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

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.