Abstract
Olive mill waste (OMW), a byproduct from the extraction of olive oil, causes serious environmental problems for its disposal, and extensive efforts have been made to find cost-effective solutions for its management. Biochars produced from OMW were applied as soil amendment and found in many cases to successfully increase plant productivity and suppress diseases. This work aims to characterize biochars obtained by pyrolysis of OMW at 300 °C to 1000 °C using 13C NMR spectroscopy, LC-ESI-Q-TOF-MS and SEM (Scanning Electron Microscopy). Chemical characterization revealed that biochar composition varied according to the increase of pyrolysis temperature (PT). Thermal treated materials showed a progressive reduction of alkyl C fractions coupled to the enrichment in aromatic C products. In addition, numerous compounds present in the organic feedstock (fatty acids, phenolic compounds, triterpene acids) reduced (PT = 300 °C) or completely disappeared (PT ≥ 500 °C) in biochars as compared to untreated OMW. PT also affected surface morphology of biochars by increasing porosity and heterogeneity of pore size. The effects of biochars extracts on the growth of different organisms (two plants, one nematode and four fungal species) were also evaluated. When tested on different living organisms, biochars and OMW showed opposite effects. The root growth of Lepidium sativum and Brassica rapa, as well as the survival of the nematode Meloidogyne incognita, were inhibited by the untreated material or biochar produced at 300 °C, but toxicity decreased at higher PTs. Conversely, growth of Aspergillus, Fusarium, Rhizoctonia and Trichoderma fungi was stimulated by organic feedstock, while being inhibited by thermally treated biochars. Our findings showed a pattern of association between specific biochar chemical traits and its biological effects that, once mechanistically explained and tested in field conditions, may lead to effective applications in agriculture.
Highlights
Cultivation of olive and oil production are important agricultural activities in countries of the Mediterranean basin [1, 2], with Spain, Italy, Greece, Turkey and Tunisia representing the world biggest producers
The results found in our present study are consistent with the positive effects of biochar material to work as a soil amendment to improve plant growth and a stabilizer of agro-ecosystems [21], further experiments are necessary to confirm our results at field scale
This study provides evidence that pyrolysis induced radical changes in the chemical composition of solid Olive mill waste (OMW), and that the properties of the resulting biochars were influenced by thermal process
Summary
Cultivation of olive and oil production are important agricultural activities in countries of the Mediterranean basin [1, 2], with Spain, Italy, Greece, Turkey and Tunisia representing the world biggest producers. Oil extraction from olives generates large amounts of waste by-products that can adversely affect soil microbes, water resources and plants when disposed in the environment [3]. Since the early ‘90s, the modern two-phase technology has almost replaced the traditional pressing and the three-phase centrifugation system, reducing by 75% the olive mill wastes [3]. This system guaranteed less water consumption and generated a solid waste ( called “alperujo”, thereafter indicated as OMW), showing particular properties, such as high moisture and carbohydrate concentration [6]. Different methods have been proposed to valorize this solid OMW, including physicochemical treatments for second extraction of oil or energy recovery [6, 7], direct soil application as amendment [8], solid state fermentation for production of enzymes, livestock feed or fuel [9, 10], composting [11], or extraction of valuable compounds, such as pectins, polyphenols and fibers [12]
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