ABSTRACT
The olive oil extraction system generates a residue known as dry olive residue (DOR) in huge amounts. Among the strategies suggested for its revalorization is its use as a soil organic amendment, since it contains a high content in organic matter as well as in important nutrient cations. This strategy can be especially interesting for soils from the Mediterranean region, where many of them are suffering from important degradation problems. Likewise, the use of DOR as an organic amendment has been proposed as a useful approach for the recovery of soils contaminated with heavy metals. However, DOR needs a pre-treatment before its soil application, since it presents microtoxic and phytotoxic activities, which have mainly been related to its phenolic fraction. The incubation of DOR with soil saprophytic or white-rot fungi has been demonstrated to be an appropriate method for the generation of a transformed product with the suitable properties to be used as a soil organic amendment. In this way, after fungal biotransformation, a new product is generated with a high content in stabilized organic matter as well as in nutrients and without phytotoxic effects due to the removal of the phenolic fraction. Likewise, biotransformed DOR has been demonstrated to have good sorption characteristics for heavy metals. The soil application of biotransformed DOR has a beneficial effect on plant growth and determines the enrichment of soil with fresh organic matter. This enrichment in easily decomposable C sources results in increased microbial abundances (bacteria and fungi) as well as moderate shifts in the structure of both bacterial and fungal communities in concomitance with changes in soil microbial functionality (e.g. enhanced potential microbial enzyme activities). Soil biotransformed DOR application has also been demonstrated to be an effective strategy for the recovery of soil microbial functionality of multi-metal polluted soils.
