Root-derived carbon in soil respiration and microbial biomass determined by 14C and 13C

Two approaches to quantitatively estimating root-derived carbon in soil CO2 efflux and in microbial biomass were compared under controlled conditions. In the 14C labelling approach, maize (Zea mays) was pulse labelled and the tracer was chased in plant and soil compartments. Root-derived carbon in CO2 efflux and in microbial biomass was estimated based on a linear relationship between the plant shoots and the below-ground compartment. Since the maize plants were grown on C3 soil, in a second approach the differences in 13C natural abundance between C3 and C4 plants were used to calculate root-derived carbon in the CO2 efflux and in the microbial biomass. The root-derived carbon in the total CO2 efflux was between 69% and 94% using the 14C labelling approach and between 86% and 94% in the natural 13C labelling approach. At a 13C fractionation measured to be 5.2‰ between soil organic matter (SOM) and CO2, the root-derived contribution to CO2 ranged from 70% to 88% and was much closer to the results of the 14C labelling approach. Root-derived contributions to the microbial biomass carbon ranged from 2% to 9% using 14C labelling and from 16% to 36% using natural 13C labelling. At a 3.2‰ 13C fractionation between SOM and microbial biomass, both labelling approaches yielded an equal contribution of root-derived C in the microbial biomass. Both approaches may therefore be used to partition CO2 efflux and to quantify the C sources of microbial biomass. However, the assumed 13C fractionation strongly affects the contributions of individual C sources.