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Rhizosphere Conference

Root Exudation, Phosphorus Acquisition, and Microbial Diversity in the Rhizosphere of White Lupine as Affected by Phosphorus Supply and Atmospheric Carbon Dioxide Concentration

^a Creative Research Initiative ‘Sousei’ (CRIS), Hokkaido University, N21W10, Kita-ku, Sapporo 001-0021, Japan
^b Institute of Soil Science, University of Hohenheim, Emil-Wolff-Strasse 27, 70599 Stuttgart, Germany
^c Institute of Plant Nutrition, University of Hohenheim, Fruwirthstrasse 20, 70599 Stuttgart, Germany
^d Graduate School of Agriculture, Hokkaido University, N9W9, Kita-ku, Sapporo 060-8589, Japan

^* Corresponding author (junw{at}chem.agr.hokudai.ac.jp)

Received for publication November 9, 2004. White lupine (Lupinus albus L.) was used as a phosphorus (P)-efficient model plant to study the effects of elevated atmospheric CO₂ concentrations on (i) P acquisition, (ii) the related alterations in root development and rhizosphere chemistry, and (iii) the functional and structural diversity of rhizosphere microbial communities, on a P-deficient calcareous subsoil with and without soluble P fertilization. In both +P (80 mg P kg^–1) and –P treatments (no added P), elevated CO₂ (800 µmol mol^–1) increased shoot biomass production by 20 to 35% and accelerated the development of cluster roots, which exhibit important functions in chemical mobilization of sparingly soluble soil P sources. Accordingly, cluster root formation was stimulated in plants without P application by 140 and 60% for ambient and elevated CO₂ treatments, respectively. Intense accumulation of citrate and increased activities of acid and alkaline phosphatases, but also of chitinase, in the rhizosphere were mainly confined to later stages of cluster root development in –P treatments. Regardless of atmospheric CO₂ concentrations, there was no significant effect on accumulation of citrate or on selected enzyme activities of C, N, and P cycles in the rhizosphere of individual root clusters. Discriminant analysis of selected enzyme activities revealed that mainly phosphatase and chitinase contributed to the experimental variance (81.3%) of the data. Phosphatase and chitinase activities in the rhizosphere might be dominated by the secretion from cluster roots rather than by microbial activity. Alterations in rhizosphere bacterial communities analyzed by denaturing gradient gel electrophoresis (DGGE) were related with the intense changes in root secretory activity observed during cluster root development but not with elevated CO₂ concentrations.

Abbreviations: DAT, days after transplanting • DGGE, denaturing gradient gel electrophoresis • PCR, polymerase chain reaction

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