Effect of biochar on the co-conversion of carbon-nitrogen and humification of kitchen waste composting process

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Composting was an effective technique to treat kitchen waste into high-quality fertilizer or soil amendment. During the composting process of kitchen waste, different degradation rates of carbon and nitrogen resulted in carbon loss, nitrogen emission and low product quality. Consequently, the aim of this study was to investigate the mechanisms and effects of biochar (5%, 10% and 15%) on synergistic conversion of nitrogen and carbon and humification processes in kitchen waste composting. The findings revealed that the addition of 10% and 15% biochar prolonged the thermophilic phase (>55°C) by a minimum of three days compared to CK. Additionally, the final NH4+-N concentration in the 10% and 15% treatment groups exhibited a 66.7% reduction in comparison to the control. Besides, the 15% addition proved more efficacious in reducing the NH4+-N content. The increase in biochar facilitates the adsorption of ammonia nitrogen and influences the structure of the microbial community, especially Bacteroides and Pseudoxanthomonas, thus promoting nitrogen mineralisation and humification, which provides energy and carbon for the survival and metabolism of more microorganisms. The compost with 15% biochar exhibited higher levels of SBR1031, resulting in a 73.8% increase in the conversion of nitrogen to NO3--N compared to the control. The aforementioned behaviour facilitates a synergistic conversion of nitrogen and carbon. Therefore, the addition of biochar promotes synergistic carbon and nitrogen transformations in terms of nitrogen conservation, organic matter decomposition, microbial species and their interactions, which is essential for the production of value-added compost fertilizers.