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

Substantial manure have been produced from the dairy farming industry in recent years. Aerobic composting can serve as an effective treatment. However, dairy manure, such as high moisture content and dense structure, has posed great challenges, leading to low efficiency decomposition and serious nutrient loss. Fortunately, biological, physical, or chemical measures can be expected to enhance aerobic composting efficiency. This study aims to investigate the synergistic effects of the combined physical and biological regulation on the humification and nutrient enrichment. A composting mixture was utilized with 85% dairy manure and 15% straw (wet weight basis). An L9(34) orthogonal array was designed to systematically evaluate the effects of the rapeseed cake addition ratio (5%, 10%, and 15% wet weight), mature compost addition ratio (0, 10%, and 20% wet weight), microbial inoculum ratio (0, 0.05%, and 0.10% dry weight), and rapeseed cake addition timing (initial, cooling, and maturation phases) on the humification and nutrient enrichment. The results demonstrated that the addition of the rapeseed cake promoted the temperature elevation, although the delayed addition also increased the heating frequency. Concurrently, the ammonia (NH₃) emissions were elevated by 37.3%-59.4%, and hydrogen sulfide (H₂S) emissions by 29.6%-459.9%. The final content of the organic matter increased by 6.6%-15.1%. The humification was then enhanced. The seed germination index (GI) was significantly reduced by 30.9-98.9%. The addition of the mature compost effectively reduced the NH₃ and H₂S emissions by 24.5%-80.0% and 7.9%-55.8%, respectively. While there was some increase in the humification (humic acid/fulvic acid (HA/FA) ratio increased by 21.6%-61.8%) and nutrient enrichment. Analysis of variance identified the mature compost ratio and rapeseed cake ratio as statistically significant factors (p < 0.01) influencing the humic acid content and total nutrient retention. The optimal combination of the parameters was achieved, with the 10% mature compost, 0.05% microbial inoculum, and 10% rapeseed cake added during the initial phase. Superior compost quality was characterized by the total nutrient content of 8.2%, humic substances (HS) of 120.88 mg/g, fulvic acid (FA) of 30.0 mg/g·DM, humic acid (HA) of 90.79 mg/g, HA/FA ratio of 3.03, the maximum GI of 167.9%, and organic matter content of 52.4%. All critical parameters (total nutrients, organic matter, HA/FA ratio, and HS content) were ranked among the top three performance groups. Therefore, the integrated physical-biological regulation can simultaneously optimize the composting efficiency, emission reduction, and product quality in the dairy manure systems. The findings can also provide valuable insights for the efficient utilization of the dairy manure.