Pelletization Conditions Reduce Microbial Viability in Biochar-Based Biofertilizers

The conversion of biowaste into biofertilizer offers a sustainable alternative to synthetic fertilizers by supporting nutrient recycling and agricultural productivity. However, industrial pelletization can compromise the viability of microorganisms essential for biofertilizer function. In this study, a 40/60 (dry wt%) blend of biochar and commercial potting mix (biowaste blend) was used to produce a biochar biofertilizer (BCBF) through pelletization. Microbial population dynamics were then assessed at different stages of the BCBF pelletization process and under variations in key pelleting parameters—moisture content (15–35%), die surface temperature (70–180 °C), and feed rate (75–150 lb/h). The results showed that fungal and protozoan populations increased during the composting stage of BCBF, but declined to undetectable levels following drying and coating of the BCBF pellets. Bacterial populations increased after composting, but decreased substantially after pelleting and subsequent storage of the BCBF, while actinobacteria remained low throughout the pelletization process. Elevated temperatures and moisture loss were identified as major contributors to microbial inactivation during pelletization. These findings demonstrate that careful control of pelletization parameters is essential for maintaining microbial viability, thereby supporting the development of higher-quality, microbially active biochar-based biofertilizers.