Continuous monitoring of rhizosphere respiration after labelling of plant shoots with 14CO2

The present work describes an original method to follow rate of ¹⁴CO₂ and total CO₂ production from rhizosphere respiration after plant shoots had been pulse-labelled with ¹⁴CO₂. We used a radioactivity detector equipped with a plastic cell for flow detection of beta radiation by solid scintillation counting. The radioactivity detector was coupled with an infrared gas analyser. The flow detection of ¹⁴CO₂ was compared to trapping of ¹⁴CO₂ in NaOH and counting by liquid scintillation. First, we demonstrated that NaOH (1 M) trapped 95% of the CO₂ of a gaseous sample. Then, we determined that the counting efficiency of the radioactivity flow cell was 41% of the activity of gaseous samples as determined by trapping in NaOH (1 M) and by counting by static liquid scintillation. The sensitivity of the ¹⁴CO₂- flow detection was 0.08 Bq mL⁻¹ air and the precision was 2.9% of the activity measured compared to 0.9% for NaOH trapping method. We presented two applications which illustrate the relevance of ¹⁴CO₂-flow detection to investigations using 14C to trace photoassimilates within the plant-soil system. First, we examined the kinetics of 14CO₂ production when concentrated acid is added to NaH¹⁴CO₃. This method is the most commonly used to label photoassimilates with ¹⁴C. Then, we monitored ¹⁴CO₂ activity in rhizosphere respiration of 5-week old maize cultivated in soil and whose shoots had been pulse-labelled with ¹⁴CO₂. We conclude that alkali traps should be used for a cumulative determination of ¹⁴CO₂ because they are cheap and accurate. On the other hand, we demonstrated that the flow detection of ¹⁴CO₂ had a finer temporal resolution and was consequently a relevant tool to study C dynamics in the rhizosphere at a short time scale. This revised version was published online in June 2006 with corrections to the Cover Date.