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The Influence of Organic Complexing Agents on Iron Mobility in a Simulated Rhizosphere¹

ABSTRACT

The effect of complexing agents on Fe mobility was investigated using a simulated root-soil system that consisted of 0.24-mm o.d. cellulose fibers embedded in a clay-agar matrix. Complexing agents diffused into the clay-agar matrix from a solution flowing through the fibers and their effect on Fe flux to the fiber surface was determined by following Fe concentration in the effluent solution from the fibers. Experiments with the chelating agents ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), ethylenediamine di-(o-hydroxyphenylacetic acid) (EDDHA), and ethylene glycol-bis-(2-amino-ethylether) tetraacetic acid (EGTA) demonstrated that solubilization of Fe qualitatively followed patterns predicted from known thermodynamic stability constant-pH relationships. The concentration of Fe in the fibers 24 h after introduction of either EDTA, DTPA, or EDDHA was increased from close to 0 to 30 to 100 g Fe L^–1, whereas EGTA had no measurable effect on Fe solubility. The measured fluxes of Fe were compared with calculations based on diffusion and reaction. An equilibrium model, where the complexing agent was assumed to equilibrate instantaneously with the soil (reaction rate constant K_r = 1), predicted concentrations of Fe 10³ times greater than those observed. A first-order kinetic model, which was solved by numerical methods, gave results similar to those found experimentally when K_r was on the order of 10^–7. The partial success of the thermodynamic stability constant approach was reconciled with the kinetic model by considering K_r to combine both kinetic and equilibrium factors.

¹ Contribution from the Center for Root-Soil Research, Cornell Univ., Ithaca, NY 14853.

² Former Graduate Research Assistant, Associate Professor and Professor, Dep. of Agronomy, and Research Scientist, USDA-ARS, respectively. The senior author is currently Assistant Professor, Dep. of Plant and Soil Sciences, Univ. of Maine, Orono, ME 04469.

Received for publication March 24, 1986.