Beyond CO₂ mineralization: non-isotopic carbon partitioning of PBS and PBS/orange peel biocomposites during mesophilic composting with standard and bioaugmented inoculation

CO₂ evolution is the standard endpoint for assessing aerobic biodegradation, but it does not resolve the fate of non-mineralized carbon. Here, a non-isotopic carbon-partitioning framework was developed in carbon-free activated vermiculite to evaluate the mesophilic biodegradation of poly(butylene succinate) (PBS) and PBS/orange peel (PBS/OP) biocomposites under composting-like conditions. Two inoculation strategies were compared: a mature compost extract and the same extract bioaugmented with a PBS-degrading bacterial consortium. The framework combined cumulative CO₂ respirometry, matrix-matched GC–MS quantification of residual PBS-derived carbon after acid methanolysis, and total organic carbon analysis of post-biodegradation vermiculite. Matrix-matched calibration revealed a 27% decrease in GC–MS sensitivity in vermiculite, confirming the need to account for matrix effects. After 133 days at 28 °C, residual PBS-derived carbon remained low in neat PBS but increased in PBS/OP systems. On a common initial total-material-carbon basis, the non-residual organic-carbon pool represented 3.7, 8.5, 10.3 and 13.6% for PBS, PBS+, PBS/OP and PBS/OP+, respectively. For neat PBS and PBS+, where PBS was the only intentionally added organic carbon source after blank correction, the sum of CO₂ mineralization and non-residual organic carbon was interpreted as overall PBS biodegradation, reaching 47.3 and 81.9%, respectively. For PBS/OP and PBS/OP+, the corresponding operational total-material carbon conversions were 90.2 and 89.2%.