Topography and land use change effects on the soil organic carbon stock of forest soils in Mediterranean natural areas

Abstract

In Mediterranean areas, through the years, natural forest (NF) areas have suffered land use change (LUC) mainly for economic reasons. This is the case of olive groves (OG) in Andalusia (south Spain), the main olive oil production area in the world. This LUC from NF to OG has caused soil erosion and the loss of soil quality. Also, this effect has been aggravated by climatic conditions and poor soil management practices. A field study was conducted to examine the effects of LUC from NF to OG in a toposequence (summit-SM, backslope-BS and toeslope-TS) and to determine soil organic carbon (SOC), nitrogen (N) and the stratification ratio (SR) of SOC in the Torredelcampo–Jaen–Spain (Mediterranean area) in a Calcisols–Regosols–Vertisols sequence.Total SOC (T-SOC) was low for both NF and OG soils. In the case of NF, there was a pattern of decrease from the highest (SM) to the lowest topographical position (TS); whereas for OG, the highest T-SOC was found in TS (50.2±1.1gkg−1). T-SOC was more than 40% higher in NF than in OG in SM and BS. The total SOC stock (T-SOCS) in NF soils remained evenly distributed in the three topographical positions. However, the trend for the OG soils was an increase of T-SOCS along the hillside (SM: 68.9±4.8Mgha−1; BS: 82.3±12.3Mgha−1 and TS: 158.7±15.4Mgha−1). N followed a pattern of decrease with depth in the OG soils, but not in NF soils. As for N stock, it was higher in NF than in OG, with the exception of NF-TS and OG-TS. SR values in NF had a general trend of increasing from higher to lower topographical positions, despite the fact that T-SOC decreased from higher to lower topographical positions. This indicates that soil quality is higher in lower topographical positions even though there is less T-SOC. SR values in OG soils had a similar trend as in NF soils (but with lower values). However, T-SOC increased as well in the same way, which was opposite to the NF soils case. Therefore, soil quality was higher in lower topographical positions for both land uses.