Improving pig manure compost quality and increasing bioavailability of Cd by substituting insoluble carbon with easily degradable organic carbon

Composting is a widely used method for managing livestock manure, yet the role of carbon sources in this process is often overlooked. This study investigated the effects of easily degradable organic carbon (EDOC) on composting by replacing insoluble carbon while maintaining a constant C/N ratio (30:1) and total carbon content. Three types of EDOC—Glucose, sucrose, and starch were supplemented at three levels (1/75, 2/75, 1/25), designated as G1-G3, SC1-SC3, and ST1-ST3, respectively. The results indicated that higher EDOC levels promoted increased composting temperatures and extended the thermophilic phase. Total nitrogen content was significantly elevated in the treatment groups (2.59%-2.77%) compared to the control group (2.51%, p < 0.05). EDOC addition also increased the final humic acid (HA + FA) content (8.362%-9.809% vs. 8.00%) and GI value (105.7%-114.3% vs. 100.7%, p < 0.05), indicating improved compost quality. Furthermore, the oxidisable and residual forms of Cd in the treatment groups significantly decreased compared to the control group (19.52%-24.85% vs. 32.60%, p < 0.05), which corresponded to an increase in Cd bioavailability. Pearson correlation analysis revealed a significant relationship between Cd bioavailability and Pb bioavailability (R²=0.71, p = 0.020), and an inverse correlation with compost pH (R²=-0.66, p = 0.039). These findings suggest that higher EDOC levels in the substrate could mitigate nitrogen loss, enhance compost quality, and increase Cd bioavailability. Additionally, higher glucose levels in the substrate may promote lignocellulose degradation during composting (p < 0.05). This study underscores the pivotal role of EDOC in the composting process, providing useful insights for optimizing compost management practices.