Abstract A natural‐ 13 C‐labeling approach—formerly observed under controlled conditions—was tested in the field to partition total soil CO 2 efflux into root respiration, rhizomicrobial respiration, and soil organic matter (SOM) decomposition. Different results were expected in the field due to different climate, site, and microbial properties in contrast to the laboratory. Within this isotopic method, maize was planted on soil with C 3 ‐vegetation history and the total CO 2 efflux from soil was subdivided by isotopic mass balance. The C 4 ‐derived C in soil microbial biomass was also determined. Additionally, in a root‐exclusion approach, root‐ and SOM‐derived CO 2 were determined by the total CO 2 effluxes from maize ( Zea mays L.) and bare‐fallow plots. In both approaches, maize‐derived CO 2 contributed 22% to 35% to the total CO 2 efflux during the growth period, which was comparable to other field studies. In our laboratory study, this CO 2 fraction was tripled due to different climate, soil, and sampling conditions. In the natural‐ 13 C‐labeling approach, rhizomicrobial respiration was low compared to other studies, which was related to a low amount of C 4 ‐derived microbial biomass. At the end of the growth period, however, 64% root respiration and 36% rhizomicrobial respiration in relation to total root‐derived CO 2 were calculated when considering high isotopic fractionations between SOM, microbial biomass, and CO 2 . This relationship was closer to the 50% : 50% partitioning described in the literature than without fractionation (23% root respiration, 77% rhizomicrobial respiration). Fractionation processes of 13 C must be taken into account when calculating CO 2 partitioning in soil. Both methods—natural 13 C labeling and root exclusion—showed the same partitioning results when 13 C isotopic fractionation during microbial respiration was considered and may therefore be used to separate plant‐ and SOM‐derived CO 2 sources.
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