II. Micronutrients, pH, and Eh variations in the rhizosphere of two maize genotypes

Abstract Soil-root interface (rhizosphere) is the unavoidable link in the transfer of nutrients from soil to plant. Field measurements of two maize genotypes ( Zea mays L. , cv. Pioneer-3732 and − 3737) have shown differences in plant nutrient uptake which appeared to be accentuated in eroded calcareous soils. Rhizotron studies were conducted to: (1) determine the effect of soil CaCO 3 on plant micronutrients (Cu, Fe, Mn, and Zn) availability in the rhizosphere; and (2) characterize the rhizosphere properties of these two maize genotypes. Maize seedlings were transferred to rhizotrons containing the Ap horizon of clay loam soils with low (5 g kg − 1 ) or high (204 g kg − 1 ) levels of CaCO 3 . Rhizophere pH, Eh, and CO 2 were monitored with pH-glass, platinum, and ρCO 2 microelectrodes. At the six-leaf stage, the soil was thin-sectioned at 1-, 2-, and 3-mm distances from the root. The soluble soil ions and plant nutrient uptakes were determined. Higher Mn 2+ and Fe 2+ concentrations were found in the rhizosphere of P-3737 relative to P-3732. Higher Mn 2+ , Cu 2+ , and Zn 2+ uptake for Pioneer-3737 were observed in the high CaCO 3 soil relative to Pioneer-3732. The plant shoot/root ratio was reduced by 21.4% for P-3732 while increased by 15.7% for P-3737 in the calcareous soil compared to those in the non-calcareous soil. The rhizosphere pH, Eh and ρCO 2 differences correlated to nutrient uptake and were dependent on plant genotype in addition to bulk soil concentration. As rhizosphere physicochemical properties differ from bulk soil, bulk soil micronutrients values do not reliably indicate plant nutrient availability. 248 words