Variation of soil organic carbon, stable isotopes, and soil quality indicators across an erosion–deposition catena in a historical Spanish olive orchard

Abstract

Abstract. This study compares the distribution of bulk soil organic carbon (SOC), its fractions (unprotected and physically, chemically, and biochemically protected), available phosphorus (Pavail), organic nitrogen (Norg), and stable isotope (δ15N and δ13C) signatures at four soil depths (0–10, 10–20, 20–30, and 30–40 cm) between a nearby open forest reference area and a historical olive orchard (established in 1856) located in southern Spain. In addition, these soil properties, as well as water stable aggregates (Wsagg), were contrasted at eroding and deposition areas within the olive orchard, previously determined using 137Cs. SOC stock in the olive orchard (about 40 t C ha−1) was only 25 % of that in the forested area (about 160 t C ha−1) in the upper 40 cm of soil, and the reduction was especially severe in the unprotected organic carbon. The reference and the orchard soils also showed significant differences in the δ13C and δ15N signals, likely due to the different vegetation composition and N dynamics in both areas. Soil properties along a catena, from erosion to deposition areas within the old olive orchard, showed large differences. Soil Corg, Pavail and Norg content, and δ15N at the deposition were significantly higher than those of the erosion area, defining two distinct areas with a different soil quality status. These overall results indicate that the proper understanding of Corg content and soil quality in olive orchards requires the consideration of the spatial variability induced by erosion–deposition processes for a convenient appraisal at the farm scale.