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ABSTRACT
Two nutrient addition experiments were carried out within a program of vegetation ecology of ultramafic soils in Tuscany, Italy. Nitrogen and especially phosphorus induced a slight increase in plant species richness and a significant increase in ground cover, but their interaction was not significant. The addition of nitrogen, phosphorus and potassium together greatly promoted ground cover and showed a partial effect in promoting species richness. The addition of calcium had the same effect on species richness, but did not affect ground cover or have any synergistic effect with nitrogen, phosphorus and potassium. Species composition was not significantly influenced by any of the treatments and the ground cover increases observed concerned species already growing in the plots. Availability of soil elements differed for calcium and potassium in the plots where they were added. The addition of calcium also reduced the availability of manganese, nickel and magnesium, as well as the Mg/Ca ratio, and increased the pH. The response of vegetation to the reduction in nickel availability did not differ from control plots, but it varied markedly in the plots fertilised with nitrogen, phosphorus and potassium, suggesting that this vegetation is affected more by nutritional stress than by soil metal content.