Terrestrial ecosystems comprise above- and belowground compartments that interact with each other through plants. Analysing the impact of these aboveground-belowground relationships on ecosystem functioning is a major aim of ecology. While there is ample evidence of linkages between the two compartments, indirect interactions mediated via plants, have been very scarcely addressed. Belowground, plants supply important sources of organic matter (either by root or litter) to soil micro-organisms involved in key ecological processes for soil functioning. Aboveground, plants are involved in various interactions with animals such as pollinators and herbivores. Here, we focused on links between soil functioning and plant attractiveness to pollinators.

Using a two factorial experimental design, we tested single and combined effects of plant association and litter addition on vegetative and flowering traits of three plant species: Lamium amplexicaule, Mimulus guttatus, Medicago sativa. Here, we especially investigated the impact of the identity, specificity and diversity of litter input on plant resource allocations between growth and reproduction. Eight experimental communities were created by manipulating plant species diversity  (control without plants, mono-specific, bi-specific and tri-specific communities) and five litter treatments (control without litter, mono-specific and tri-specific litter) were applied to each type of plant community. Litters incorporated to soil were obtained from dry shoot biomass of each of these three plant species.

Results/Conclusions

Our results indicated that both the type of litter added and the diversity of the plant community modify vegetative (biomass, shoot C/N ratio) and reproductive traits (phenology, nectar volume and nectar sugar composition). Considering mono-specific communities, plants that had grown on their own litter produced more  biomass and flowered later compared  to plants that had grown on litter from different plant species. Growth-reproduction trade-offs may thus be influenced by soil functioning processes, that in turn depend on the type of litter input. This also suggests that inter-specific competition between plants for pollination may indirectly occur through litter deposition. Analyses of soil microbial activity and diversity are still in progress and will allow us to identify the underlying soil processes. This inter-disciplinary study (community ecology, microbial ecology, biogeochemistry) stresses the importance of considering belowground and aboveground interactions simultaneously to better understand the functioning of terrestrial ecosystems.