THE PARTITIONING OF PHOTOSYNTHETICALLY FIXED CARBON WITHIN THE RHIZOSPHERE OF MATURE WHEAT

The distribution of photosynthetically-fixed carbon between plant and soil was measured for wheat grown for 63 days in a (CO2)-C-14 atmosphere from early tillering to anthesis and at two different moisture regimes. Rhizosphere CO2-C released from soil columns (100 mm dia x 1 m), each containing a single wheat plant, ranged between 447 and 1423 mg C and was evolved with a maximum rate 1.6-7.3 times that of the unplanted soil columns. The amount of C translocated below ground was equivalent to 1765 kg C ha-1 and was not significantly affected by the moisture treatment. Between 31 and 38% of the soil C-14 residue was found incorporated into the soil microbial biomass, with differences not significantly related to the different moisture regimes. Humic acid and humin fractions, essentially free from protein and carbohydrate components, contained 38 42% of the soil C-14 residue.