Theoretical background for partitioning of root and rhizomicrobial respiration by δ13C of microbial biomass

Separate estimation of sources of root-derived CO2 efflux from the soil into actual root respiration and microbial respiration of rhizodeposits is very important for determining the carbon (C) and energy balance of soils and plants, C sources for rhizosphere microorganisms, sources of soil organic matter (SOM), etc. Besides component integration, to date, only four adequate methods based on the pulse labeling of shoots in a 14CO2 atmosphere and subsequent monitoring of 14CO2 efflux from the soil have been suggested: 1) the isotope dilution, 2) the model rhizodeposition technique, 3) modeling of 14CO2 efflux dynamics, and 4) the exudate elution procedure. These methods are based on different assumptions and principles that are very difficult to check experimentally and have different results. Therefore, none of these methods can be accepted as a standard procedure allowing quantitative separate estimation of root respiration and rhizomicrobial respiration. This contribution provides an elaboration of the theoretical background of a procedure allowing quantitative separate estimation of root respiration and rhizomicrobial respiration in non-sterile soils. The method is based on 13C natural abundance by growing C4 plant on C3 soil or vice versa. Four δ13C values are necessary: of the SOM, of the roots, of soil microbial biomass, and of CO2 efflux from the soil. The advantages and assumptions of the new approach, as well as possible applications including FACE systems and continuous labeling experiments are discussed.