Benoît Jaillard

Origins of root-mediated pH changes in the rhizosphere and their responses to environmental constraints: A review

The aim of the present review is to define the various origins of root-mediated changes of pH in the rhizosphere, i.e., the volume of soil around roots that is influenced by root activities. Root-mediated pH changes are of major relevance in an ecological perspective as soil pH is a critical parameter that influences the bioavailability of many nutrients and toxic elements and the physiology of the roots and rhizosphere microorganisms. A major process that contributes root-induced pH changes in the rhizosphere is the release of charges carried by H+ or OH− to compensate for an unbalanced cation–anion uptake at the soil–root interface. In addition to the ions taken up by the plant, all the ions crossing the plasma membrane of root cells (e.g., organic anions exuded by plant roots) should be taken into account, since they all need to be balanced by an exchange of charges, i.e., by a release of either H+ or OH−. Although poorly documented, root exudation and respiration can contribute some proportion of rhizosphere pH decrease as a result of a build-up of the CO2 concentration. This will form carbonic acid in the rhizosphere that may dissociate in neutral to alkaline soils, and result in some pH decrease. Ultimately, plant roots and associated microorganisms can also alter rhizosphere pH via redox-coupled reactions. These various processes involved in root-mediated pH changes in the rhizosphere also depend on environmental constraints, especially nutritional constraints to which plants can respond. This is briefly addressed, with a special emphasis on the response of plant roots to deficiencies of P and Fe and to Al toxicity. Finally, soil pH itself and pH buffering capacity also have a dramatic influence on root-mediated pH changes.

Acquisition of phosphorus and other poorly mobile nutrients by roots. Where do plant nutrition models fail

BackgroundIn the context of increasing global food demand, ecological intensification of agroecosystems is required to increase nutrient use efficiency in plants while decreasing fertilizer inputs. Better exploration and exploitation of soil resources is a major issue for phosphorus, given that rock phosphate ores are finite resources, which are going to be exhausted in decades from now on.ScopeWe review the processes governing the acquisition by plants of poorly mobile nutrients in soils, with a particular focus on processes at the root–soil interface. Rhizosphere processes are poorly accounted for in most plant nutrition models. This lack largely explains why present-day models fail at predicting the actual uptake of poorly mobile nutrients such as phosphorus under low input conditions. A first section is dedicated to biophysical processes and the spatial/temporal development of the rhizosphere. A second section concentrates on biochemical/biogeochemical processes, while a third section addresses biological/ecological processes operating in the rhizosphere.ConclusionsNew routes for improving soil nutrient efficiency are addressed, with a particular focus on breeding and ecological engineering options. Better mimicking natural ecosystems and exploiting plant diversity appears as an appealing way forward, on this long and winding road towards ecological intensification of agroecosystems.

Dynamics of phosphorus in the rhizosphere of maize and rape grown on synthetic, phosphated calcite and goethite

In calcareous soils the dynamics of phosphorus is controlled by calcite and iron oxides such as goethite which strongly retain P and consequently maintain low P concentrations in soil solution. Plants can drastically change chemical conditions in the rhizosphere, in particular by releasing H+ or OH− or by excreting organic anions. By modifying the dissolution/precipitation and desorption/adsorption equilibria, roots can influence the mobility of soil P. The aim of this work was to test whether H+ or OH− release can induce the mobilization of P in the rhizosphere of maize and rape supplied with NO3-N or NH4-N and grown on synthetic phosphated calcite or goethite as sole source of P. With P-calcite, the mobilization of P was generally related to the acidification of the rhizosphere. With P-goethite, rhizosphere acidification induced some increase of DTPA-extractable Fe and hence dissolution of goethite. Rhizosphere P was concomitantly depleted but the mechanisms involved are less clear. The difference in behavior of the two species is discussed.

pH mapping in transparent gel using color indicator videodensitometry

The colored pH indicator method introduced by Weisenseel et al. (1979) is particularly useful for localizing the zones along roots where acidification/alkalinization occurs. It can also be used to assess the direction and intensity of the proton fluxes. Because the method has not been quantitatively evaluated, however, it is nowadays little used or used in conjunction with other such as potentiometry. In the present study we examine the theoretical basis underlying this method of colorimetric visualization and show its similarity to spectrodensitometry. It thus becomes possible to quantify the luminous information and express it in terms of environmental pH. We describe the method used, emphasizing in particular the conditions required to achieve maximum accuracy of measurement, and an appropriate experimental device. pH distribution around roots can be mapped with a relative error of 0.03 pH units. The experimental device is easy to use and incorporates a computer-controlled video camera, thanks to which al acquisition and calculation procedures can be automated.

Agarose as a suitable substrate for use in the study of Al dynamics in the rhizosphere

Because of experimental difficulties, few authors have studied the dynamics of aluminium in the rhizosphere. The aim of this paper is to present a suitable method for studying rhizosphere Al dynamics. It is based on the use of agarose as a substrate for plant growth. Agar and agarose gels are often used in rhizosphere studies, but most are poorly characterized and occasionally give rise to experimental artefacts, especially with low mobility elements like Al. The results reported here show that agarose is a relatively pure substrate, nearly devoid of phosphorus and other Al-complexing substances. Aqueous extracts of agarose also exhibit Al phytotoxicity equivalent to that of a nutrient solution. Since this substrate has the properties of a variable charge exchange complex, it can be considered as a physico-chemical model for organic matter. Finally, its Al adsorption capacity is high enough for the Al reserve in the substrate not to exert a limiting effect on plants and low enough to allow accurate measurement of Al depletion in the rhizophere.

