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Abbreviations CFI = comparative fit index; RLD = root length density; RMSEA = root mean square error of approximation; RTD = root tissue density; SRL = specific root length; SEM = structural equation model. NomenclatureAbstract 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 spectrum theory.

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We analyzed the linearity of relationships between soil test P (STP) and cumulative phosphorus (P) budget using data from six long-term fertilized grassland sites in four countries: France (Ercé and Gramond), Switzerland (Les Verrières), Canada (Lévis), and Finland (Maaninka and Siikajoki). STP was determined according to existing national guidelines. A linear-plateau model was used to determine the presence of deflection points in the relationships. Deflection points with (x, y) coordinates were observed everywhere but Maaninka. Above the deflection point, a significant linear relationship was obtained (0.33 < r2 < 0.72) at four sites, while below the deflection point, the relationship was not significant, with a negligible rate of STP decrease. The relationship was not linear over the range of STP encountered at most sites, suggesting a need for caution when using the P budget approach to predict STP changes in grasslands, particularly in situations of very low P fertilization. Our study provides insights and description of a tool to improve global P strategies aimed at maintaining STP at levels adequate for grassland production while reducing the risk of P pollution of water.

Soil acidification due to N supplied from fertilizer or aerial deposition in grasslands is well recognized and documented; however, little is known about its consequences on soil P availability. This question was addressed on a long‐term field experiment done on grassland in Ercé (Central Pyrenees, France), where plots were treated for 6 yr with a factorial combination of mineral N added as NH4NO3 (0 and 190 kg ha−1 yr−1) and P (0 and 50 kg ha−1 yr−1) and managed with a frequent defoliation regime (3 cuts yr−1). Diffusive PO4 ion dynamics at the soil–solution interface was determined by coupling sorption studies with 32P labeling of PO4 ions (Pi) and dilution analysis. Results showed that the soil pH did not change in zero‐N treatments (5.9 ± 0.1) in comparison with the initial soil (5.9 ± 0.1), whereas it decreased to 5.3 ± 0.1 on N‐treated soils. Meanwhile the cation exchange capacity and exchangeable Ca2+ and Mg2+ contents decreased significantly and the saturation percentage of the adsorption complex by Al3+ increased from 2 to 15; no changes were observed for untreated soils. Furthermore, the gross amount of diffusive Pi (Pr) was accurately described as a function of the concentration of PO4 ions in the solution (Cp) and time (t) by a Freundlich kinetic equation; these dynamics were significantly modified by N supply but not by P.