Background and aims

Only the carbon (C) isotope pulse labeling approach can provide time-resolved data concerning the input and turnover of plant-derived C in the soil, which are urgently needed to improve the performance of terrestrial C cycle models. However, there is currently very limited information about the point in time after pulse labeling at which the distribution of tracer C accurately represents the usage of photosynthates in different components of the plant-soil system. This should be the case as soon as the tracer has disappeared from the mobile C pool due to respiration, incorporation into the structural C pool of shoot and root tissue and exudation into the soil (rhizodeposition).

Methods

Following  14 CO 2  ¹⁴CO₂ pulse labeling in laboratory and outdoor experiments with spring rye, the ¹⁴C dilution rates of soluble fractions and different substances from the structural C pool of the shoot (molecular level), the release of labeled CO₂ by belowground respiration (component level), and the ¹⁴C kinetics of shoot respiration and ¹⁴C remaining in the plant-soil-soil gas continuum (system level) were analyzed during different stages of plant development.

Results

At all three levels investigated, ¹⁴C kinetics indicated that the C tracer levels changed very little between 15 and 21 days after labeling. Results also showed increasing tracer depletion in the mobile C pool. Consequently, only 0.42 % and 0.06 % of all ¹⁴C was still available for shoot respiration 15 and 21 days after labeling, respectively.

Conclusions

The similarities between ¹⁴C tracer kinetics at the three investigated levels indicate that tracer disappearance from the mobile pool and distribution throughout the plant-soil system was nearly complete between 15 and 21 days after labeling. Therefore, this appears to be the point at which the pulse labeling approach provides sufficiently precise data concerning the use of C (assimilated during labeling) for root growth, rhizodeposition, root respiration and the microbial turnover of rhizodeposits.