13C isotope fractionation during rhizosphere respiration of C3 and C4 plants

Abstract

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 δ13C value as root biomass. Here we investigated the magnitude of 13C 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 δ13C 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 13C (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 13C (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 13C-depleted than root biomass. Therefore, accounting for this 13C fractionation is required for accurately partitioning total soil CO2 efflux into root-derived and soil-derived components using natural abundance stable carbon isotope methods.