The current carbon dioxide (CO) evolution-based standard method for determining biodegradable microplastics (MPs) degradation neglects its priming effect on soil organic matter decomposition, which misestimates their biodegradability. Here, a C natural abundance method was used to estimate the mineralization of poly(lactic acid) (PLA) MP in various agricultural soils, and to trace its utilization in different microbial groups. In alkaline soils, the PLA-derived CO emissions increased with increasing soil carbon/nitrogen (C/N) ratios, and the mineralization of PLA MP concentrations ranged from 3-33 %, whereas the CO evolution method probably over- or under-estimated the mineralization of PLA in alkaline soils with different soil C/N ratios. Low PLA mineralization (1-5 %) were found in the acidic soil, and the standard method largely overestimated the mineralization of PLA MP by 1.3- to 3.3-fold. Moreover, the hydrolysate of PLA MP was preferentially assimilated by Gram-negative bacteria, but Gram-positive bacterial decomposition mainly contributed to the release of PLA-derived CO at low MP concentrations (≤ 1 %). Overall, the C natural abundance method appears to be suitable for tracking the mineralization and microbial utilization of biodegradable PLA in soils, and the PLA-derived C is mainly assimilated and decomposed by bacterial groups.