In order to study the effect of cotton stalk particle size on humification, cotton stalks of different lengths (5 cm, 10 cm, 15 cm) were co-composted with pig manure for 49 days. The results showed that the 10 cm treatment (T2) maintained a high-temperature stage for 8 days, and the total organic carbon decreased by 60.0%. T2 showed the highest cellulose (57.0%) and hemicellulose (77.1%) degradation rate, the lowest lignin accumulation (69.8%), and the highest humus content (34.94 g/kg), which was 88.1% higher than that in the initial stage. FTIR analysis revealed significant changes in functional groups. The aromatic C=C stretching vibration (1650 cm-1) in the T2 treatment group increased by 79.8%, showing a better aromatization degree than the other two groups. Two-dimensional FTIR spectroscopy analysis showed that phenolic and alcohol hydroxyl groups were first involved in humification, followed by polysaccharides, hemicellulose, lignin, and aromatic structures, and the T2 treatment group enhanced this material transformation pathway. Microbial diversity analysis identified seven main phyla, among which T2 showed higher Planctomycetota and Acidobacteria abundance, which was closely related to the degradation of hemicellulose, cellulose, and phenolic compounds. Second, the abundance of characteristic species such as Planifilum fulgidum also showed certain advantages in the T2 treatment group. In summary, the particle size of 10 cm optimized the microbial activity and organic matter transformation and effectively regulated the composting humification process.
