Pattern of carbon allocation in the rhizosphere of rye grass (Lolium perenne) after defoliation

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English. It is shown that defoliation could have controversial effects on plant soil C allocation pattern and on the component of the soil food web [1] depending of experimental conditions. The interactions between plant, root exudates and micro-organisms can only be understood in relation to soil fauna activity, indicating that the soil fauna has an important function in regulating rhizosphere microbial processes and therefore significantly affects plant growth [2]. In this context, this work aimed to measure pattern of C allocation to the rhizosphere of ryegrass plant submitted to defoliation under field conditions. Undisturbed soil monoliths with rye grass plant were excavated from the field trial, 2 and 4 days after defoliation for plant and micro-organisms analyses. Soil food web analyses were performed on soil monoliths excavated six days after defoliation.Defoliation decreased significantly root biomass especially 4 days after defoliation. Carbon concentration of shoot tissue (expressed in g C.g shoot-1) was significantly decreased whereas carbon concentration of roots tissue (g C.g root-1) was significantly increased by defoliation. Therefore, it is likely that C allocation to the roots was increased by defoliation. Total soil C content, density (colony forming units CFU) and activity (estimated from the short-term utilisation of 14C glucose by rhizosphere micro-organisms) of adhering soil micro-organisms were not affected. In agreement, the number of bacterivores, known to respond very well to changes in bacterial growth, was not affected by defoliation. The lower number of herbivores, fungivores and omnivores in defoliated plots was probably directly related to changes in root biomass, root quality or mycorrhizal fungi. It is concluded, that on a short-term scale, defoliation does not affect the patterns of C allocation in the rhizosphere of ryegrass.