Carbon allocation in grassland communities under drought stress followed by 14C pulse labeling

Although extreme climatic events such as drought have important consequences for belowground carbon (C) cycling, their impact on the plant-soil system of mixed plant communities is poorly understood. Our objective was to study the effect of drought on C allocation and rhizosphere-mediated CO2 fluxes under three plant species: Lolium perenne, Festuca arundinacea and Medicago sativa grown in monocultures or mixture. The conceptual approach included 14CO2 pulse labeling of plants grown under drought and optimum water conditions in order to be able to follow above- and belowground C allocation. After 14C pulse labeling, we traced 14C allocation to shoots and roots, soil and rhizospheric CO2, dissolved organic carbon (DOC) and microbial biomass. Drought and plant community composition significantly affected assimilate allocation in the plant-soil system. Drought conditions changed the source sink relationship of monocultures, which transferred a relatively larger portion of assimilates to their roots compared to water sufficient plants. In contrast, plant mixture showed an increase in 14C allocation to shoots when exposed to drought. Under drought stress, root respiration was reduced for all monocultures except under the legume species. Microbial respiration remained similar in all cases showing that microbial activity was less affected by drought than root activity. This may be explained by strongly increased assimilate allocation to easily available exudates or rhizodeposits under drought. In conclusion, plant community composition may modify the impact of climatic changes on carbon allocation and belowground carbon fluxes. The presence of legume species attenuates drought effects on rhizosphere processes.