Increased Plant Carbon Translocation Linked to Overyielding in Grassland Species Mixtures

Plant species richness and productivity often show a positive relationship, but the underlying mechanisms are not fully understood, especially at the plant species level. We examined how growing plants in species mixture influences intraspecific rates of short-term carbon(C-) translocation, and determined whether such short-term responses are reflected in biomass yields. We grew monocultures and mixtures of six common C3 grassland plant species in outdoor mesocosms, applied a C-13-CO2 pulse in situ to trace assimilated C through plants, into the soil, and back to the atmosphere, and quantified species-specific biomass. Pulse derived C-13 enrichment was highest in the legumes Lotus corniculatus and Trifolium repens, and relocation (i.e. transport from the leaves to other plant parts) of the recently assimilated C-13 was most rapid in T. repens grown in 6-species mixtures. The grass Anthoxanthum odoratum also showed high levels of C-13 enrichment in 6-species mixtures, while C-13 enrichment was low in Lolium perenne, Plantago lanceolata and Achillea millefolium. Rates of C loss through respiration were highest in monocultures of T. repens and relatively low in species mixtures, while the proportion of C-13 in the respired CO2 was similar in monocultures and mixtures. The grass A. odoratum and legume T. repens were most promoted in 6-species mixtures, and together with L. corniculatus, caused the net biomass increase in 6-species mixtures. These plant species also had highest rates of C-13- label translocation, and for A. odoratum and T. repens this effect was greatest in plant individuals grown in species mixtures. Our study reveals that short-term plant C translocation can be accelerated in plant individuals of legume and C3 grass species when grown in mixtures, and that this is strongly positively related to overyielding. These results demonstrate a mechanistic coupling between changes in intraspecific plant carbon physiology and increased community level productivity in grassland systems.