Spent mushroom substrate (SMS) is an excellent conditioner for livestock manure composting. However, existing studies have confirmed that it is difficult to achieve the desired effect by directly mixing SMS with manure. Coarse (≥2 mm) and fine (<2 mm) of SMS particles from an edible fungus (Auricularia auricula) were obtained after sieving and used for cow manure composting. In our study, the appropriate ratio of coarse SMS to fine SMS particles added to the manure was explored. Four treatments were designed, adding 20% coarse SMS (T1), 15% coarse SMS + 5% fine SMS (T2), 5% coarse SMS + 15% fine SMS (T3), and 20% fine SMS (T4) to cow manure for composting, respectively. The physicochemical properties, maturity, and nutrient content of the composts were analyzed in a 35-day composting trial. The optimal treatment was determined through a comprehensive evaluation using the entropy-weighted TOPSIS method. The results showed that the highest composting temperature reached 65.13 °C in T3, and the duration of the thermophilic phase of T2 was the longest. The relative germination rate was not affected, and the relative radicle growth (RRG) reflected the variation in phytotoxicity during composting. After composting, the pH of the finished composts was between 8.78 and 9.05. The electric conductivity was between 2207 and 2513 μS cm⁻¹. The ammonium nitrogen content was less than 150 mg kg⁻¹, which was at the level found in mature compost. The RRG was no less than 80%, indicating the compost was mature and had no phytotoxicity. The available phosphorus and potassium contents increased by 4.8% to 59.1% compared with that before composting. The comprehensive evaluation showed that the treatment supplemented with 15% coarse SMS and 5% fine SMS was optimal.

Compost maturity is commonly assessed using the seed germination test, which mainly reflects below-ground radicle growth. However, composts that meet conventional maturity criteria may still inhibit above-ground seedling development. This study assessed the deep maturity of compost using relative cotyledon chlorophyll (RCC) in the seedling establishment test as a complementary indicator in full-scale cow manure composting. Windrow composting and membrane-covered composting were conducted, with Chinese cabbage (Brassica rapa L. subsp. pekinensis) seeds used for phytotoxicity testing. The relative seed germination (RSG) remained above 90% throughout both composting processes, indicating that RSG alone was insufficient to indicate maturity. The relative radicle growth (RRG) and germination index (GI) exceeded 80% after day 14 in windrow composting and after day 21 in membrane-covered composting, confirming full maturity of both products. In contrast, RCC exceeded 100% only after day 28 in both technologies, suggesting that a longer time is required to achieve deep maturity that promotes above-ground plant growth. RCC was significantly correlated with chemical indices (dissolved organic carbon and ammonium nitrogen) but showed no correlation with RRG or GI, indicating its independent and complementary role in phytotoxicity assessment. These findings demonstrate that composts considered mature based on radicle elongation may still inhibit cotyledon greening. The RCC indicator thus provides a simple and complementary tool for deep maturity assessment, particularly when compost is used for nursery substrate.