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.
Highlights
Research on soil organic carbon (SOC) sequestration and the potential of agricultural soils to store carbon has increased since the declaration of the 4 per 1000 program (Lal, 2015), which seeks to increase global soil organic carbon stocks by 0.4 % per year as compensation for global anthropogenic C emissions
The moderate differences in Corg and the homogeneity in δ15N and δ13C isotopic signals between the eroding and deposition areas may be due to several processes, some of which were discussed above, such as spatial variability of carbon input due to biomass in the plot, surface soil operations in the orchard, and fertilization. We found it both interesting and worth exploring in future studies that significant correlations between erosion and deposition rates and Corg, δ15N, and δ 13C-related variables were found for samples from the 10–20 and 20–30 cm layers, indicating that short-term disturbance by surface processes can mask experimental determination of the impact of erosion deposition processes in this olive orchard for these variables
The results indicate that erosion and deposition within the investigated old olive orchard have created a significant difference in soil properties along the catena, which is translated into different soil Corg, Pavail, and Norg content, δ15N, and contrasting soil quality status
Summary
Research on soil organic carbon (SOC) sequestration and the potential of agricultural soils to store carbon has increased since the declaration of the 4 per 1000 program (Lal, 2015), which seeks to increase global soil organic carbon stocks by 0.4 % per year as compensation for global anthropogenic C emissions. Under this program, special emphasis is given to combating soil degradation due to its strong impact on the global carbon cycle because of the depletion of SOC stock. The effects of soil erosion and the fate of the specific SOC fraction transported by erosion in specific agricultural systems such as olive groves remain poorly understood; agroenvironmental impacts of SOC dynamics and variability require more site- and crop-specific research
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