Carbon Loss from the Roots of Forage Rape (Brassica napus L.) Seedlings Following Pulse-labelling with 14CO2

The loss of organic material from the roots of forage rape ( Brassica napus L.,) was studied by pulse-labelling 25-d-old non-sterile sand-grown plants with 14 CO 2 . The distribution of 14 C within the plant was measured at 0, 6 and 13 d after labelling whilst 14 C accumulating in the root-zone was measured at more frequent intervals. The rates of 14 C release into the rhizosphere, and loss of 14 CO 2 from the rhizosphere were also determined. These data were used to estimate the accumulative loss of 14 C from roots and loss respiratory 14 CO 2 from both roots and associated micro-organisms. Approximately 17-19% of fixed 14 CO 2 was translocated to the roots over 2 weeks, of which 30-34% was released into the rhizosphere, and 23-24% was respired by the roots as 14 CO 2 . Of the 14 C released into the rhizosphere, between 35-51% was assimilated and respired by rhizosphere micro-organisms.