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 C-14 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 oil C-3 Soil, in a second approach the differences in C-13 natural abundance between C-3 and C-4 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 C-14 labelling approach and between 86% and 94% in the natural C-13 labelling approach. At a C-13 fractionation measured to be 5.2 parts per thousand 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 C-14 labelling approach. Root-derived contributions to the microbial biomass carbon ranged from 2% to 9% using C-14 labelling and from 16% to 36% using natural C-13 labelling. At a 3.2 parts per thousand C-13 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 C-13 fractionation strongly affects the contributions of individual C sources. (c) 2007 Elsevier Ltd. All rights reserved.