Land spreading of olive mill wastewater: Effects on soil microbial activity and potential phytotoxicity

Extremely high organic load and the toxic nature of olive mill wastewater (OMW) prevent their direct discharge into domestic wastewater treatment systems. In addition to the various treatment schemes designed for such wastewater, controlled land spreading of untreated OMW has been suggested as an alternative mean of disposal. A field study was conducted between October 2004 and September 2005 to assess possible effects of OMW on soil microbial activity and potential phytotoxicity. The experiment was carried out in an organic orchard located on a Vertisol-type soil (Jezre’el Valley, Israel) and included two application levels of OMW (36 and 72 m3 ha−1). Total microbial counts, and to less extent the hydrolytic activity and soil respiration were increased following the high OMW application level. A bench-scale lab experiment showed that the rate of OMW mineralization was mainly dependent on the general status of soil activity and was not related to previous acclimatization of the soil microflora to OMW. Soil phytotoxicity (% germination and root elongation) was assessed in soil extracts of samples collected before and after each OMW application, using germinating cress (Lepidium sativum L.) seeds. We found direct short-term effect of OMW application on soil phytotoxicity. However, the soil was partly or completely recovered between successive applications. No further phytotoxicity was observed in treated soils as compared with control soil, 3 months after OMW application. Such short-term phytotoxicity was not in correlation with measured EC and total polyphenols in the soil extracts. Overall, the results of this study further support a safe controlled OMW spreading on lands that are not associated with sensitive aquifers.