Synergistic co‑amendment of phosphate rock and lignite improves comprehensive quality and agronomic performance of...

Composting is an effective approach for agricultural waste valorization, but high carbon and nitrogen losses and low humification efficiency critically constrain its application. Although phosphate rock and lignite have been separately applied as compost additives, their inherent limitations remain and synergistic mechanisms are poorly understood. Herein, aerobic composting was conducted using straw mixed with chicken manure (SC), with amendments of phosphate rock (P), lignite (L), and their combination (P + L). A comprehensive compost quality index (CQI) integrating nutrients, microbes, and heavy metal ecological risk was established for holistic evaluation. Compared with the control, single P and C increased CQI by 24.54% and 57.56%, respectively, while the combined addition enhanced CQI by 160.26%. Structural equation modeling revealed a cascade mechanism linking C/N loss, nutrient retention, key microbial groups, and risk mitigation, confirming synergistic rather than additive effects. The composite compost outperformed single additives in promoting vegetable seedling growth and increased tomato fruit quality index (FQI) by 160.61%. These findings provide theoretical and practical insights for developing safe, high-efficiency composting strategies to advance agricultural waste valorization.