C isotope fractionation during rhizosphere respiration of C3 and C4 plants

Stable carbon isotopes are used extensively to partition total soil CO2 efflux into root-derived rhizosphere respiration or autotrophic respiration and soil-derived heterotrophic respiration. However, it remains unclear whether CO2 from rhizosphere respiration has the same δC value as root biomass. Here we investigated the magnitude of C isotope fractionation during rhizosphere respiration relative to root biomass in six plant species. Plants were grown in a carbon-free sand-perlite medium inoculated with microorganisms from a farm soil for 62 days inside a greenhouse. We measured the δC value of rhizosphere respiration using a closed-circulation 48-hour CO2 trapping method during 40~42 and 60~62 days after sowing. We found a consistent depletion in C (0.9~1.7‰) of CO2 from rhizosphere respiration relative to root biomass in three C3 species (Glycine max L. Merr., Helianthus annuus L. and Triticum aestivum L.), but a relatively large depletion in C (3.7~7.0‰) in three C4 species (Amaranthus tricolor L., Sorghum bicolor (L.) Moench and Zea mays L. ssp. mays). Overall, our results indicate that CO2 from rhizosphere respiration is more C-depleted than root biomass. Therefore, accounting for this C fractionation is required for accurately partitioning total soil CO2 efflux into root-derived and soil-derived components using natural abundance stable carbon isotope methods.