Short-term effects in soil microbial community following agronomic application of olive mill wastewaters in a field of olive trees

Olive mill wastewater (OMW) creates a disposal problem. The large amounts generated, combined with the high phenol and chemical oxygen demand concentrations, are the main difficulties in finding a solution for the management of these wastewaters. We investigated the short-term effect of spreading OMW on the soil surface of an olive grove on the soil microbial communities. Analyse of ester-linked fatty acid methyl ester (EL-FAME) were used to assess variation in soil microbial community structure after agronomic application of OMW. EL-FAME analysis showed significant shifts of specific groups of fatty acids 30 days after application of OMW to a field of olive trees at rates of 0 (control soil), 30, 60, 100, and 150 m3 ha−1 of OMW. In particular, the branched saturated fatty acids indicative of Gram-positive bacteria decreased and the unsaturated fatty acids commonly found in Gram-negative bacteria and fungi increased. The fungal/bacterial ratio measured increased significantly with increasing OMW. Lower cy19/18:1ω7c and cy17/16:1ω7c ratios were found in the amended soil than the control soil, and we interpret that as an indication that nutrient availability may be more limiting in the control soil. Similarly, the relative abundances of monounsaturated fatty acids increased with added OMW, and this is consistent with the presence of high substrate availability in OMW-treated soil. Principal Components Analysis of the FAME profiles showed discrimination between the control soil and OMW amended soil. Differences in fatty acid profiles between OMW-treated soil and control soil suggests that amendment of soil with OMW favors specific groups of organisms. To our knowledge, this is the first report of alterations in the FAME profile in soils due to agronomic application of OMW.