Qualitative Assessment and Interrelation of Organic Carbon Transport and Bioactivity Characteristics During Natural Composting and Vermicomposting: Various Pig Manure/Straw Pellets Ratios

The transformation pathways of organic carbon (OC) fractions and their interrelationship with microbial activity during natural composting (NC) and vermicomposting (VC) remain poorly understood across pig manure (PM)/straw pellets (SP) ratios. Therefore, the objective of this study was to elucidate the regulatory mechanisms of substrate mixing ratios on carbon fraction transformation and microbial functional networks during these processes. To achieve this, five PM/SP ratios [100:0 (T1), 75:25 (T2), 50:50 (T3), 25:75 (T4), and 0:100 (T5)] were composted with or without earthworms, revealing the T2 (75:25) ratio had most efficient composting performance within 60 days due to the suitable initial C/N ratio (31.65 ± 0.99). Consequently, the T2 treatment reached the highest organic degradation, including TOC reduction (58.6%), TN accumulation (63.9%), and C/N decline (74.8%) in the VC. Vermicomposting markedly stimulated functional microbial groups—nitrogen-fixing, phosphate-solubilizing, and potassium-solubilizing bacteria—thereby enhancing nutrient (N, P, K) bioavailability. The prominence of the optimal C/N ratio across multiple hydrolytic and oxidative enzymes in the VC-T2 further proved that this ratio provided an optimal nutrient and structural balance for both earthworms and microbial consortia. Strong correlations between bacterial abundance and enzyme activities (r ≥ 0.98), lignin and dissolved OC (r ≈ −0.81), and particulate organic carbon and mineral-associated carbon (r > 0.9) highlighted microbially mediated carbon stabilization through enzymatic mineralization, aggregation, and redistribution of carbon from active pools toward mineral-associated OC. This work identifies the critical PM-SP ratio for waste valorization and mechanistically links earthworm–bacteria interactions to carbon sequestration pathways.