Agronomical and Microbiological Effects of Olive Mill Waste Compost Used as a Potting Substrate for Ornamental Plants.

In the frame of the SAN-SOIL project, a high-quality compost obtained from olive mill waste (OMW) using a static procedure was tested as a peat surrogate for nursery cultivation of potted plants. A farm-scale trial was implemented using three ornamental tree species, namely Viburnum lucidum, Prunus laurocerasus and Cupressus sempervirens, and substituting peat with 33%, 66% and 100% of OMW compost with or without chemical fertilization. The potting substrates containing the OMW compost showed higher pH and electrical conductivity than those containing only peat. Interestingly, after 6 months of cultivation, the growth (assessed as fresh weight) of plants potted with 33% and 66% of OMW compost, especially in the presence of chemical fertilization, was much higher than those potted in the traditional peat-based substrate. The effects of OMW compost on the structure and abundance of microbial communities of both the potting substrates and the plant roots were assessed using culture-independent molecular methods such as denaturing gradient gel electrophoresis (DGGE) profilingand qPCR analysis of ribosomal (rRNA) genes. While qPCR analysis indicated that replacing peat with OMW compost did not inhibit bacterial growth, DGGE profiling clearly showed that these substrates profoundly modified bacterial diversity. Taken together these results clearly indicate that OMW compost can represent a valuable alternative to peat for nursery cultivation of potted ornamental trees and suggest that the modulation of soil and endophytic microbial communities may mediate the observed beneficial effects on plant growth.