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A poster accepted : Grassland root functional parameters vary according to a community-level resource acquisition-conservation trade-off

Florian Fort¹, Pablo Cruz², Eric Lecloux², Claire Jouany²

¹ Montpellier SupAgro, UMR 5175 CEFE, Université de Montpellier – Université Paul Valéry – EPHE, 1919 route de Mende, F-34293 Montpellier Cedex 5, France; ² INRA, UMR1248 AGIR, INRA – Université de Toulouse – INPT, F-31326 Castanet-Tolosan, France

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 communities’ adaptation to environmental factors. As a result we ask: i) How do fertility and soil depth influence the communities’ position along the root acquisition-conservation trade-off? ii) How does root position along the soil profile influence its functional parameter?

We have assessed grassland botanical composition and measured communities’ root functional parameters (at plot and soil-layer levels) in 16 plots differentiated by the amounts of N and P fertilisers applied over 16 years and a soil depth gradient. Structural equation models were used to analyse relations among environmental factors, botanical composition and root functional parameters.

Botanical composition and plot-level root functional parameters vary according to fertility and soil depth. Communities from low fertility plots display high root tissue density, low specific root length (SRL) and low root length density (RLD), while communities from high fertility plots display opposite parameter values. Roots display different parameter values with soil depth. Roots in the surface horizon have small diameters and high SRL and RLD, while roots in deep horizons have large diameters and low SRL.

Our results demonstrate that the root resource management strategy varies according to fertility, communities from fertile plots are displaying more acquisitive strategies than the others. Root parameters variation with soil depth suggests intensive soil exploration, a high nutrient acquisition capacity in the surface horizon and a high water transport capacity per root length unit in deep horizon. We hypothesise that root specialisation enables species to be competitive for both nutrient and water uptakes. Our study provides evidence that fertility and soil depth modified root functional parameters in agreement with predictions from the economics spectrum theory.