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Maintaining adequate levels of soil organic matter in Mediterranean agro-ecosystems is a pressing need due to the increasing evidence of climate change. The use of by-products of the olive oil industry as organic amendments could contribute to this goal. We report the results of a 2-year research carried out in southern Italy on a clay loam soil for evaluating the effects of different olive oil industry by-products on soil organic carbon and other related soil characteristics. The treatments were: (i) Olive mill wastewater (OMW), (ii) compost from olive pomace (CP1), (iii) compost from olive pomace in double quantity (CP2), and (iv) organo-mineral fertilizer (OMF). Soil samples, collected at a depth of 0–20 cm, were analyzed for total organic carbon (TOC), its extractable (TEC) and humic fractions (HC), and aggregate stability (Ist). In addition, soil macroporosity, water retention, and penetration resistance (PR) were evaluated. CP1 induced the largest increase in soil TOC, TEC, and HC content, and a significant improvement in Ist; the addition of a large quantity of organic carbon (CP2) did not determine a proportional increase in soil organic matter content. The aggregate stability of the CP2 was the lowest; nevertheless, the characterization of macroporosity indicated an improvement of soil structure functionality. With respect to control (OMF), OMW had a significant decrease in Ist and an increase in PR of the uppermost soil layer.
Vignozzi et al. (Fri,) studied this question.