Continuous monitoring of rhizosphere respiration after labelling of plant shoots with (CO2)-C-14

The present work describes an original method to follow rate of (CO2)-C-14 and total CO2 production from rhizosphere respiration after plant shoots ha d been pulse-labelled with (CO2)-C-14. We used a radioactivity detector equ ipped with a plastic cell for flow detection of beta radiation by solid sci ntillation counting. The radioactivity detector was coupled with an infrare d gas analyser. The flow detection of (CO2)-C-14 was compared to trapping o f (CO2)-C-14 in NaOH and counting by liquid scintillation. First, we demons trated 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 (CO2)-C-14- 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 p resented two applications which illustrate the relevance of (CO2)-C-14-flow detection to investigations using C-14 to trace photoassimilates within th e plant-soil system. First, we examined the kinetics of (CO2)-C-14 producti on when concentrated acid is added to (NaHCO3)-C-14. This method is the mos t commonly used to label photoassimilates with C-14. Then, we monitored (CO 2)-C-14 activity in rhizosphere respiration of 5-week old maize cultivated in soil and whose shoots had been pulse-labelled with (CO2)-C-14. We conclu de that alkali traps should be used for a cumulative determination of (CO2) -C-14 because they are cheap and accurate. On the other hand, we demonstrat ed that the flow detection of (CO2)-C-14 had a finer temporal resolution an d was consequently a relevant tool to study C dynamics in the rhizosphere a t a short time scale.