mediate production of complex arrays of exudates, including organic acids, siderophores, enzymes, and other organic compounds, which alter nutrient equilibria in soils, leading to increased access to phosphorus (P) and other macro- and micronutrients. As a metaorganism, the tripartite ectomycorrhizas increase the ecological breadth of host trees and influence the structure and function of forested ecosystems. <![CDATA[<b>Improving selenium status in plant nutrition and quality</b>]]> Selenium (Se) is an essential micronutrient for human health due to its antioxidant capabilities. The Se content around the world is highly variable from 0.005 mg kg-1 in areas from China and Finland to 8,000 mg kg-1 in seleniferous soils fromTuva-Russia. However, about one billion of people in the worldwide are Se deficient. During the last decade, studies related with strategies for Se biofortification in food plants for human nutrition have significantly increased because this metalloid is incorporated into human metabolism mainly as a constituent of food plants. Similarly, Se biofortification is important in pastures for increasing the Se content in cattle to enrich meat and to prevent disease associated to Se deficiency as white muscle disease. In China, two endemic diseaseshave been relatedto Se deficiency: Keshan and Keshin-Beck diseases. Agronomic biofortification by using inorganic Se sources is a current practice in countries as China, Finland, and USA. In Chile, fertilization by using chemical compounds with Se is an uncommon practice due the edaphoclimatic characteristics of Andisols, which represent around 60% of agricultural soils of southern Chile. Recent studies showed that microorganisms as bacteria and arbuscularmycorrhizal fungi play an important role in the transformations and Se availability, representing an interesting biotechnological alternative to Se biofortification. This review is focalized to describing Se behavior in soil-plant system and the possible strategies to improving Se content, including the use of microorganisms as biotechnological tools for increasing plant nutrition and quality. Specific attention will be devoted to volcanic soils of Southern Chile, where different factors concur to enhance the Se-deficiency problem. <![CDATA[<b>Current overview on the study of bacteria in the rhizosphere by modern molecular techniques</b>: <b>a mini‒review</b>]]> The rhizosphere (soil zone influenced by roots) is a complex environment that harbors diverse bacterial populations, which have an important role in biogeochemical cycling of organic matter and mineral nutrients. Nevertheless, our knowledge of the ecology and role of these bacteria in the rhizosphere is very limited, particularly regarding how indigenous bacteria are able to communicate, colonize root environments, and compete along the rhizosphere microsites. In recent decades, the development and improvement of molecular techniques have provided more accurate knowledge of bacteria in their natural environment, refining microbial ecology and generating new questions about the roles and functions of bacteria in the rhizosphere. Recently, advances in soil post‒genomic techniques (metagenomics, metaproteomics and metatranscriptomics) are being applied to improve our understanding of the microbial communities at a higher resolution. Moreover, advantages and limitations of classical and post‒genomic techniques must be considered when studying bacteria in the rhizosphere. This review provides an overview of the current knowledge on the study of bacterial community in the rhizosphere by using modern molecular techniques, describing the bias of classical molecular techniques, next generation sequencing platforms and post‒genomics techniques. <![CDATA[<b>Rhizosphere-induced heavy metal(loid) transformation in relation to bioavailability and remediation</b>]]> Soil is the sink and source of heavy metals (both geogenic and anthropogenic) and plants are the ecosystem regulators, balancing the chemistry of life on earth. However, roots are the only connection between soil and plants, which are the real engineers of ecosystem dynamics responsible for environmental balance and stability. The plant-soil interface termed as ‘rhizosphere’ is a typical zone of soil where the physical, chemical and biological characteristics are different from bulk soil (outside the rhizosphere region). This is mainly controlled by physiological response from plants to the environmental changes through exudation of chemicals from root region and the cascade of chemical (changes in pH and redox potential, release of anions and nutrient transformation) and biological (microbial association) events that follow. The other adaptive mechanisms include root length and area as affected by temperature, moisture and nutrient content of the soil. In the recent years, advanced technologies have lead to significant findings at the micro-level in rhizosphere research, targeting the role of root-soil interface towards nutrient availability and agricultural productivity. However, with increasing human activities (including agriculture), undesirable quantites of heavy metals are being added to the environment thereby resulting in soil contamination. This review will discuss in detail on the processes involved in the (im)mobilisation of heavy metals in and around the root region as affected by chemical (pH and root exudates) and biological (microorganisms) components.