CHARACTERIZATION OF THE DYNAMICS OF C-PARTITIONING WITHIN LOLIUM-PERENNE AND TO THE RHIZOSPHERE MICROBIAL BIOMASS USING C-14 PULSE-CHASE

The dynamics of C partitioning with Lolium perenne and its associated rhizosphere was investigated in plant-soil microcosms using C-14 pulse-chase labelling. The CO2(C-14) pulse was introduced into the shoot chamber and the plants allowed to assimilate the label for a fixed period. The microcosm design facilitated independent monitoring of shoot and root/soil respiration during the chase period. Partitioning between above- and below-ground pools was determined between 30 min and 168 h after the pulse, and the distribution was found to vary with the length of the chase period. Initially (30 min after the pulse), C-14 was predominantly (99%) in the shoot biomass and declined thereafter. The results indicate that translocation of recent photoassimilate is rapid, with C-14 detected below ground within 30 min of pulse application. The translocation rate of C-14 below ground was maximal (6.2% h-1) between 30 min and 3h after the pulse, with greatest incorporation into the microbial biomass detected at 3 h. After 3 h, the microbial biomass C-14 pool accounted for 74% of the total C-14 rhizosphere pool. By 24 h, approximately 30% of C-14 assimilate had been translocated below ground; thereafter C-14 translocation was greatly reduced. Partitioning of recent assimilate changed with increasing CO2 concentration. The proportion of C-14 translocated below ground almost doubled from 17.76% at the ambient atmospheric CO2 concentration (450 ppm) to 33.73% at 750 ppm CO2 concentration. More specifically, these changes occurred in the root biomass and the total rhizosphere pools, with two- and threefold C-14 increases at an elevated CO2 concentration compared to ambient, respectively. The pulse-labelling strategy developed in this study provided sufficient sensitivity to determine perturbations in C dynamics in L. perenne, in particular rhizosphere C pools, in response to an elevated atmospheric CO2 concentration.