Beyond the C/N Ratio: The Critical Role of Carbon Bioavailability in Aerobic Composting of Agricultural Waste

The initial carbon-to-nitrogen (C/N) ratio is a fundamental parameter for aerobic composting, with a generally recommended optimal range of 25:1 to 30:1. However, in practical applications, the optimal C/N ratio often deviates from the recommended value. We attribute this discrepancy to the limitations of traditional stoichiometric methods in assessing the bioavailability of carbon and nitrogen sources. This study investigated how carbon bioavailability governs composting efficiency and product quality. Laboratory-scale aerobic composting experiments were conducted using six types of raw crop straws and two physically pretreated straws, representing a biodegradability gradient. Results demonstrated that carbon bioavailability significantly modulated the composting performance. Substrates rich in labile carbon pool (LCP), such as wheat straw and extruded cassava plant residue, demonstrated superior thermogenesis, humification, and seed germination indices compared to those dominated by recalcitrant carbon pool (RCP), such as untreated cassava plant residue. Principal component analysis confirmed a strong positive correlation between LCP content and key quality indicators. Microbiological analysis revealed that carbon source variations shaped bacterial succession: Bacteroidota abundance correlated positively with LCP, driving rapid initial degradation, whereas Pseudomonadota were more abundant in RCP-rich treatments, suggesting a role in complex polymer breakdown. This study confirmed that carbon bioavailability, rather than the bulk C/N ratio alone, is a critical limiting factor. This finding logically extends to the role of nitrogen bioavailability, suggesting that a “biochemical C/N ratio”—accounting for the lability of both carbon and nitrogen—could be a more accurate predictor of aerobic composting performance.