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Iron toxicity influence of phosphorus potassium calcium and magnesium fertilization on rhizoflora redox potential and iron uptake by different rice varieties oryza sativa



Iron toxicity influence of phosphorus potassium calcium and magnesium fertilization on rhizoflora redox potential and iron uptake by different rice varieties oryza sativa



Landwirtschaftliche Forschung 36(3-4): 285-299



To evaluate the effects of N, P, K, Ca + Mg (applied alone or in combination) of Fe reduction (measured by Fe(II)-formation), dehydrogenase activity (DHA, an index for microbial bioamss), pH and redox dynamics (results of microbial activity) in the rhizosphere and on Fe-uptake (Fe toxicity), a greenhouse experiment with 2 different soils was designed. A variety relatively susceptible to Fe toxicity ('TR 22') and another relatively Fe-tolerant ('TR 42') were kept overnight in 2% ZnO2 suspension (to prevent Zn-deficiency) and transplanted (2 seedlings/Wagner pot). Each pot (3 parallels) was filled either with a typical Fe-toxic soil (silty loam from San Dionisio, Philippines, pH = 4, 7; CEC [cation exchange capacity] = 8.8 meq/100 g soil, deficient in P, K, Ca and Mg) or with a fertile clay (taken from the experimental farm of the International Rice Research Institute, Manila, Philippines; pH = 6.6; CEC = 27, 4 meq/100 g soil, low in N but sufficient in P, K, Ca and Mg). With both rice varieties, fertilization enhanced plant growth and decreased Fe uptake. These effects were more significant with the susceptible variety IR 22. Especially K and Ca + Mg reduced Fe uptake of Fe significantly. These effects were ascribed to a decrease in root exudation caused by decreased permeability of the root membranes. Both DHA and FE(II)-formation (expressed as the ratio of planted to unplanted soil = rhizosphere effect) decreased in either variety as a result of fertilization. Particularly during maximum tillering stage (phase of intensive metabolic activity), the influence of nutrients on DHA and FE(II) formation was clearly expressed. These results indirectly confirm the essential role of nutrients (P, K, Ca + Mg) in lowering microbial activity in the rice rhizophere by improving root permeability and diminishing organic exudation. In the fertilized pots the drop in pH and Eh (redox potential) in the rhizosphere in IR 22 and IR 42 was more intensive than in the untreated controls. Fe toxicity is interpreted as a function of a multiple nutritional stress rather than low pH and high amount of easily reducible Fe.

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