Soil organic matter of Iberian open woodland rangelands as influenced by vegetation cover and land management

Spanish rangelands occupy more than 90,000 km2 of land grazed by millions of domestic and wild animals. Organic matter content of soil is an essential component for fertility and productivity in both, natural and human-induced ecosystems. Despite its importance to soil quality, soil organic carbon has been little studied in these rangelands. The main goal of this study is to assess the amounts and characteristics of soil organic matter in grazed open woodlands of SW Iberia as influenced by vegetation cover and land management. Soil properties were analysed from 22 environmental units selected from 10 farms distributed throughout the region of Extremadura being representative of different rangeland types: dehesas (wooded rangelands), scrublands of Retama sphaerocarpa and treeless grasslands. Some of the soil samples were used for a chemical fractioning of soil carbon. Farmer interviews were conducted in order to quantify livestock densities and stocking rates and to assess land management. Results showed that soils are generally shallow, acidic, with low soil organic carbon contents and poor in nutrients. Areas with higher soil carbon stocks corresponded to units encroached by Retama sphaerocarpa shrubs and dehesas with 49.2 T · ha− 1 and 46.0 T · ha− 1, respectively, particularly those developed on schist. Soil organic carbon concentrated mainly in the top 5 cm soil layer, displaying large variability (mean: 2.30%). SOC was relatively high beneath the tree canopy (mean: 3.35%) and less concentrated and reduced in variability with depth. Regardless of the variation of total amount of carbon among cover types, the proportion of each carbon fraction from the total carbon was kept relatively constant whatever the cover type. A multivariate analysis performed and other results suggest that land mismanagement and excessive stocking rates could be related with the appearance of bare soil areas, therefore negatively affecting soil organic matter by removing the sensitive topsoil materials through water erosion.