Eggshell-zeolite composite additive enhances humification and mitigates CH4 emissions during cow-manure composting: Insights from metagenomic analysis

Aerobic composting is a sustainable approach to livestock manure valorization, yet the recalcitrant lignocellulose in compost materials limits humification efficiency and induces excessive methane (CH 4 ) emissions. Here, eggshell (EP) and zeolite (ZP) composite additives were incorporated into a cow manure-cotton stalk composting system, and their impacts on gaseous emissions, humus formation, functional microbial communities, and carbohydrate-active enzyme (CAZyme) gene potential were investigated via a 30-day aerobic composting experiment with four treatments: control (CK), 5% EP, 5% ZP, and 2.5% EP + 2.5% ZP (EZ). The EZ treatment significantly promoted organic matter mineralization, with the highest degradation rates of cellulose (36.56%) and hemicellulose (25.79%) among all treatments. Concurrently, EZ mitigated cumulative CH 4 emissions possibly by maintaining a favorable aerobic microenvironment, while increasing cumulative CO 2 emissions due to enhanced lignocellulose degradation. The EZ treatment also exhibited the highest humic substance (HS) content and humic acid/fulvic acid (HA/FA) ratio, indicating optimal compost maturity. Metagenomic analysis revealed that Actinomycetota , Pseudomonadota , and Bacillota were the core phyla contributing to cellulose-degrading functional enzymes, and the EZ additive enriched these functional microbial communities. Redundancy analysis (RDA) identified pH as a key environmental factor driving the succession of functional microbial communities and humification. Overall, the eggshell-zeolite composite additive optimized the bacterial-enzyme system in the compost pile, accelerating lignocellulose degradation and humification while reducing greenhouse gas emissions. This study provides an additive strategy for improving manure composting efficiency and mitigating its environmental impacts.