Frédéric Gérard

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.

A mechanistic model for understanding root-induced chemical changes controlling phosphorus availability

BACKGROUND AND AIMS Plant nutrition models do not properly account for the effects of root-induced chemical changes in the rhizosphere, e.g. pH changes, on the availability of nutrients such as phosphorus (P). As a result, they underestimate the actual P uptake, i.e. P bioavailability to the plant, in low-P soils. The present study aims at simulating root-induced chemical mechanisms controlling P nutrition in a P-limited soil. METHODS In this work a mechanistic description for the adsorption of cations and anions by soil constituents (1pK-Triple Plane Model, ion-exchange and Nica-Donnan) was used to simulate changes induced by durum wheat (Triticum durum turgidum) in the P availability of the soil, as measured by water and CaCl2 extraction. Calcium (Ca) availability was also measured and simulated. KEY RESULTS The simulations were found to be in close agreement with experimental data. In the rhizosphere, the goodness-of-fit required to account for the measured uptake of Ca by plants, in addition to the measured uptake of P and root-induced alkalization, were satisfactory. Calcium uptake significantly increased P availability, as assessed through water extraction, by decreasing the promoting effect of Ca adsorption on P adsorption. The study thus enabled P and Ca availability to be related to their bioavailability for durum wheat under experimental conditions. It was also shown that P was primarily adsorbed onto Fe oxides and clay minerals (kaolinite and illite) depending on soil pH. The major source of P for durum wheat nutrition was P desorbed from goethite and kaolinite. CONCLUSIONS In addition to confirming the validity of our approach to model P availability, the present investigation suggested that in the studied soil, a novel root-induced chemical process was controlling P nutrition under P-deficient conditions, namely the uptake of Ca.

Processes controlling silica concentration in leaching and capillary soil solutions of an acidic brown forest soil (Rhône, France)

Abstract Chemical analysis of leaching and capillary soil solutions collected at different soil depths was performed on a monthly basis for several years at the Vauxrenard site (Rhone, France). The seasonal variations in dissolved silica (Si) indicated considerable differences whether contained in leaching or capillary soil solutions. In capillary solutions, the maximum and minimum Si concentrations occurred in the summer and winter, respectively, while the opposite trend was observed with leaching soil solutions. In both solutions types, significant relationships may be obtained between Si concentration and soil temperature (T) and, to a lesser extent, H+ concentration. Evaporation or evapotranspiration had little effect on Si in capillary solutions, limited to the upper soil layers. An inverse relationship between Si and T found in leaching solutions indicated that weathering did not control Si concentration. In contrast, a positive relationship between Si and T found in capillary solutions was consistent with this process. This was reinforced by a significant relationship obtained between logSi and pH, which was consistent with surface-controlled and proton-promoted weathering. Calculated apparent activation energy and reaction order with respect to pH were both consistent with muscovite at the laboratory scale. It is suggested that Si concentration in leaching solutions was controlled mainly by diffusion of aqueous silica (essentially orthosilicic acid) from capillary solutions in relation to soil drainage. Thermodynamic calculations showed that the temperature-dependence of the solubility of Si-containing secondary phases did not significantly control Si concentration in both soil solution types. However, it was calculated that the reversible formation of some hypothetical siliceous phases (Si/A1>1) proceeded at relatively slow rates, thus limiting their impact on Si concentration. Kinetic calculations showed excellent results by correlating Si concentration in capillary solutions to specific weathering rate for primary soil silicates. In agreement with most of the statistical analysis, soil temperature appeared to be the main driving force for chemical weathering. Protons (H+) had a significant influence in the deeper soil horizons as well as in the seasons corresponding to lesser soil temperature variations. An important effect of organic ligands and particularly of low molecular weight compounds on weathering may explain larger Si concentration observed in the upper soil layers.

