Abstract
The long-term effects of the olive mill wastewater (OMWW) spreading on soil chemical properties, microbial community, and olive tree parameters have been far poorly investigated. Therefore, this study aims to evaluate the effect of OMWW application on an olive orchard, and samples were collected at two different depths (0–20 cm and 20–40 cm) and 14 days, one year and two years from the end of the OMWW spreading on soil chemical characteristics and soil microbial structures. Variations of soil chemical parameters (pH, salinity, available P, and water-extractable organic C) were observed particularly at 14 days after spreading at both depths. All these parameters reached similar values to the soil after two years, except for available phosphorus. Firmicutes, Proteobacteria, and Actinobacteria are the most abundant phyla: only Firmicutes were negatively affected by the OMWW spreading after 14 days, suggesting that Gram-positive bacteria were probably negatively influenced by the addition of OMWW. The abundance of bacterial taxa in the soil is restored along time, except for the decrease of Firmicutes. This evidence suggests that the OMWW spreading in the long term does not affect the endemic soil bacterial community of the olive grove, as well as leaf net photosynthesis, the olive tree vegetative activity, yield, and fruits characteristics.
Highlights
The olive oil extraction requires a considerable amount of water and produces vast quantities of oil mill wastes (OMW) in a limited period with a significant environmental impact [1,2,3,4,5]
The results of soil chemical characteristics of unamended control, 14 days after spreading, one year and two years from the end of the olive mill wastewater (OMWW) spreading are reported in Tables 2 and 3 for 0–20 and 20–40 cm, respectively
This work has demonstrated that the application of OMWW affected in the first two weeks the pH, salinity, available P, water-extractable organic C (WEOC) parameters at both depths
Summary
The olive oil extraction requires a considerable amount of water and produces vast quantities of oil mill wastes (OMW) in a limited period with a significant environmental impact [1,2,3,4,5]. The type of OMW produced depends on the extraction system adopted. The OMW from the three-phase centrifugation is solid pomace and olive mill wastewater (OMWW). Only a wet pomace is generated from the two-phase centrifugation oil extraction system [6,7,8]. The chemical-physical characteristics of the OMWW depend on the climatic conditions of the cultivation area, the cult, the state of ripeness of the olives, and especially the type of processing adopted [9,10,11,12]. Since OMWW derives exclusively from the mechanical extraction process, no synthetic chemicals, additives, pathogenic microorganisms, and viruses are contained or could occur for the urban sewage [18]
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