C-13 Fractionation At The Root-Microorganisms-Soil Interface: A Review And Outlook For Partitioning Studies

Natural variations of the C-13/C-12 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-C-13-labelling approach, i.e. natural delta C-13 variation after C-3-C-4 vegetation changes. In this review, we focus on C-13 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 C-13 enriched compared to shoots by +1.2 +/- 0.6 parts per thousand and +0.3 +/- 0.4 parts per thousand respectively. The CO2 released by root respiration was C-13 depleted by about -2.1 +/- 2.2 parts per thousand for C-3 plants and -1.3 +/- 2.4 parts per thousand for C-4 plants compared to root tissue. However, only a very few studies investigated 13C fractionation by root respiration. This urgently calls for further research. In soils developed under C-3 vegetation, the microbial biomass was C-13 enriched by +1.2 +/- 2.6 parts per thousand and microbial CO2 was also C-13 enriched by +0.7 +/- 2.8 parts per thousand compared to SOM. This discrimination pattern suggests preferential utilization of C-13-enriched substances by microorganisms, but a respiration of lighter compounds from this fraction. The delta C-13 signature of the microbial pool is composed of metabolically active and dormant microorganisms; the respired CO2, however, derives mainly from active organisms. This discrepancy and the preferential substrate utilization explain the delta C-13 differences between microorganisms and CO2 by an 'apparent' C-13 discrimination. Preferential consumption of easily decomposable substrates and less negative delta C-13 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 (Delta = +1.1 parts per thousand) and respired lighter C (Delta = 1.1 parts per thousand) than SOM. Temperature and precipitation had no significant effect on the C-13 fractionation in these processes in C-3 soils. Increasing temperature and decreasing precipitation led, however, to increasing delta C-13 of soil C pools. Based on these C-13 fractionations we developed a number of consequences for C partitioning studies using C-13 natural abundance. In the framework of standard isotope mixing models, we calculated CO2 partitioning using the natural-C-13-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 CO2. Disregarding any C-13 fractionation processes, the calculated results deviated by up to 10% from the assumed fractions.Accounting for C-13 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 CO2 poos. Including C-13 fractionations directly into the mass balance equations reproduced the assumed CO2 partitioning exactly. At the end, we therefore give recommendations on how to consider C-13 fractionations in research on carbon flows between plants, microorganisms, and soil. (C) 2010 Elsevier Ltd. All rights reserved.