Consideration on the occurrence of the Al13 polycation in natural soil solutions and surface waters

Abstract Equilibrium speciation calculations were performed (1) for soil solutions and streamwaters collected in central and eastern France and (2) for simulated waters at 0 and 25°C, to assess the highest concentration of Al13 that could be reached in waters in the absence of complexing ligands other than OH−. A comprehensive and updated set of aqueous Al species, including polymeric hydroxyaluminosilicates (HAS), and their corresponding thermodynamic formation constants, were used. Results suggest that the concentration of the Al13 polycation in natural waters has been largely overestimated in some past studies using equilibrium models to calculate Al speciation, owing to oversimplification (many Al ligands not considered) and the unrecognised temperature dependence of some formation constants. The Al13 concentration in mildly acidic natural waters may not exceed a few μmol l−1 at AlT on the order of 10−4 mol l−1 and should be less than 1 μmol l−1 at AlT=10−5 mol l−1. Monomeric Al–Si species may not significantly interfere with the formation of Al13, but the formation of both HAS polymers (proto-imogolite precursors) and organo-Al complexes have a marked detrimental effect on the Al13 concentration. The maximum concentration of Al13 decreased upon increasing temperature from 0 to 25°C. In contrast, the pH range wherein Al13 may occur increases slightly with temperature and the most acidic pH value above which Al13 may be formed has been underestimated. At T=25°C, the Al13 polycation may be a significant Al species (4 to 5% of AlT) at pH 10−4 mol l−1. The results of this study and the use of HAS polymers to calculate Al speciation in moderately natural acidic soil solutions were in better accordance with soil mineralogy. This research suggests strongly that Al13 should be negligible in natural soil and surface waters and may not control either Al3+ activity or Al-trihydroxide formation through polymerisation/depolymerisation steps. Also, from a biological point of view, the toxicity of Al13 to plants and aquatic organisms in natural conditions may be considered to be very low.

Root-induced processes controlling phosphate availability in soils with contrasted P-fertilized treatments

AimsIn this study we identified the nature of the root-induced chemical processes controlling changes in phosphate (P) availability in a soil with two P loadings resulting from long-term fertilization treatments.MethodsWe used a set of mechanistic adsorption models (surface complexation and ion exchange) within the framework of the component additive approach to simulate the effect of durum wheat roots on P availability. We had to consider the influence of adsorption of other ions to ensure the goodness-of-fit of the simulations.ResultsWe found that Ca2+ uptake, in addition to P uptake and root-induced alkalization, controlled P availability in the rhizosphere regardless of the fertilization level. The relative influence of these three processes depends primarily on the extractant used to estimate P availability. Calcium uptake was the most significant process in water extracts, whereas P uptake was the dominant root-induced chemical process in CaCl2 extracts. Under low Ca concentrations, Ca2+ uptake decreased the promoting influence of Ca2+ adsorption on P adsorption.ConclusionsIn addition to confirming the validity of our approach to model P availability, the present investigation indicated that root-induced processes markedly affect P availability irrespective of the fertilization level.

The use of root growth and modelling data to investigate amelioration of aluminium toxicity by silicon in Picea abies seedlings.

Three-week-old Picea abies seedlings were grown for 7 days in 100 microM aluminium (Al), combined with 1000 or 2000 microM silicon (Si). Solution pH was adjusted to 4.00, 4.25, 4.50, 4.75, or 5.00. In the absence of Si, solution pH had no effect on the decrease in root growth caused by 100 microM Al. Silicon did not ameliorate toxic effects of Al on root growth at pH 4.00, 4.25 and 4.50, whereas significant, and apparently complete, amelioration was found at pH 4.75 and 5.00. An equilibrium speciation model (EQ3NR), with a current thermodynamic database, was used to predict the behaviour of Al and Si in growth solutions. When Si was not present in the 100 microM Al solutions, Al(3+) declined from 92.4% of total Al at pH 4.00 to 54.6% at pH 5.00, and there was a concomitant increase in hydroxyaluminium species as pH increased. The addition of 1000 microM Si to the 100 microM Al solutions caused a reduction in Al(3+) content over the whole pH range: at pH 4.00 Al(3+) fell from 92.4 to 83.3% in the presence of Si; and at pH 5.00 the fall was from 54.6 to 17.7%. These falls were attributed to the formation of hydroxyaluminosilicate (HAS) species. Similar, but somewhat greater, changes were observed in solutions containing 2000 microM Si. The match between root growth observations and the modelling data was not very good. Modelling predicted that change in Al(3+) content with pH in the presence of Si was gradual, but root growth was markedly increased between pH 4.50 and 4.75. Differences between root growth and modelling data may be due to the model not correctly predicting solution chemistry or to in planta effects which override the influence of solution chemistry.

