Analysis of δ13C of CO2 distinguishes between microbial respiration of added C4-sucrose and other soil respiration in a C3-ecosystem

The main aim of this study was to test various hypotheses regarding the changes in delta(13)C of emitted CO2 that follow the addition of C-4-sucrose to the soil of a C-3-ecosystem. It forms part of an experimental series designed to assess whether or not the contributions from C-3-respiration (root and microbial) and C-4-respiration (microbial) to total soil respiration can be calculated from such changes. A series of five experiments, three on sieved (root-free) mor-layer material, and two in the field with intact mor-layer (and consequently with active roots), were performed. Both in the experiments on sieved mor-layer and the field experiments, we found a C-4-sucrose-induced increase in C-3-respiration that accounted for between 30% and 40% of the respiration increase 1 h after sucrose addition. When the course of C-3-, C-4- and total respiration was followed in sieved material over four days following addition of C-4-sucrose, the initially increased respiration of C-3-C was transient, passing within less than 24 h. In a separate pot experiment, neither ectomycorrhizal Pinus sylvestris L. roots nor non-mycorrhizal roots of this species showed respiratory changes in response to exogenous sucrose. No shift in the delta(13)C of the evolved CO2 after adding C-3-sucrose to sieved mor-layer material was found, confirming that the sucrose-induced increase in respiration of endogenous C was not an artefact of discrimination against C-13 during respiration. Furthermore, we conclude that the C-4-sucrose induced transient increase in C-3-respiration is most likely the result of accelerated turnover of C in the microbial biomass. Thus, neither respiration of mycorrhizal roots, nor processes discriminating against delta(13)C were likely sources of error in the field. The estimated delta(13)C of evolved soil CO2 in three field experiments lay between -25.2 parts per thousand and -23.6 parts per thousand. The study shows that we can distinguish between CO2 evolved from microbial mineralisation of added C-4-sucrose, and CO2 evolved from endogenous carbon sources (roots and microbial respiration).