RHIZODEPOSITION BY (CO2)-C-14-PULSE-LABELED SPRING BARLEY GROWN IN SMALL-FIELD PLOTS ON SANDY LOAM

I examined the below-ground transfer of C in spring barley (Hordeum vulgare L. cv. Alexis) grown in the field in soil cores contained in stainless steel cylinders, and compared results obtained by C-14 pulse-labelling and root washing. At four growth stages, shoots of three cylinders were labelled with (CO2)-C-14 for 8-10 h, and the distribution of C-14 determined 6 days after labelling (day 6). At maturity (127 days after sowing), the yield of grain, straw and macro-roots (isolated by root washing) was 273.2, 261.7 and 45 g C m-2 respectively. Throughout the growing season, 67-73% of the macro-root C and 45-78% of the C-14 retained below-ground (day 6) was in the 0-15 cm of soil layer. Generally, the proportion of below-ground C-14 recovered in the 0-15 cm layer decreased throughout the growing season. At early elongation, 36.7% of C-14 recovered was translocated below-ground with 7.6, 20.7 and 8.3% being recovered in macro-roots, macro-root free soil and as rhizosphere respiration (root and microbial), respectively, the corresponding values at late grain filling being only 0.7, 1.1 and 3.0%. The half life of C-14 deposited in the soil (macro-root free soil) was estimated to 6-42 days depending on the intensity of root growth. The total below-ground transfer of C during the growth of spring barley was estimated to 165.2 g C m-2 (1652 kg C ha-2). Rhizosphere respiration and macro-root C isolated by root washing at maturity accounted for 23.3 and 27.2%, respectively. The amount of C translocated below-ground corresponds to one-third of the above-ground C harvested at maturity.