Sustainable production of mature and stable amendments through biochar-enhanced vermicomposting of cocoa pod husks

A major waste resulting from cocoa production is cocoa pod husk (CPH), which is often piled up on-farm and left to rot. This study aims to valorise CPH through a vermicomposting process to produce stable and mature amendments. Fresh CPH were mixed with cattle manure (ratio 1:1), to this mixture 4% and 8% CPH biochar were added. The earthworm species, Eudrilus eugeniae, which is native to the Ghana region and is readily accessible to local farmers, was used for the vermicomposting over a 60-day duration. Chemical parameters (pH, electrical conductivity, organic C, total N, P, K, Ca, Mg) and microbial characteristics (culturable bacteria and fungi and DNA sequencing) were monitored during the vermicomposting. The pH, total N and K of the different matrices raised over time and with increasing amounts of biochar. In all treatments, bacterial counts decreased in the first 10 days (between 1.79 and 2.16 times) and then stabilised throughout the process. Biochar inhibited the number of fungi in the first 40 days, but subsequently stimulated their growth. The biochar addition had a significant impact on the dynamics of the bacterial communities, although Streptomyces, Pelagibacterium, Rhabdothermus, Lysinibacillus, Micromonospora, and Mesorhizobium were the dominant genera in all samples. All treatments promoted an increase in microbial taxa involved in carbon and nitrogen cycle. The results highlighted that CPH can be valorised through the production of vermichar, although scalability and efficiency assessments need to be performed, to ensure the applicability of this bioprocess on large scale in cocoa-growing regions.