Chromium-Driven Changes in Heavy Metal Resistance Genes During Pig Manure Composting

Composting is an effective method for stabilizing and valorizing pig manure, which is rich in nutrients but also contains heavy metals such as chromium (Cr). These heavy metals can promote the development of heavy metal resistance genes (MRGs) during composting, posing environmental and health risks. In this study, pig manure composting supplemented with pyridine carboxylate chromium was applied to investigate its effects on heavy metal speciation and MRG abundance and explore the influence factors of the dynamics of MRGs during composting. The results showed that the addition of Cr significantly influenced the composting process, including temperature fluctuations and nutrient dynamics. Specifically, the addition of Cr weakened the impact of water addition on temperature, as evidenced by the failure of the Cr-amended treatment to re-enter the thermophilic phase after water addition. The speciation of Cr changed during composting, with a significant reduction in high-bioavailable Cr forms (e.g., a 54.56% reduction in high-bioavailable Cr in the Cr-amended treatment) and an increase in low-bioavailable forms. The abundance of MRGs, particularly copper resistance genes, increased over time, with more pronounced fluctuations in the Cr-amended treatment. The primary factors influencing the dynamics of these MRGs during composting were identified as heavy metal speciation, microbial community structure, and specific physicochemical properties such as pH, electrical conductivity, and dissolved organic carbon. The present study offers valuable insights into the complex interactions between heavy metals and microbial communities during composting and provides inspiration for managing heavy metals to minimize the spread of MRGs.