Repeated application of diluted olive mill wastewater induces changes in the structure of the soil microbial community

Repeated application of diluted olive mill wastewater (OMW) is a common disposal method which allows the application of large amounts of OMW and improves the organic matter and nutrient status of Mediterranean soils. However, there is lack of information regarding the effects of this practice on the soil microbial community. A study was carried out to investigate these effects on the structure of the bacterial and fungal community of a loamy sand (LS) and a sandy loam (SL) soil, using denaturating gradient gel electrophoresis (DGGE) fingerprinting. OMW was daily applied as aqueous solutions of 0, 2 and 4%, in the presence or absence of nitrogen fertilization, for a three-month period. Multivariate analysis of the DGGE profiles showed that OMW applications resulted in marked changes in the fungal community in both soils, while nitrogen fertilization diminished these effects. Small effects were evident for the bacteria only in the LS soil and this was attributed to the higher availability of OMW-derived phenolics in this soil which resulted in a direct impact on bacteria. Nitrogen fertilization alleviated the effects of OMW on the bacterial community. We suggest that the impact of OMW on the structure of the soil microbial communities was mostly a result of its indirect effect on the soil nutritional status, which becomes enriched in organic substrates and poor in available nitrogen.