Effect of different types of additives on carbon and nitrogen loss and humification during Chinese medicine residue - cow manure composting

Residues generated during traditional Chinese medicine (TCM) production are rich in organic matter and trace elements, serving as valuable biomass raw material for organic fertilizer production with broad prospects for resource utilization. To achieve waste valorization and leverage the complementary decomposition characteristics of different organic materials, this study prepared organic fertilizer using TCM residues. The residues were thoroughly mixed with cow manure at a 3:10 wet weight ratio as the base composting material. Three commonly used additives — biochar (produced at different temperatures, 7% dry weight), clay minerals (montmorillonite, 7% dry weight), and a microbial inoculant (Aspergillus oryzae) — were incorporated. A 35-day aerobic composting experiment was conducted in a 50 L closed reactor to investigate the effects of different additive types on nutrient transformation during organic fertilizer preparation, aiming to provide technical support for producing efficient bio-organic fertilizers. Nine treatments were established: no additive (CK); biochar produced at 300 °C (300C); biochar produced at 500 °C (500C); biochar produced at 700 °C (700C); montmorillonite (M); A. oryzae (W); biochar produced at 500 °C + montmorillonite (500CM); biochar produced at 500 °C + A. oryzae (500CW); biochar produced at 500 °C + montmorillonite + A. oryzae (500CMW). The results demonstrated that adding biochar, montmorillonite, and A. oryzae, either individually or in combined, facilitated pile temperature increase and accelerated the composting process. Compared to CK treatment, the treatments of 300C, 500C, 700C, M, 500CM and 500CMW increased humus content and reduced carbon and nitrogen losses. Specifically, carbon losses were reduced by 19.52%, 20.07%, 18.55%, 7.06%, 27.12%, and 9.45%, respectively. N losses were reduced by 61.76%, 50.79%, 39.22%, 35.16%, 63.16%, and 20.31%, respectively. Among these, the W treatment yielded the highest increase in humus content (27.00% higher than CK). The additives biochar, montmorillonite, and A. oryzae promoted compost maturation and humification. Biochar addition enhanced the organic matter content of the fertilizer. Combining biochar with montmorillonite was particularly effective in reducing C and N losses, with synergistic effects observed. Conversely, adding A. oryzae was more effective in accelerating the humification of organic materials. Thus, biochar and montmorillonite conserved nutrients by minimizing C and N losses, while A. oryzae enabled rapid and efficient organic fertilizer preparation by expediting humification. Under the experimental conditions of this study, the optimal additive combination was the co-application of 500 °C biochar and A. oryzae (500CW), which enhanced the organic matter and nutrient content, increased the degree of humification, and produced a high-quality organic fertilizer.