Short-term utilisation of C-14-[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 o f carbon availability in soils is a key point to better understand nutrient flows. We investigated the short-term utilisation of C-14-[U]glucose by so il microorganisms to determine whether it could evidence differences in car bon availability between unplanted and maize-planted soils. In unplanted so ils, the kinetics of glucose uptake indicated a multicomponent carrier-medi ated transport. The lower affinity constant (K-m) for glucose uptake was de termined at 74 muM of glucose. The kinetics of (CO2)-C-14 production from u nplanted soil amended with 0.07 mug C-glucose g(-1) soil demonstrated a pea k of respiratory activity during the first 15 min. One hour after C-14-gluc ose addition, the mineralisation of the substrate represented 7.8% of the C -14 that was unrecovered by 0.5 M K2SO4 extraction and which was assumed to be absorbed by soil microorganisms (97% of the C-14 amended). Three days a fter 14C-glucose addition, (CO2)-C-14 increased to 28% of the absorbed gluc ose in unplanted soils. The C-14 extractable by K2SO4 after fumigation with chloroform remained constant with time (about 25%). The apparent high effi ciency of utilisation of C-14 suggested that under conditions of carbon lim itation, the substrate is rather stored by microorganisms than used for gro wth. We proposed a model derived from the one of Bremer and Kuikman (Soil B iol. Biochem 26 (1994) 511) for the short-term uptake and utilisation of gl ucose by soil microorganisms. Furthermore, we found that the utilisation of C-14-glucose by microorganisms was significantly different between unplant ed and maize-planted soils. In planted soils, microorganisms mineralised a significant larger proportion of the absorbed glucose (32%) and the C-14-ac tivity in the fraction released after fumigation by chloroform vapours was lower (22%). This indicated a greater availability of carbon in maize-plant ed soils, which was related to the release of organic compounds by roots. ( C) 2001 Elsevier Science Ltd. All rights reserved.