Storage of Alperujo influences composting performance: Insights into gaseous emissions and functional metagenomics

Alperujo (AL), the primary by-product of olive oil extraction, poses a significant environmental challenge in the Mediterranean region. Understanding the AL composting process is essential for controlled aerobic revalorisation to obtain a stable and good quality organic amendment with the minimum environmental impact. Our approach assumes that full-scale pond storage duration modifies the degree of hydrolytic fermentation of AL, affecting the subsequent stages of the composting process. In this work, AL raw materials for composting were stored for 3 or 6 months. Subsequently, during pilot-scale composting, the loss of organic matter (OM) not only induced key changes in the solid mass but also in gaseous emissions, which decreased along with the storage time. Consequently, the initial C/N ratio decreased from 25.76 to 22.24, and composting yields relative to the AL initially mixed with a bulking agent (3/1, wt./wt.) were 76.4 % and 41.7 %, respectively. Phenolic compounds were effectively degraded throughout the composting process under both initial conditions, enhancing the potential value of the final products. Metagenomic analysis revealed differences in the raw material bacteriome, variations that also became evident throughout composting. The thermophilic stage fostered the selection of a range of thermotolerant microorganisms, many of them with lignocellulosic activity, which is essential for the decomposition of OM. Then, at the final mesophilic phase, a significant increase in bacterial diversity and metabolic activity was observed. This study contributes to better understand the functional role of the microbiome in AL composting, particularly regarding the bacterial community dynamics and gaseous emissions to the atmosphere.