Accelerating carbon conversion in garden waste composting with food waste-expanding microbial inoculants

The expansion of urbanization has resulted in the generation of a large amount of garden waste (40 million tons per year in China), while traditional treatment methods (incineration and landfill) tend to cause serious environmental pollution and waste of resources. Composting is an effective way to realize resource utilization of garden waste. However, the high lignocellulose content of garden waste limits its resource utilization. Accelerating the degradation of lignocellulose in the composting process is of great significance for achieving effective resource utilization of garden waste. Inoculation with exogenous microorganisms is considered an environmentally friendly and cost-effective method to accelerate lignocellulose degradation, which would further reduce the cost of inoculum production and improve inoculation efficiency. In this study, food residues (apple pomace and bean dregs) were used instead of conventional carbon and nitrogen sources (glucose and peptone) to propagate lignocellulose-degrading fungi. The number of viable fungi in the multiplication product reached 3.7×10¹⁰ cfu∙mL⁻¹, which increased by 46.2% compared with the traditional industrial medium. The effects of different inoculum amounts (0, 2%, 4%, and 8%, dry weight) on carbon conversion during garden waste composting were also discussed. The inoculation treatments significantly increased lignocellulose degradation (P<0.05), according to the results. The total lignocellulose degradation rates of the 2%, 4%, and 8% inoculation treatments (2%IM, 4%IM, and 8%IM) increased by 6.3%, 9.2%, and 23.0%, respectively, compared with CK. Dynamic changes in humus precursors (reducing sugars and polyphenols) and humus components were further analyzed. The 8%IM treatment accelerated the complete mineralization of carbon, resulting in the complete degradation of the humus precursors (polyphenols and reducing sugars) into CO₂, which inhibited humification. Compared with CK, 2%IM, and 4%IM, the cumulative CO₂ emissions of 8%IM increased by 21.9%, 22.3%, and 26.0%, respectively. The 4%IM treatment accelerated lignocellulose degradation while promoting the synthesis of humic acid (HA). The final HA content reached 91.3 g∙kg⁻¹, which was 24.9%, 10.7%, and 35.8% higher than that of CK, 2%IM, and 8%IM treatments, respectively. These results indicate that appropriate inoculation is beneficial to the directional transformation of lignocellulose to humic acid, whereas excessive inoculation would lead to an excessive loss of organic matter due to the high metabolic activity of microorganisms; and the degradation efficiency of lignocellulose is lower when inoculated with a small amount, which was further confirmed by the partial least squares path analysis model in this study. The conversion of lignocellulose to dissolved organic carbon increased with increasing inoculation amount (correlation coefficients of CK, 2%IM, 4%IM, and 8%IM were 0.59, 0.70, 0.75, and 0.85, respectively), while the correlation coefficient of 4%IM from DOC to HA was −0.85, which was higher than 2%IM (−0.76) and 8%IM (−0.34). Therefore, the growth and propagation of lignocellulose-degrading fungi can be completely realized by using food residues as a culture substrate. A 4% inoculation amount was more conducive to the humification of garden waste compost and the preservation of carbon. This study provides a reference for the garden waste composting inoculation process and a theoretical basis for multi-source waste-efficient collaborative treatment.