Carbon turnover in the rhizosphere under continuous plant labeling with 13CO2: Partitioning of root,microbial, and rhizomicrobial respiration

The input of labeled C into the pool of soil organic matter, the CO₂ fluxes from the soil, and the contribution of root and microbial respiration to the CO₂ emission were studied in a greenhouse experiment with continuous labeling of oat plants with ¹³CO₂ using the method of the natural ¹³C abundance in the air. The carbon of the microbial biomass composed 56 and 39% of the total amounts of ¹³C photoassimilates in the rhizosphere and in the bulk soil, respectively. The contribution of root respiration to the CO₂ emission from the soil reached 61–92%, including 4–23% of the rhizomicrobial respiration. The contribution of the microbial respiration to the total CO₂ emission from the soil varied from 8 to 39%. The soil organic matter served as the major carbon-containing substrate for microorganisms in the bulk soil and in the rhizosphere: 81–91% of the total amount of carbon involved in the microbial metabolism was derived from the soil organic matter.