Saturday, 15 July 2006

Maize Genotype Responses to CaCO3 in Soils.

Hero Gollany, Columbia Plateau Conservation Research Center, USDA-ARS-PWA, PO Box 370, Pendleton, OR 97801 and Thomas Schumacher, South Dakota State Univ, NPB 247A, P.O. Box 2140C, Brookings, SD 57007.

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 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 CaCO3 level on rhizosphere properties. Plexiglass-lined rhizotrons (45 x 13 x 2.5 cm) were filled with soil from the Ap horizon of a Beadle clay loam (fine, montmorillonitic, mesic Typic Argiustoll) containing low (5 g kg-1) or high (204 g kg-1) levels of CaCO3. Seedlings of the two genotypes were transplanted into the rhizotrons and positioned at a 45o angle on a stand in a growth chamber. Plants were kept on a night/day regime of 8/16hours, 18 - 25 oC, relative humidity 65 - 75%, and photon flux density (400 - 700 nm) of 460 Ám m-2 s-1. Rhizosphere pH, pe and ρCO2 were monitored at 1-, 2- and 3-mm distances from the root surface with pH-glass, platinum, and ρCO2 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 ρCO2 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 CaCO3 soil. Accumulation of Ca2+ and Mg2+ at the rhizoplane (< 1 mm distance from the root surface) was observed for all treatments. The rhizosphere Ca2+ content however were greatly reduced relative to the bulk soil. Pioneer-3732 produced more dry matter and higher shoot/root ratio than did Pioneer-3737 in the low CaCO3. Higher H2PO4- and K+ uptake and lower Ca2+ and Mg2+ 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 Ca2+ and Mg2+ uptake in the high CaCO3. In the calcareous soil, the plant K+/( Ca2+ and Mg2+) 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 ρCO2 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.

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