Change in soil properties and the soil microbial community following land spreading of olive mill wastewater affects olive trees key physiological parameters and the abundance of arbuscular mycorrhizal fungi

Olive mill wastewater (OMW) constitutes a major environmental problem for Mediterranean countries, where most of the world olive oil production takes place. The recycling of the OMW and its use as water for irrigation in agriculture, provided that its impact on soil and plant is established, is an attractive possibility for the Mediterranean countries. Investigations were performed on the influence of agronomic application of OMW (amount applied: 30, 60, 100 and 150 m3 ha-1) in a field of olive trees on trees characters (photosynthesis, root-soluble carbohydrate and root colonisation), soil properties, and soil microbial community structure. Specific attention was paid to arbuscular mycorrhizal (AM) fungi. The soil fatty acid methyl ester (FAME) 16:1ω5 was used to quantify biomass of AM fungi and the root FAME 16:1ω5 analysis was used as index for the development of colonisation in the olive trees roots. A significant increase in organic C, C/N ratio, extractable phosphorus and exchangeable potassium was found after one year of agronomic application of OMW. The development of saprophytic fungi was significantly higher in the OMW amended soils, whereas the abundance of the soil FAME 16:1ω5, root FAME 16:1ω5, photosynthetic rates and the amount of the total root-soluble carbohydrate were decreased significantly after agronomic application of OMW. A principal component analysis (PCA) of the trees characteristics profiles showed discrimination between the nonirrigated and the OMW irrigated olive trees. These findings suggest that the altering functioning of arbuscular mycorrhizas should be considered as potential factors mediating olive trees responses to agronomic application of OMW when the OMW dose applied is higher than 30 m3 ha-1. To our knowledge, this is the first report of alterations in the soil FAME 16:1ω5 and root FAME 16:1ω5 due to land spreading of OMW.