Short-term utilisation of 14C-[U]glucose by soil microorganisms in relation to carbon availability

The release of organic compounds into the soil by plant roots increases the availability of carbon in the rhizosphere and consequently stimulates the growth and the activities of microorganisms. Therefore, the determination of carbon availability in soils is a key point to better understand nutrient flows. We investigated the short-term utilisation of 14C-[U]glucose by soil microorganisms to determine whether it could evidence differences in carbon availability between unplanted and maize-planted soils. In unplanted soils, the kinetics of glucose uptake indicated a multicomponent carrier-mediated transport. The lower affinity constant (K(m)) for glucose uptake was determined at 74 micromolar of glucose. The kinetics of 14CO2 production from unplanted soil amended with 0.07 microgram C-glucose g-1 soil demonstrated a peak of respiratory activity during the first 15 min. One hour after 14C-glucose addition, the mineralisation of the substrate represented 7.8% of the 14C that was unrecovered by 0.5 M K2SO4 extraction and which was assumed to be absorbed by soil microorganisms (97% of the 14C amended). Three days after 14C-glucose addition, 14CO2 increased to 28% of the absorbed glucose in unplanted soils. The 14C extractable by K2SO4 after fumigation with chloroform remained constant with time (about 25%). The apparent high efficiency of utilisation of 14C suggested that under conditions of carbon limitation, the substrate is rather stored by microorganisms than used for growth. We proposed a model derived from the one of Bremer and Kuikman (Soil Biol. Biochem 26 (1994) 511) for the short-term uptake and utilisation of glucose by soil microorganisms. Furthermore, we found that the utilisation of 14C-glucose by microorganisms was significantly different between unplanted and maize-planted soils. In planted soils, microorganisms mineralised a significant larger proportion of the absorbed glucose (32%) and the 14C-activity in the fraction released after fumigation by chloroform vapours was lower (22%). This indicated a greater availability of carbon in maize-planted soils, which was related to the release of organic compounds by roots.