A carbon-14-glucose assay to compare microbial activity between rhizosphere samples

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English. The present work investigated to what extent the utilisation of 14C-glucose by soil microorganisms could be used to compare microbial activity between rhizosphere samples. A very low amount of 14C-glucose was added to the soil to be tested. After incubation for 6 days at a constant temperature in the dark, we determined the mineralised fraction (14CO2) and the 14C of the microbial biomass rendered extractable after fumigation with chloroform vapour (14CFE). In unplanted soils, we observed that the 14CO2/14CFE ratio increased when microbial growth was stimulated by addition of unlabelled glucose (20-500 µg C g-1 soil) compared to the control (no unlabelled glucose added). When 14C-glucose was added immediately after unlabelled glucose amendments, the 14CO2/14CFE ratio increased with the rate of added C (20-500 µg C g-1 soil). When the labelled glucose was added 48 h or 72 h after the unlabelled glucose, the rate of added C had no effect on the ratio. However, the 14CO2/14CFE ratio was greater in amended soils compared to control soils. The14C-glucose assay was performed on soil samples collected from maize-planted cores. The 14CO2/14CFE ratio was significantly greater in the soil surrounding the apex+ root hair zone compared to the soil of the branching zone and to the bulk soil. Moreover, partial defoliation of maize plants significantly reduced the 14CO2/14CFE ratio of the rhizosphere compared to that of control plants, suggesting that leaf removal decreased the availability of C around roots within a short time-scale. The use of the assay to investigate the flux of C released from roots is discussed.