Effects of olive mill wastewater spreading on the physico-chemical and microbiological characteristics of soil

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Abstract

Olive mill wastewater (OMW) is the liquid by-product obtained from olive processing to extract virgin olive oil by mechanical means. The acid pH and the high amounts of organic matter and phenols make it very difficult to further purify this material. The question is then what to do with it? A solution would be to spread it on the soil. The objective of this study was to explore the effects of different OMW amounts on the chemical characteristics and on microbial community of the soil cultivated with olive trees (Olea europaea L.). The increase of the organic matter and phenols, due to the treatment with OMW, was found in the soil as well as the microbial biomass, as estimated by direct cell counting and by measuring soil-induced respiration. It was observed that the OMW spreading in high amounts increased the soil-denitrifying community while nitrifying populations decreased slightly, probably owing to the reductive effect of phenols. In the present paper we investigated the effects of OMW spreading on the chemical characteristics and on different groups of bacteria and fungi within microbial community, in soil cultivated with olive trees, at different depth of 10–20 cm and 20–40 cm.

Introduction

The olive fruit processing by mechanical means allows to obtain virgin olive oil but also by-products such as olive pomace and olive mill wastewater (OMW). The pomace is evaluated economically because it is generally destined to the industry for extraction of pomace oil by solvent, while the elimination of OMW represents one of the main environmental problems related to the olive oil industry of Mediterranean countries for its chemical and physical characteristics. The olive mill wastewater composition is highly variable and depends, in particular, by variety, ripeness and type of the oil extraction technology (traditional and continuous systems). Generally, OMW has high values of biological oxygen demand (BOD) and chemical oxygen demand (COD), high contents of organic matter, suspended solids (Andrich et al., 1986, Di Giovacchino et al., 1988, Proietti et al., 1995), inhibiting substances (lipidic and phenolic compounds), and a medium–high content of the mineral fraction, in particular of potassium, phosphorus and calcium. Many studies have been published on the opportunity to utilize OMW in the Mediterranean countries, both by spreading it on agricultural land and by its depuration with different techniques (Montedoro et al., 1988, Amirante and Di Renzo, 1990, Brunetti et al., 1991, Petruccioli et al., 1991), but the results obtained were not easily practicable. Other authors pointed out the beneficial effects of the spreading of fresh, stored or treated OMW on the cultivated soil obtaining good results thanks to the fertilizing action of OMW on crops grown (Albi Romeo and Fiestas Ros De Ursinos, 1960, Vitagliano et al., 1975, Morisot, 1979, Janer Del Valle, 1980). In particular recent papers have been published about the effects of spreading OMW on the soil cultivated with cereals and other annual crops (Di Giovacchino and Seghetti, 1990, Bonari and Ceccarini, 1991, Galoppini et al., 1992, Bonari et al., 1993, Caporali et al., 1996, Di Giovacchino et al., 1996), grapevine (Catalano and De Felice, 1989, Di Giovacchino et al., 1996, Di Giovacchino et al., 2001, Rinaldi et al., 2002, Marsilio et al., 2006) and with olive trees (Catalano et al., 1985, Lombardo et al., 1988, Lombardo et al., 1993, Proietti et al., 1988, Briccoli Bati et al., 1990, Briccoli Bati et al., 2002, Marsilio et al., 1990, Rinaldi et al., 2002), evidencing the positive effects of utilization of OMW. Besides, it was observed an olive production (kg/tree) and trunk circumference (cm) increasing (Marsilio et al., 2006). Furthermore, the chemical characteristics of the soil treated with different amounts of OMW (Della Monica, 1979, Potenz et al., 1985, Marsilio et al., 1991, Senette et al., 1991, Levi-Minzi et al., 1992, Saviozzi et al., 1993, Proietti et al., 1995), the humification index (Alianello et al., 1998), the structure of the soil (Favi et al., 1993), the biology and photosynthesis of the plants (Paliotti and Proietti, 1992, Briccoli Bati et al., 2002) were also studied. The results indicated that the controlled spreading of OMW increased the fertility of the soil, offering the opportunity to recycle the various compounds. Because of the high amounts of organic matter and macronutrients (especially potassium), OMW could be considered as a useful, low cost amendment and fertilizer (Proietti et al., 1988, Bonari et al., 1993). There are few studies on impact of OMW on soil microbial composition (Lombardo et al., 1988, Briccoli Bati et al., 1990, Marsilio et al., 1990, Cicolani et al., 1992, Picci and Pera, 1993). Soils are estimated to harbor up to 1010 bacteria of about 104 different ribotypes per g of which more than 95% cannot be cultured by present methods. However, knowledge about microbial community structure and its variants is very important to predict nutrient fluxes and to analyze the effects of xenobiotica.

In the present paper we investigated the effects of OMW spreading on the chemical characteristics and on different groups of bacteria and fungi within microbial community, in soil cultivated with olive trees, at different depth of 10–20 cm and 20–40 cm. The possibility to reduce the use of chemical fertilizers was also considered.

Section snippets

Experimental plot

The experiment was carried out in 2006 in a 16 years old olive orchard, planted in Rossano (CS), Italy, co-ordinate; North latitude 39°16′21.43″ and East longitude 16°38′32.70″. The orchard is cultivated with two different varieties, Carolea (most vigorous plants) and Nocellara del Belice (little vigorous plants) and the trees were trained using the free vase system with a planting density of 6 × 7 m. The planted not were pruned, fertilized and irrigated in the years 2005–2006.

The OMW utilised in

Results and discussion

The experimental period was characterized by scarce or no rainfall in the spring. The daily minimum air temperature generally increased from 9 to 11 °C in April to 22 °C at the end of June and the maximum air temperature from 19 °C in April to 35 °C at the end of June (Fig. 1).

The freshly collected OMW was dark-red brown in colour, had a cloudy appearance and a slight smell of olive oil. The composition of olive mill wastewater (OMW) utilised for the treatments is reported in Table 1.

In Table 2 the

Acknowledgements

This research was supported by grants from the Italian Ministry of Agriculture and Forestry – Project “RIOM- WP5”. The authors wish to thank ARSSA – Servizio Agrometeorologia “Villa Margherita”, Cutro (KR), for providing meteorological informations.

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      Increased soil microbial community after OMW application has also been reported by Di Serio et al. (2008), Magdich et al. (2013), and Ayoub et al. (2014), which was also proportional to the wastewater dose as reported by Magdich et al. (2013). Some studies have also reported an increase in respiration activity after OMW application, attributed to the organic matter contained in the wastewater (Kotsou et al., 2004; Di Serio et al., 2008). However, dose-dependent toxicity results are also ascribed to decrease of biomass (Bargougui et al., 2019).

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