The above-belowground coupling of the C cycle: fast and slow mechanisms of C transfer for root and rhizomicrobial respiration

Background and aims The coupling of photosynthesis with belowground processes appears to be much faster than the time needed for assimilate translocation with the phloem flow. Pressure/concentration waves have been hypothesized to release belowground C already present in the phloem, resulting in a very fast feedback of rhizosphere processes to photosynthesis changes. We evaluate the speed of aboveground-rhizosphere coupling under maize by two mechanisms: pressure/concentration waves and direct phloem transport. Methods We combined two isotopic approaches: 1) the speed of direct phloem transport was evaluated by labeling shoots in 14CO2 and tracing 14C in the nutrient solution and in the CO2 flux, 2) pressure/concentration waves were evaluated by labeling the solution with [13C] glucose and tracing the isotope dilution during photoassimilation. Results 14C shoot labeling of maize plants showed that 12 h were needed for 14C to peak in root-derived CO2. In contrast, in the solution labeling approach, CO2 flux increased within 2 h after switching on the light. Pressure/concentration waves contributed 5% to diurnal respiration efflux and affected only root respiration. Root exudation was independent of the fast mechanism of above-belowground coupling. Conclusions Photosynthesis affected root and rhizomicrobial respiration on variable time-scales: root respiration within the first 2 h by pressure/concentration waves, whereas rhizomicrobial respiration may depend on internal circadian cycles in regulating exudation rather than on light directly.