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

The present work investigated to what extent the utilisation of C-14-glucose by soil microorganisms could be used to compare microbial activity between rhizosphere samples. A very low amount of C-14-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 ((CO2)-C-14) and the C-14 of the microbial biomass rendered extractable after fumigation with chloroform vapour (C-14(FE)). In unplanted soils, we observed that the (CO2)-C-14/C-14(FE) ratio increased when microbial growth was stimulated by addition of unlabelled glucose (20-500 mug C g(-1) soil) compared to the control (no unlabelled glucose added). When C-14-glucose was added immediately after unlabelled glucose amendments, the (CO2)-C-14/C-14(FE) ratio increased with the rate of added C (20-500 mug 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 (CO2)-C-14/C-14(FE) ratio was greater in amended soils compared to control soils. The C-14-glucose assay was performed on soil samples collected from maize-planted cores. The (CO2)-C-14/C-14(FE) 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 (CO2)-C-14/C-14(FE) 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.