Currently there are two proposed mechanisms to link assimilation to belowground respiration via the phloem:
1) the transport of assimilates basipetally according to the Münch theory, and 2) pressure-concentration waves. The transport of assimilates through the phloem by mechanism 1 is often quantified through isotopic labelling studies. Thus, the time between isotopic labelling in the canopy and when the labeled carbon is respired from the rhizoshpere characterizes the degree of coupling between aboveground and belowground metabolism. The timing between the aboveground uptake and belowground respiration of the labelled carbon is termed the “speed of link”. Based on statistical approaches, recent studies have reported a speed of link on the order of one day or less in mature forests, which is too fast for phloem transport by molecular diffusion or classical sink-source
dynamics. These studies often cite mechanism 2 to support their conclusions, there is however, absolutely no empirical evidence of pressure-concentration waves affecting soil respiration. In this presentation, we report results from experiments to test the mechanisms behind the speed of link on Douglas-fir saplings of the same provenance.
We kept the plants for several days in the dark to create a large carbon source-sink gradient with the intention of inducing a strong pressure-concentration wave. Following the no light treatment, in a controlled growth chamber, we introduced labelled CO2 prior to exposing the plant to light. Upon exposing the plants to light, the labeled CO2 is carboxylated through photosynthesis and after transport, is respired belowground. During the experiment we monitored the carbon isotopic composition of CO2 and the CO2
flux from the soil. We hypothesized that if the pressure concentration-wave effect on soil respiration existed then we would see an increase in CO2 flux prior to the arrival of CO2 derived from labelled assimilates.