Differences in rhizosphere carbon-partitioning among plant species of different families

Interspecific variations in carbon ( C) allocation and partitioning in the rhizosphere were investigated on 12 Mediterranean species belonging to different family groups ( grasses, legumes, non-legume forbs) and having different life cycles. Plants grown individually in artificial soil, in a greenhouse and inoculated with rhizosphere microflora were labelled with (CO2)-C-14 for 3 h at the vegetative stage. Rhizosphere respiration was measured during 6 days after which labelled C partitioning between shoots, roots, soil, root washing solution and respiration was estimated. The percentage of assimilated C-14 allocated below ground differed significantly between species ( 41 - 76%) but no significant difference was found between grasses, legumes and non-legume forbs. When expressed as percentage of below-ground C-14, rhizosphere respiration was significantly smaller for non-legume forbs (42%) than for grasses (46%) and legumes (51%). Consequently more C-14 was incorporated into root biomass in the former. Half-life of (CO2)-C-14 evolution through respiration ranged from 23 h in legumes to 27 h for non-legume forbs and 37 h for grasses. This suggested differences in microbial activities due to quantities and quality of root exuded C. Rhizosphere respiration was positively correlated with the amount of C-14 in the solution used to wash the roots on one hand, and root N concentration on the other hand. This led to a functional hierarchy between plant family groups of the overall rhizosphere activity.. It went from non-legume forbs being the less active ( except Crepis sancta) in terms of respiration and exudation, to grasses and then legumes, the most active but also the richest in nitrogen.