Long-term application of olive-mill wastewater affects soil chemical and microbial properties

Abstract

Disposal of untreated olive-mill wastewater (OMW) is a major environmental problem in many Mediterranean countries. This study assessed the impact of OMW application on soil microbiological properties and explored the relationship to soil chemical properties during a 9-month, periodical soil-sampling campaign in a pilot study area in Crete, South Greece. Cases studied involved: direct application of OMW on soil; OMW disposal in active evaporation ponds; sites hosting evaporation ponds that have been inactive for the past 9 years; sites downstream of active evaporation ponds; and control soils, upstream of the waste-disposal ponds. Long-term OMW disposal on land affected the main soil chemical properties. Applicability of the results from the systematic monitoring was confirmed by results obtained in other OMW disposal sites around the pilot area. Soil microbial properties (microbial activity, microbial biomass carbon, and metabolic quotient) were considerably affected by OMW disposal. Moreover, seasonal changes of soil properties revealed short- and long-term residual effects due to OMW disposal. Significant correlations were observed among soil microbial characteristics and soil chemical properties, clearly indicating a close relationship between chemical properties and the transformation of microbial communities in soil after OMW land spreading. The determination of a key set of chemical and microbiological parameters that can be used as indicators for monitoring soil quality at olive-mill waste-disposal areas will verify the efficiency of the techniques used for the land disposal of OMW and will consequently promote their sustainable management.