Submitted to: World Congress of Soil Science
Publication Type: Abstract Only
Publication Acceptance Date: 2/15/2006
Publication Date: 6/14/2006
Citation: Gollany, H.T., Schumacher, T.E. 2006. Maize genotype responses to CaCo3 in soils. World Congress of Soil Science.
Technical Abstract: The chemical, biological and physical properties of the soil-root interface are critical to root growth and nutrient uptake. The rhizosphere is the unavoidable link in the transfer of nutrients from soil to plant. Field observations of two similar maize genotypes (Zea mays L., cv. Pioneer-3732 and -3737) have shown different nutrient uptake responses. These differences in response appeared to be accentuated in calcareous soils. There are limited published studies, however, of rhizosphere physicochemical properties and nutrient availability for maize genotypes. Progress in this field is restricted by a lack of convenient methods to examine this opaque and structurally complex medium without disturbing the soil-root interface. Rhizotron studies were conducted to: (1) determine whether the rhizosphere physicochemical properties of these two maize genotypes are different, and (2) evaluate the effect of bulk soil CaCO_3 level on rhizosphere properties. Plexiglass-lined rhizotrons (45 x 13 x 2.5 cm) were filled with soil containing low (5 g kg^-1) or high (204 g kg^-1) levels of CaCO3 from the Ap horizon of a Beadle clay loam (fine, montmorillonitic, mesic Typic Argiustoll). Seedlings of the two genotypes were transplanted into the rhizotrons and positioned at a 45' angle on a stand in a growth chamber. Plants were kept on a night/day regime of 8/16hours, 18 - 25 'C, relative humidity 65 - 75%, and photon flux density (400 - 700 nm) of 460 'm m^-2 s^-1. Rhizosphere pH, pe and 'CO_2 were monitored at 1-, 2- and 3-mm distances from the root surface with pH-glass, platinum, and 'CO_2 microelectrodes. At the 6-leaf stage, the rhizosphere soil was thin-sectioned at 1-, 2- and 3-mm distances from the root surface. Soil-water was extracted at the container capacity by an immiscible displacement method. Dissolved nutrients within the rhizosphere were determined by ICP. The roots and shoots were separated and total nutrient uptake was determined. Pioneer-3737 acidified the rhizosphere more than did Pioneer-3732. The higher acidification corresponded to a higher 'CO2 in the rhizosphere. Pioneer-3737 had more soluble P and K in the rhizosphere, especially in the high CaCO3 soil, relative to the other treatments. The P depletion zone extended to over 3-mm for all treatments except Pioneer-3737 in the high CaCO_3 soil. Accumulation of Ca^2+ and Mg^2+ at the rhizoplane (< 1 mm distance from the root surface) was observed for all treatments. The rhizosphere Ca^2+ content however was greatly reduced relative to the bulk soil. Pioneer-3732 produced more dry matter and had higher shoot/root ratio than did Pioneer-3737 in the low CaCO_3. Higher H_2PO_4- and K+ uptake and lower Ca^2+ and Mg^2+ uptake for Pioneer-3737 were observed in the high CaCO3 soil relative to Pioneer-3732. Reduced K+ uptake for Pioneer-3732 suggests an antagonism from higher Ca^2+ and Mg^2+ uptake in the high CaCO_3. In the calcareous soil, the plant K^+ /( Ca^2+ and Mg^2+) ratio were reduced to 47% and 78% for Pioneer-3732 and -3737 respectively, compared to those in the noncalcareous soil. The difference in K+ uptake between the two genotypes was related to differences in rhizosphere pH and 'CO2 as well as differences in Ca2+ and Mg2+ uptake. The results revealed that nutrient uptake is dependent not only on bulk soil concentration but also on plant genotype. The roots of the two maize genotypes altered the nutrient composition within the rhizosphere and responded differently to the CaCO3 content of the bulk soil. The physicochemical properties of the rhizosphere were different from that of the bulk soil. Bulk soil nutrient values therefore cannot provide a highly reliable indication of the nutrient availability.