Résumé
Questions: The fundamental trade-off between fine root trait attributes related to resource acquisition and conservation is well documented at species and community levels. However, relations remain unclear between this trade-off and community adaptation to environmental factors. As a result we ask: (1) how do fertility changes influence community position along the root acquisition–conservation trade-off; (2) how does root position along the soil profile influence its functional parameter; and (3) do fertility and soil depth have an interactive effect on community root parameters ? Location: Native multi-species temperate grassland in the French Massif Central. Methods: We assessed botanical composition and measured community root functional parameters (at plot and soil layer levels) in 16 plots differentiated by the amounts of N and P fertilizers applied over 16 yr and a soil depth gradient. Structural equation models were used to analyse relations among environmental factors, botanical composition and root functional parameters. Results: Botanical composition and plot-level root functional parameters vary according to fertility and soil depth. Communities from low fertility plots display a resource conservation strategy, i.e. high root tissue density, low specific root length (SRL) and low root length density (RLD), while communities from high fertility plots display a resource acquisition strategy, i.e. opposite parameter values. This demonstrates the importance of root resource management strategies in community adaptation to resource acquisition. Roots display different parameter values with soil depth. Roots in the surface horizon have small diameters and high SRL and RLD, suggesting intensive soil exploration and a high nutrient acquisition capacity. Roots in deep horizons have large diameters and low SRL, suggesting high water transport capacity per root length unit. This pattern is affected by plot fertility, i.e. communities from high fertility plots show higher root strategy differentiation with depth than communities from low fertility plots. We hypothesize that this root specialization along the soil profile enables species to be competitive for nutrient uptake in shallow soil layers and water uptake and transport in deep soil layers. Conclusion: Our study provides evidence that fertility and soil depth modified root functional parameters, in agreement with predictions from the economics spectrumtheory.