Effects of material blending and bioaugmentation on humification and nutrient enrichment in dairy manure composting

The rapid development of the dairy farming industry has led to substantial manure production. While aerobic composting serves as an effective treatment method, the inherent characteristics of dairy manure—such as high moisture content and dense structure—pose challenges including inefficient decomposition and significant nutrient loss, While current approaches often employ biological, physical, or chemical measures to enhance aerobic composting efficiency, few studies have investigated the synergistic effects of combined physical and biological regulation on humification and nutrient enrichment. This study utilized a composting mixture consisting of 85% dairy manure and 15% straw (wet weight basis) in an L₉(3⁴) orthogonal array design to systematically evaluate the effects of rapeseed cake addition ratio (5%, 10%, 15% wet weight), mature compost addition ratio (0%, 10%, 20% wet weight), microbial inoculum ratio (0.00%, 0.05%, 0.10% dry weight), and rapeseed cake addition timing (initial, cooling, maturation phases) on humification and nutrient enrichment, with results demonstrating that rapeseed cake addition promoted temperature elevation, though delayed addition increased heating frequency and concurrently elevated ammonia (NH₃) emissions by 37.3-59.4% and hydrogen sulfide (H₂S) emissions by 29.6-459.9%, while also shortening the organic matter decomposition cycle by 18-32%, increasing final organic matter content by 6.6-15.1%, and enhancing humification (humic acid carbon increased by 22-40%), it significantly reduced the seed germination index (GI) by 30.9-98.9%; mature compost addition effectively reduced NH₃ emissions by 24.5-80.0% and H₂S emissions by 7.9-55.8% while improving humification (humic acid/fulvic acid (HA/FA) ratio increased by 21.6-61.8%) and nutrient enrichment (total N+P₂O₅+K₂O increased by 12-28%). Analysis of variance identified mature compost ratio and rapeseed cake ratio as statistically significant factors (p < 0.01) influencing humic acid content and total nutrient retention, with the optimal combination being 10% mature compost, 0.05% microbial inoculum, and 10% rapeseed cake added during the initial phase, which yielded superior compost quality characterized by total nutrient content of 8.2%, humic substances (HS) of 120.88 mg/g·DM, fulvic acid (FA) of 30.0 mg/g·DM, humic acid (HA) of 90.79 mg/g·DM, HA/FA ratio of 3.03, maximum GI of 167.9%, and organic matter content of 52.4%, with all critical parameters (total nutrients, organic matter, HA/FA ratio, and HS content) ranking among the top three performance groups, demonstrating that integrated physical-biological regulation can simultaneously optimize composting efficiency, emission reduction, and product quality in dairy manure management systems. The findings provide valuable insights for the efficient utilization of dairy manure.