The mechanism of mobilization of iron from soil minerals in the rhizosphere of Cicer arietinum L

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Characterization of ferric reducing activity in the rhizosphere of chickpea plants (Cicer arietinum L)

Chickpea seedlings (cv. ILC-195) were precultured in an iron free nutrient solution for 7 days. Thereafter, iron was supplied to one set of plants (Treatment A) and the second set of plants (Treatment B) continued to grow in a -Fe nutrient solution. Measurements were carried out to investigate the activity of the two reduction systems (extra cellular reductants and reductase) and the accumulation of organic acids. Fe-stress appeared to enhance slightly the release ofreductants capable ofreducing Fe+3. The quan-tity of Fe +3reduced by reductants was however extremely low compared to the capacity of the intact root system to reduce iron. The rate of Fe reduction by the basal roots was significantly greater than that by root tips. The use of agar plate technique provided another evidence that iron reduction was all over the root system and was restricted to the apical zones and so was the acidification of the rhizosphere. Regardless of the iron nutritional status, the concentrations of organic acids were much higher in the shoots than roots. The distribution of organic acids between tops and basal shoots was in favour of basal shoots. There was little difference between the different zones ofthe roots. The accumulated organic acids were mostly citric and malic in the shoots whereas, malonic and oxalic in the roots

Phosphorus mobility in a karst landscape under pasture grazing system

The spatial distribution and partitioning of water dissolved phosphorus fractions in the soil profile of a grazed karst sinkhole landscape were investigated. We also measured P fractions in surface runoff entering a sinkhole drain and in karst spring flow draining the study area. Grazing increased total N, C, and all forms of P of soil. Dissolved inorganic orthophosphate (DPi) was the highest in the surface soil layer and diminished significantly with depth. The proportion of dissolved unreactive phosphorus (DPu) increased with soil depth. Changes in DPu with landscape position and depth were closely correlated with changes in dissolved organic carbon (DOC) suggesting that the mobility and transport of DPu was mediated by DOC. Landscape position sampling showed molybdate reactive phosphorus (MRP) and DPu increased toward the bottom and center of sinkholes. The distribution of DPi and DPu in surface runoff and that occurring in underground watercourses confirms the significance of DPu transported into karst groundwater.Mobilité du phosphore dans une zone karstique paturée. La distribution spatiale et la partition des fractions de phosphore dissoutes dans le profil de sol d'un paysage pâturé de dolines karstiques ont été étudiées. Nous avons également mesuré les fractions de P dans les eaux de surface entrant dans un drain de doline et dans l'écoulement ressortant du karst drainant le secteur d'étude. Le pâturage a augmenté N total, C, et toutes les formes de P de sol. L'orthophosphore inorganique dissous (DPi) était le plus élevé dans la couche de surface du sol et réduit significativement avec la profondeur. La proportion de phosphore non réactif dissous (DPu) a augmenté avec la profondeur de sol. Des changements de DPu en fonction de la position dans le paysage et de la profondeur ont été étroitement corrélés avec des changements du carbone organique dissous (DOC) suggérant que la mobilité et le transport du DPu ont été facilités par le DOC. Un échantillonnage dans le paysage a montré une augmentation de phosphore réactif au molybdate (MRP) et de DPu vers le fond et le centre des dolines. La distribution de DPi et DPu dans l'écoulement de surface et ce qui se produit lors du transit de l'eau sous terre confirme la valeur significative du DPu transporté dans les eaux souterraines du karst

Minjingu phosphate rock availability in low-pH highly weathered soil as affected by added salts

Concentrations and identity of ions in the soil solution may affect soil phosphorus (P) reactions and P availability. In this study, the magnitude of these reactions was evaluated following the application of Minjingu phosphate rock (MPR) combined with chloride and carbonate salts of Na and Ca within an incubation experiment. Twenty-one days later NaOH-P and HCl-P were determined. This investigation was undertaken with the aim of identifying the role of Ca-ion activity in the liquid phase on the solubilization of MPR and formation of insoluble Ca-P phases. The increase in pH was higher with Na2CO3 than with CaCO3, while both CaCl2 and NaCl resulted in slight decreases in pH. The dissolution of MPR was higher overall when MPR was applied singularly than for the combined application of the phosphate rock with salts of calcium or sodium after 60 days of incubation. Dissolution of MPR decreased as levels of CaCO3 or CaCl2 increased but the decrease was more pronounced in CaCO3-treated than in CaCl2-treated soils. Ca-ion activity in the liquid phase is the main factor responsible for the insolubilization of MPR and the formation of insoluble Ca-P phases (HCl P). The formation of Ca-P solid phases increased with the concentration of Ca-ions, and was governed by the pH and nature of the accompanying anion. For soils with low levels of exchangeable cations and where liming is a recommended intervention measure, Ca from lime will form insoluble P phases and reduce the dissolution of PR and P availability to plants