Evaluation of chemical properties and humification process during co-composting of spent mushroom substrate (Pleurotus ostreatus) and pig manure under different mass ratios

Spent mushroom substrate (SMS) from Pleurotus ostreatus is high in salinity, susceptible to deterioration caused by mold, and not suitable for direct application in soil. Its high C/N ratio and low pH inhibit the humification process during composting. Pig manure (PM) possesses high density and low porosity, impeding microbial metabolism. Co-composting SMS and PM can overcome these challenges. A 120-day composting experiment was conducted with varying mass ratios of SMS and PM, supplemented with calcium bentonite and microbial inoculum. After composting, there was observed a significant rise in the N and K₂O contents, ranging from 11.1% to 84.1% and 55.1%–296.8%, respectively. Additionally, there was a decrease in the C/N ratio by a range of 18.9%–60.1%. The total nutrient content of SMS post-composting solely was a mere 4.9%, failing to meet the 5% required by China's Professional Standard outlined for organic fertilizers (NY/T525-2021). Composting PM alone simplified the molecular structure of humic-extracted acid (HE) and reduced the C content of humic acid (HA) by up to 18.1%. In contrast, co-composting SMS and PM in equal proportions stimulated microbial consumption of water-soluble substances, resulting in increase in C contents of HE and HA by 11.5% and 18.7%, respectively, along with a 15.4% rise in the polymerization level of the composted materials. Furthermore, this combination facilitated the conversion of inert humic component, reducing C content of humin by 79.3%, lowering the electrical conductivity (EC) value by 23.0%, and boosting GI value by 154.1%. Increasing the proportion of SMS in co-compost proved advantageous in enriching its organic matter content. The composting process involved bio-degradation of O-containing functional groups like phenols and alcohols, utilization of polysaccharides by microorganisms, and development of carboxyl groups in the HA. Based on the above index analysis, the most effective compost ratio to achieve higher humification level and compost quality was determined to be a mass ratio of SMS to PM of 5:5, followed by 6:4.