Modelling of the dynamics of Al and protons in the rhizosphere of maize cultivated in acid substrate

The object of this study was to analyze the dynamics of Al and protons in the rhizosphere of maize cultivated in a simple acid substrate, so as to allow the use of a dynamic model of the functioning of a rhizosphere consisting of an organic phase (an agarose gel) and a mineral phase (an amorphous aluminium hydroxide). Two cultivars of maize (Zea mays L.), one Al-sensitive and the other Al-tolerant, were cultivated on this substrate in the presence of different proportions of NH4+ and NO3-, which served to acidify the rhizosphere to a greater or lesser extent. The state of the agarose gel and of the cell walls of the roots were monitored using an ion exchange model which had previously been calibrated for each substrate.The experiment showed that Al and protons reduce root growth and the Ca and Mg content in the root, while relative growth varies little between pH 4.0 and pH 4.5. The model showed that competition between Al and protons for the binding sites of the cell walls might account for these results. The sensitivity of the model to the rate of Al(OH)3 dissolution and to the cation exchange capacity of the culture substrate was tested by numerical simulation. When roots release protons and dissolve Al(OH)3 in the rhizosphere, there is little possibility of Al desorption by protons on the cell walls at pHs compatible with good root growth of maize, plant specie sensitive to Al and H. Furthermore, the phytotoxicity of the different forms of Al hydroxides should be considered only in taking into account the dynamics of the whole system, in particular the solubilisation of Al in the rhizosphere.

Determining the net flux of charge released by maize roots by directly measuring variations of the alkalinity in the nutrient solution

The net flux of charge released by maize, i.e. the strong ion exchange balance between the roots and their environment, was determined in acidic and alkaline solutions, i.e. solutions with a low and a high pH buffering capacity, respectively. The work was based on direct measurement of total alkalinity in culture solutions over a period of several days.The results show there was little difference in the net flux of charge released by maize in acidic and alkaline solutions: In both cases, approximately −1 μmolc (kg DM)−1 s−1. As the maize was grown in a non-limiting nitrate solution, the charge flux was negative, corresponding to a net release of hydroxyls into the rhizosphere. In contrast, the change in the amounts of free protons in the solution was approximately 1 nmol (kg DM)−1 s−1, i.e. 3 orders of magnitude lower than the net charge flux. Moreover, it was negative in acidic media , i.e. the solution pH increased, and positive in alkaline media, i.e. the solution pH decreased. This decrease probably resulted from the release of inorganic carbon by the roots. The effect on the change in solution pH was only slight in acidic conditions but considerable in alkaline conditions, where it reduced the pH even though the culture solution was alkalinised by the roots.The difference in the way that acidic and alkaline solutions function demonstrates the importance of the pH buffering capacity of the solution in determining the net flux of charge released by the plants. It underlines the difficulty of estimating the net charge flux from pH change measurements in the rhizosphere.

Soil-Root-Microbe Interactions in the Rhizosphere: A Key to Understanding and Predicting Nutrient Bio availability to Plants

As stressed in the Millennium Ecosystem Assessment, over the last 50 years, human beings have modified the ecosystems to an unpreceded point in humankind history, in order to meet the increasing world demand in food, drinking water, wood, fibers and energy (Tilman 1999). Such changes much contributed to improving humankind well-being, but this was achieved at the expense of a degradation of numerous ecosystem services and increasing poverty of the poorest populations. Prediction models forecast further degradation of ecosystem services in the coming 50 years,

An a posteriori species clustering for quantifying the effects of species interactions on ecosystem functioning

1.Quantifying the effects of species interactions is key to understanding the relationships between biodiversity and ecosystem functioning but remains elusive due to combinatorics issues. Functional groups have been commonly used to capture the diversity of forms and functions and thus simplify the reality. However, the explicit incorporation of species interactions is still lacking in functional group-based approaches. Here we propose a new approach based on an a posteriori clustering of species to quantify the effects of species interactions on ecosystem functioning. 2.We first decompose the observed ecosystem function using null models, in which species diversity does not affect ecosystem function, to separate the effects of species interactions and species composition. This allows the identification of a posteriori functional groups that have contrasting diversity effects on ecosystem functioning. We then develop a formal combinatorial model of species interactions in which an ecosystem is described as a combination of co-occurring functional groups, which we call an assembly motif. Each assembly motif corresponds to a particular biotic environment. We demonstrate the relevance of our approach using datasets from a microbial experiment and the long-term Cedar Creek Biodiversity II experiment. 3.We show that our a posteriori approach is more accurate, more efficient and more parsimonious than a priori approaches. The discrepancy between a priori and a posteriori approaches results from the way each clustering is set up: a priori approaches are based on ecosystem or species properties, such as ecosystem size (number of species or functional groups) or species’ functional traits, whereas our a posteriori approach is based only on the observed interaction and composition effects on ecosystem functioning. 4.Our findings demonstrate that an a posteriori approach is highly explanatory: it identifies who interacts with whom, and quantifies the effects of species interactions on ecosystem functioning. They also highlight that a combinatorial modelling of ecosystem functioning can predict the functioning of an ecosystem without any hypothesis about the biotic or environmental determinants or any information on species functional traits. It only requires the species composition of the ecosystem and the observed functioning of others that share the same assembly motif. This article is protected by copyright. All rights reserved.

Correction: A combinatorial analysis using observational data identifies species that govern ecosystem functioning

[This corrects the article DOI: 10.1371/journal.pone.0201135.].