عنوان مقاله [English]
Introduction: Redox potential is one of the most important factors affecting on the solubility of iron minerals in soil. Decreasing redox potential in soil reduces Fe3+ to Fe2+, thereby affecting on solubility of Fe minerals. Application of organic matter to soil under waterlogging condition, decrease redox potential and as a consequence, accelerate the transformation of Fe minerals. The objectives of this study were: 1- The effect of waterlogging on the soluble total Fe concentration and transformation of Fe minerals in different soil pH values. 2- The indirect effects of organic matter on solubility of Fe minerals by changing the redox potential of the soils.
Materials and Methods: A study was conducted to determine the effects of redox potential on solubility of Fe and transformation of Fe minerals during the time. Four agricultural soils were selected from different regions of Iran. The soil samples were treated with 0 (C)and 2% (O) alfalfa powder and then incubated for 12 weeks under 60% Field capacity (F) and waterlogged conditions (S). Subsamples were taken after 1and 12 weeks of incubation and the redox potential, pH value, electrical conductivity (EC), soluble cations (such as Ca2+, Mg2+, K+ and Na+) and anions (such as Cl-, SO42-, PO43- and NO3- ) and soluble Fe concentrations in the subsamples were measured. Concentrations of Fe2+ and Fe3+ species in soil solution were also predicted using Visual MINTEQ speciation program. Mineralogical transformation of Fe minerals was also determined by X-ray diffraction (XRD) technique.
Results and Discussion: The results in 60% Field capacity condition showed that pH value by organic matter (alfalfa powder) application (OF) increased significantly (p≤ 0.05) in acid and neutral soils and decreased in calcareous soils when compared to the control (CF). Organic matter is usually capable of lowering pH of alkaline soils by releasing hydrogen ions associated with organic anions or by nitriﬁcation in an open system. On the other hand, it may cause pH to increase in the acid soils either by mineralization of organic acids to carbon dioxide (CO2) and water (thereby removing H+) or by the alkaline nature of the organic residues. This treatment increased soluble total Fe concentration in all soils. It is clear that decomposition of organic matter cause to produce soluble organic compounds and form soluble complexes with Fe, thereby increasing soluble total Fe concentration. Waterlogging (CS) decreased redox potential of the soils gradually with the incubation time, especially in the neutral soil and alfalfa powder application (OS) accelerated this decreasing redox potential. The decrease rates by waterlogging in acid, neutral and two calcareous soils were 2, 3.6, 1.5 and 1.7folds, respectively compared to the control. The soluble total Fe concentration in CS compared to CF treatment increased significantly (p≤ 0.05) in all soils except in acid soil. This increasing was continued with time in all soils except in neutral soil. An important point that OS compared to CS treatment enhanced the soluble total Fe in acid and neutral soils, whereas decreased it in both calcareous soils. However, soluble total Fe increased during the incubation time in all soils except in neutral soil. The increase rates in week 12 relative to week 1were 3.4, 2.2 and 1.8 folds in acid and two calcareous soils, respectively. The decrease of soluble total Fe in the neutral soil is probably attributed to more severe decrease of redox potential in the soil when compared to the other soils. The solubility diagrams and X-ray diffraction results confirmed the formation of pyrite in the acid and neutral soils and the formation of siderite in one of the calcareous soils.
Conclusion: In aerobic condition, organic matter application increased the concentration of soluble total Fe and changed Fe-controlling mineral from soil-Fe to amorphous Fe. Waterlogging decreased redox potential and Fe-controlling mineral changed to pyrite and/or siderite, depending on CO2 pressure and pH value of the soils. It might be pointed out that severe reduction of the redox potential decreases soluble Fe through the formation of insoluble Fe minerals such as Fe sulfides. It is concluded that waterlogging soils can provide available Fe to the plant, though severe decrease of redox potential, by application of organic mater, may decrease Fe availability.