Impact of Biochar and Compost Amendments on Rhizosphere Microbial Diversity: A Metagenomic Perspective

In recent years, the urgent need to recover urban and agricultural soil in accordance with sustainability principles and the circular economy has gained increasing attention. Biochar and compost, applied as soil amendments, have emerged as effective strategies to support this transition. The experimental site is an olive orchard located in the municipality of Fisciano, in South Italy, next to the University of Salerno. Olive trees were estimated to be 50±3 years old. This study aimed to investigate the effects of biochar and/or compost application in the olive orchard. Olive trees were selected with a minimum spacing of 6m between each other. A randomized block experiment was set up with five treatments and five replicates. The treated area around each tree was delimited by two boundaries, 0.5m and 1.5m from the trunk. The treatments included: biochar (B), compost (C), biochar+compost (B+C), commercial mineral fertilizers (M), and a control (CNT). Biochar was applied at the beginning of the study, while compost and mineral fertilizers were applied annually. At the end of the experimentation, fine roots were collected, the DNAs were extracted from their rizosphere, and v3-v4 regions of the 16SrRNA gene amplified and sequenced trough Next-Generation-Sequencing. The metagenomic results revealed that the most represented phylum, for all the experimental group, were Acidobacteria, Alphaproteobacteria and Gammaproteobacteria. Furthermore, some bacterial classes, as DA052, Coriobacteriia, MJK10, OPB56 and GKS2-174 were specific to CNT, M, B, C and B+C, respectively. However, the alpha diversity values were higher in the CNT compared to treatments with mineral fertilizers (p<0.05), B, C, and B+C (p<0.05). In summary, for this soil, the application of organic amendments and commercial mineral fertilizers did not provide any advantage in terms of species richness for rhizosphere microorganisms, but the different soil improvers affected the presence and relative frequencies of the less represented OTUs.