Effect of climate and physiography on occurrence and intensity of decarbonation in Mediterranean forest soils of Spain

The aim of this work is to model the occurrence and intensity of calcium carbonate loss in calcareous forest soils under Mediterranean conditions. We have used 193 Spanish forest soils found on limestone parent material considered highly calcareous materials. This data set came from FOREDAF, a database containing 2200 complete soil profiles and all of them covered by forest stands (mainly dominated by pine species). The research is focused on the influence of physiographic features and climatic conditions (19 parameters), together with soil characteristics (11 parameters) on soil carbonate leaching. We have considered the chemically active calcium carbonates sensu Duchaufour, i.e. associated to silt and clay fractions. Thus, we have compared the characteristics of decarbonated soils (with lower level of active carbonates than the original richness of parent material) against non-decarbonated soils. Furthermore, we consider the intensity of carbonate leaching by means of an expression that involves the relative difference between both active and inactive carbonates. Thus, several logistic regressions were performed to determine the most powerful predictors for the occurrence of soil carbonate leaching related to physiography, temperature and precipitation. In addition, linear models were carried out to determine the most powerful predictor for chemically active carbonate levels of soils. In general, the studied forest soils had loamy textures (sand 31.9%, silt 42.7% and clay 26.3%), and similar richness of carbonate in limestone parent material (78.6%). We observed a pH decreasing (pH water 7.9) and an increasing organic matter content (5.6 g 100 g−1) in surface horizons which were linked to soil carbonate leaching. Our results indicate that the most relevant factors for the occurrence of carbonate loss are a combination of slope, aspect and latitude (by means of insolation) and an estimated climatic demand for water depending on temperatures (annual potential evapotranspiration). However, the most important parameters for the intensity of calcium carbonate leaching were altitude and an estimate of soil water (annual moisture surplus). As hypothesised, several soil forming factors mainly related to water availability are responsible for this process.