Different Contribution of Olive Groves and Citrus Orchards to Soil Organic Carbon Sequestration: A Field Study in Four Sites in Crete, Greece

In this work, we evaluated the effects of cultivation practices and sites (representing four locations in Crete, Greece) on soil organic carbon sequestration in established citrus orchards, olive groves, and uncultivated fields (used as a control). Soil pH, soil texture, soil organic matter (SOM), Permanganate Oxidizable Carbon (POXC), Total Kjeldahl Nitrogen (TKN), Carbon and Nitrogen ratio (C:N), as well as soil CO2 respiration rates, and specific enzymes’ activity (i.e., N-Acetyl Glutamate (NAG), Beta Glucosidase (BG), Dehydrogenase) were determined in the upper soil layer (0–20 cm). It was shown that citrus and olive orchards under the South Mediterranean conditions could substantially increase C storage in the soil. However, soils planted with orange trees showed lower capacity than olive trees, which was related to litter chemistry (i.e., leaf C:N ratio). Sites had no significant impact on SOM. In our study, SOM had a positive relationship with TKN (and less with POXC) and the C:N ratio of the tree crop species litter. Our findings have implications for designing soil conservation practices in Mediterranean conditions and developing initiatives describing achievable targets of SOM restoration depending on soil properties and cropping systems.