Interaction between rhizosphere microorganisms and plant roots: ¹³C fluxes in the rhizosphere after pulse labeling

The input dynamics of labeled C into pools of soil organic matter and CO₂ fluxes from soil were studied in a pot experiment with the pulse labeling of oats and corn under a ¹³CO₂ atmosphere, and the contribution of the root and microbial respiration to the emission of CO₂ from the soil was determined from the fluxes of labeled C in the microbial biomass and the evolved carbon dioxide. A considerable amount of ¹³C (up to 96% of the total amount of the label found in the rhizosphere soil) was incorporated into the biomass of the rhizosphere microorganisms. The diurnal fluctuations of the labeled C pools in the microbial biomass, dissolved organic carbon, and CO₂ released in the rhizosphere of oats and corn were related to the day/night changes, i.e., to the on and off periods of the photosynthetic activity of the plants. The average contribution of the corn root respiration (70% of the total CO₂ emission from the soil surface) was higher than that of the oats roots (44%), which was related to the lower incorporation of rhizodeposit carbon into the microbial biomass in the soil under the corn plants than in the soil under the oats plants.