Olive mill wastewater affects the structure of soil bacterial communities

Abstract

Land spreading of olive mill wastewater (OMW) has been proposed as a useful means for its disposal. However, there is no information regarding the impact of OMW soil applications on the structure of bacterial communities. Two soils, a loamy sand (LS) and a sandy loam (SL), were irrigated weekly with aqueous solutions of 2 and 4% of OMW for a period of 49 days. Fluctuations in key soil physical, chemical and biological properties were measured during the irrigation period. Structural changes in functional bacterial guilds such as actinobacteria and ammonia-oxidizing bacteria (AOB) were determined by denaturating gradient gel electrophoresis (DGGE) and cloning and sequencing. OMW application caused a drastic reduction in mineral-N in both soils indicating net N immobilization. An increase in soil electrical conductivity and a late peak in soil respiration and phenolics were observed only in the LS soil treated with 4% OMW. DGGE analysis showed a clear effect of OMW on the AOB communities in both soils, whereas OMW induced consistent changes in the structure of actinobacteria community in the LS soil only. The alterations in the bacterial communities induced by OMW application are potentially the result of environmental changes including lowered oxidative conditions, strong competition for mineral-N and availability of phenolics, whereas altered C-sources and selective inhibition of other microbial groups by available phenolics might have been particularly important for the actinobacteria. Cloning of the AOB community showed that all clone sequences representing dominant bands in the DGGE profiles belonged to cluster 3 of Nitrosospira sp. Members of the community were favored in OMW-treated soils, while others were sensitive to OMW. Overall, OMW soil addition induces changes in soil physical and chemical properties and consequent alterations on the structure of the bacterial communities which were found to be soil-dependent. However these changes do not appear to relate to a general toxicity effect.