Short-term modifications of soil microbial community structure and soluble organic matter chemical composition following amendment with different solid olive mill waste and their derived composts

Production and disposal of olive-mill wastes represent a problem of environmental relevance particularly in the Mediterranean countries where they are generated in huge quantities. A field trial was carried out to evaluate the short-term effects of soil amendment with solid olive mill waste (OMW) and its compost in comparison to the olive mill waste derived from the new two-phase treatment (humid OMW) and its resulting compost. The impact of treatments on soil water extractable organic matter and soil microbiota was determined. During a 120 days-long field trial, it was observed an initial increase of water extractable organic carbon (WEOC), total reducing sugars and total phenol compounds for all the amended soils. Even if humid OMW had the highest WEOC and total phenol compounds concentration, it increased to a lesser extent their content in the soil, probably due to the rapid utilisation of this particular organic matter by microorganisms. After three days from spreading, all amendments induced rapid modifications of both fungal and bacterial communities. However, while the bacterial community restored its initial state at the end of the trial, suggesting their high resilience capacity, the fungal populations changes remained until 120 days. The overall results indicated that all types of the organic materials tested as soil amendments had an impact on the soil microbial communities, especially in the early days of the field trial, without negative effects on the soil chemical characteristics.