Nutrient stoichiometry mediates the patchy coexistence of two subalpine grassland types

Abstract

Background and Aims High mountain areas often show high soil heterogeneity that allows for the close coexistence of plant species and communities with contrasting resource requirements. This study investigates the nutritional factors driving the mosaic distribution of Nardus stricta L. grasslands and chalk grasslands dominated by forbs in the subalpine southern Pyrenees (Spain). Methods The concentrations of C, N, P, S, K, Ca and fiber fractions were analyzed in herbage in relation to soil nutrient availability, soil β–glucosidase, urease, phosphatase and arylsulfatase activity and plant species and functional type composition. Results The chalk grassland showed higher N:P ratios in herbage and higher enzyme demand for P relative to N in the soil, which indicate a greater limitation of P versus N compared to the Nardus grassland. This limitation was related to the higher soil and plant Ca levels in the chalk grassland, where the calcareous bedrock lies close to the soil surface. In the Nardus grasslands, the alleviation of P limitation translated into higher productivity and replacement of forbs with taller graminoids rich in structural carbohydrates, which was accompanied by greater β–D–glucosidase activity. The plant N:K and P:K ratios indicated potential K deficiency in both grasslands, which resulted from a decreased uptake of K due to competition from Ca, as indicated by the correlation between plant K and the soil K+:Ca2+ ratio. Conclusions Our results highlight the effect of the heterogeneity of soil nutrient constraints, as mediated by their stoichiometry and controlled by local topography, on the biodiversity of high mountain ecosystems.