An assessment of composting conditions, humic matters formation and product maturity in response to different additives: A meta-analysis

Humic matters (HMs), including humic substance (HS), humic acid (HA) and fulvic acid (FA), and germination index (GI), are essential indices of compost maturity and stability. While traditional composting practices can produce poor final products, the application of different additives (such as physical, chemical, and microbial types) is a possible way to hasten the composting process and improve the quality of the compost products. Yet, there is a deficiency of comprehensive and quantitative analysis of how additives affect physicochemical parameters, HMs formation, maturity, and stability of compost products. Here, a meta-analysis was conducted based on 494 paired observations to explore the response pattern of compost physicochemical factors and maturity indices to different additives during composting. The results indicated that applied additives increased the level of HS, HA, GI, total nitrogen (TN), temperature, and pH by 41.3%, 73.4%, 16.6%, 19.4%, 9.63%, and 7.33%, respectively, across all the observations, compared to the control composts. Meanwhile, the level of FA, total organic carbon (TOC), moisture content (MC), and C/N were lower by −37%, −20.8%, −18.9%, and −20.5% than in the control composts. Composting duration contributed to considerable degrees of variation in HMs’ response to different additives (p < 0.05). Also, composting factors (feedstock, additive types, and bulking agents) were effective regulators of the response of HMs to different additives. Accordingly, adjusting the additives, process duration, and composting factors can positively impact the amount of HMs and nutrients in the compost products. Eventually, the outcomes of this study can support the selection, improvement, and application of additives to promote the quality and stability of the compost products.