Endogeic earthworms modify soil phosphorus, plant growth and interactions in a legume–cereal intercrop

Background and aimsIntercropping of legumes and cereals appears as an alternative agricultural practice to decrease the use of chemical fertilizers while maintaining high yields. A better understanding of the biotic and abiotic factors determining interactions between plants in such associations is required. Our study aimed to analyse the effect of earthworms on the legume–cereal interactions with a focus on the modifications induced by earthworms on the forms of soil phosphorus (P).MethodsIn a glasshouse experiment we investigated the effect of an endogeic earthworm (Allolobophora chlorotica) on the plant biomass and on N and P acquisition by durum wheat (Triticum turgidum durum L.) and chickpea (Cicer arietinum L.) either grown alone or intercropped. The modifications of the different organic and inorganic P forms in the bulk soil were measured.ResultsThere was no overyielding of the intercrop in the absence of earthworms. Earthworms had a strong influence on biomass and resource allocation between roots and shoots whereas no modification was observed in terms of total biomass production and P acquisition. Earthworms changed the interaction between the intercropped species mainly by reducing the competition for nutrients. Facilitation (positive plant–plant interactions) was only observed for the root biomass and P acquisition in the presence of earthworms. Earthworms decreased the amount of organic P extracted with NaOH (Po NaOH), while they increased the water soluble inorganic P (Pi H2O) content.ConclusionsIn this experiment, earthworms could be seen as “troubleshooter” in plant–plant interaction as they reduced the competition between the intercropped species. Our study brings new insights into how earthworms affect plant growth and the P cycle.

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,

Modelling the interactions between root system architecture, root functions and reactive transport processes in soil

Background and aimsSoil-plant models always oversimplified the representation of soil chemical processes or root system. The objectives of the study were (i) to present a model overcoming such limitations, and (ii) to illustrate its relevance for the modelling of soil-plant interactions.MethodsWe coupled a root system architecture (RSA) model with a reactive transport model using a macroscopic approach. The two models were coupled sequentially using Fortran-C++ interoperability. We used the resulting model to investigate the case of phosphorus (P) acquisition from hydroxyapatite (HA) in an alkaline soil as induced by P and calcium (Ca) uptake and pH variations in the root zone. Important model parameters were issued of the literature and we tested its sensitivity to selected soil properties. Model sensitivity to grid size and time increment was evaluated as well.ResultsThe simulations revealed that HA dissolution can contribute very substantially to P nutrition in case of rhizosphere alkalisation thanks to Ca and P uptake. Root-induced acidification was much more efficient at acquiring P, suggesting that ammonium-fed plants should be more P efficient. The variations of dissolved P in the root zone partly agreed with the observations, suggesting that P release was rather controlled by desorption when alkalisation occurs. The presence of more soluble minerals as well as the increase of Ca uptake should enhance P acquisition by crops.ConclusionWe developed a new model and demonstrated the interest of the mechanistic description of geochemical processes with a spatially-explicit distribution of roots in soil while modelling soil-plant interactions. Results of its first application to P acquisition from a mineral source in an alkaline soil were overall consistent with the literature.

Effect of the soils properties on the sorption capacity of phosphorus and ammonium by alkaline soils of the semi-arid areas

Soils samples were collected at different depths (from 0 to 120 cm) from tree sites of Tunisia (Chott Mariem, Enfidha and Kondar). The minerals and physicochemical properties were analyzed. Then the capacity sorption of phosphorus and ammonium was carried only in the samples collected from the surface depth (0-25cm) using the batch processes. The results showed that most soil samples have a clayey texture. The available of nutriment and heavy metals was different and varied with the sites and depths. The sorption of phosphorus and ammonium was rapid initially and gradually diminished to attain equilibrium. The equilibrium was reached after 72 h for phosphorus sorption and 300mn for ammonium sorption. Also the levels of P adsorbed were; 6.27, 6.84 and 6.92 mg P g−1 and ammonium adsorbed were 5.83, 6.5 and 6.3 mg NH4 + g −1 respectively from the soil Chott Mariem, Enfidha and kondar. Applications of the following kinetics models: pseudo-first-order, pseudo- second-order and Elovich model to the data show that the rates of phosphorus and ammonium sorption were best predicted by the pseudo-second-order kinetic model as seen from the correlation coefficient R2 (≥0.98).