Soil carbon stocks and soil solution chemistry in Quercus ilex stands in Mainland Spain

The quantification of terrestrial carbon pools is important for the modeling of carbon fluxes in forest ecosystems. As a contribution to the understanding of the factors that influence the carbon sequestration capacity of Mediterranean forest soils, the present study focuses on the quantification of the superficial soil carbon stocks in evergreen oak stands (Quercus ilex L.) representative of its distribution area in Mainland Spain and to analyze the influence of site factors (climate and topography) and the soil chemical properties in the topsoil carbon storage capacity. For that purpose, 103 Quercus ilex stands were studied grouped in four main formations: 40 wooded grassland ecosystems with scattered oak trees (dehesas), 14 open Holm oak stands, 28 mixed Holm oak forests and 21 dense Holm oak forests. The soil organic carbon stocks (SOCS) in the upper organo-mineral soil layer ranged between 1.4–15.6 kg m -2 and total soil carbon stocks (TSCS) between 1.4–17.9 kg m -2 . Tree density was a significant factor for SOC storage in the soil. The wooded grassland dehesas presented the minimum superficial SOC stocks (3.6 kg m -2 ), while dense Holm oak forests reached the maximum average values (7.6 kg m -2 ). Maximum SOCS ([10 kg m -2 ) were measured in the mixed and dense Holm oak forests over soils with calcareous substrates. Summer mean temperature (R = -0.46; P\0.001) was the climatic variable that most influenced the SOCS. Soil properties had stronger positive correlations with SOCS than site factors: nitrogen concentrations (R = 0.70; P\0.001), clay content (R = 0.62; P\0.001), soluble calcium (R = 0.60; P\0.001) and magnesium (R = 0.42; P\0.001). Climatic and topographic variables together explained 30 % of the SOCS variability. An increase up to 63 % was obtained by including soil variables. Under Mediterranean climate conditions, the soil properties that enhance the organic matter protection achieve a notable relevance. The soil carbon storage is favored by large organic matter inputs, high soil clay contents, a calcium-saturated soil matrix and reduced summer aridity.