Interspecific control of non-symbiotic carbon partitioning in the rhizosphere of a grass-clover association: Bromus madritensis-Trifolium angustifolium.

Grass-legume interaction in the rhizosphere was investigated in a greenhouse experiment with two annual species, bromegrass Bromus madritensis (L.) and clover Trifolium angustifolium (L.) grown in mono and mixed cultures. Partitioning of below-ground carbon between roots, respiration, and soil was measured after separate 2 h-labelling of each species with 14CO2 followed by a 9 d chase period. At the time of labelling, clover nodules were not yet fixing N2. Bromegrass grew much faster than clover. Shoot biomass of bromegrass was greater in the presence of clover than in monoculture. By contrast, both shoot and root biomass of clover was less in the presence of bromegrass than in monoculture. Carbon assimilation during the period of labelling was proportional to shoot biomass and partitioning above and below-ground did not differ among treatments. Absolute amounts of labelled C allocated to rhizosphere respiration was more in bromegrass than in clover (respectively 1.38 mg C against 0.75 mg C in monoculture and 1.79 mg C and 0.63 mg C in mixed culture). However, when expressed as a percentage of below-ground C allocation, rhizosphere respiration was lower in bromegrass than in clover, respectively, 38% and 45% in monoculture. In mixed culture, this percentage increased by 7.3% for clover, and 3.5% for bromegrass, thus indicating that the interspecific effect of grass was higher than that of clover. The percentage of below-ground C in a soil solution of clover in mixed culture was more than 2-fold that measured in monoculture. It was also significantly correlated with the percentage of below-ground C in respiration. These results provided evidence that the grass-legume mixture has the potential to influence the rhizosphere processes of each species in more than an additive way and that the effect of the interaction was stronger on clover than on bromegrass. The possible implications of this in grass-legume competition are discussed.