Soil microbial characteristics and mineral nitrogen availability as affected by olive oil waste water applied to cultivated soil

Waste disposal is an urgent problem in those Italian areas devoted to olive oil production where a large amount of olive oil waste is produced in a short period of time. Given the high organic matter and macronutrient content of olive oil waste water, its use in agriculture could help to solve the disposal problem and, at the same time, to improve the physiochemical characteristics of intensely cultivated soils. However, olive oil waste water contains noteworthy quantities of phenols, which are toxic and have bacteriostatic activity. The effects of repeated spreading of olive oil waste water on alfalfa, on soil microbial properties, and on the availability of mineral nitrogen were determined. Doses of 0, 80, and 160 m3 ha(-1) of olive oil waste water (OW) were spread over experimental plots sown with 3rd year alfalfa. Soil samples were collected at various intervals after OW application. The biomass, respiratory activity, metabolic quotient, mineralization index, and the potential nitrification activity (PNA) of the soil were measured. Polyphenol, organic carbon, and mineral nitrogen contents were measured on the same soil samples. In general, the spreading of OW caused slightly positive modifications to the soil microbial biomass and metabolic quotient. These modifications were temporary and disappeared as early as a few months after the treatment. The PNA, as well as the nitrate and nitrite contents, were lower in the treated plots during the vegetative cycle of alfalfa and after the last harvest, and appeared to be negatively correlated with polyphenols. The results of this experiment confirmed that the spreading of olive oil waste water on cultivated soil could be an eco-compatible practice. Despite their origin, the OW had no toxic effects on the soil microflora, and in fact, stimulated growth and reproduction of cells. Only the PNA proved to be slightly reduced, but the yield was not negatively affected.