13C fractionation at the root–microorganisms–soil interface: A review and outlook for partitioning studies

Abstract Natural variations of the 13 C/ 12 C ratio have been frequently used over the last three decades to trace C sources and fluxes between plants, microorganisms, and soil. Many of these studies have used the natural- 13 C-labelling approach, i.e. natural δ 13 C variation after C 3 –C 4 vegetation changes. In this review, we focus on 13 C fractionation in main processes at the interface between roots, microorganisms, and soil: root respiration, microbial respiration, formation of dissolved organic carbon, as well as microbial uptake and utilization of soil organic matter (SOM). Based on literature data and our own studies, we estimated that, on average, the roots of C 3 and C 4 plants are 13 C enriched compared to shoots by +1.2 ± 0.6‰ and +0.3 ± 0.4‰, respectively. The CO 2 released by root respiration was 13 C depleted by about −2.1 ± 2.2‰ for C 3 plants and −1.3 ± 2.4‰ for C 4 plants compared to root tissue. However, only a very few studies investigated 13 C fractionation by root respiration. This urgently calls for further research. In soils developed under C 3 vegetation, the microbial biomass was 13 C enriched by +1.2 ± 2.6‰ and microbial CO 2 was also 13 C enriched by +0.7 ± 2.8‰ compared to SOM. This discrimination pattern suggests preferential utilization of 13 C-enriched substances by microorganisms, but a respiration of lighter compounds from this fraction. The δ 13 C signature of the microbial pool is composed of metabolically active and dormant microorganisms; the respired CO 2 , however, derives mainly from active organisms. This discrepancy and the preferential substrate utilization explain the δ 13 C differences between microorganisms and CO 2 by an ‘apparent’ 13 C discrimination. Preferential consumption of easily decomposable substrates and less negative δ 13 C values were common for substances with low C/N ratios. Preferential substrate utilization was more important for C 3 soils because, in C 4 soils, microbial respiration strictly followed kinetics, i.e. microorganisms incorporated heavier C (∆ = +1.1‰) and respired lighter C (∆ = −1.1‰) than SOM. Temperature and precipitation had no significant effect on the 13 C fractionation in these processes in C 3 soils. Increasing temperature and decreasing precipitation led, however, to increasing δ 13 C of soil C pools. Based on these 13 C fractionations we developed a number of consequences for C partitioning studies using 13 C natural abundance. In the framework of standard isotope mixing models, we calculated CO 2 partitioning using the natural- 13 C-labelling approach at a vegetation change from C 3 to C 4 plants assuming a root-derived fraction between 0% and 100% to total soil CO 2 . Disregarding any 13 C fractionation processes, the calculated results deviated by up to 10% from the assumed fractions. Accounting for 13 C fractionation in the standard deviations of the C 4 source and the mixing pool did not improve the exactness of the partitioning results; rather, it doubled the standard errors of the CO 2 pools. Including 13 C fractionations directly into the mass balance equations reproduced the assumed CO 2 partitioning exactly. At the end, we therefore give recommendations on how to consider 13 C fractionations in research on carbon flows between plants, microorganisms, and soil.