Compostable plastics are often considered as a solution for minimizing plastic contamination due to their biodegradation capacity in composting processes. However, the standards evaluating their biodegradability do not encompass the variability of uncontrolled composting systems, especially regarding moisture content, and information on the microorganisms or enzymes required for the processes is scarce. To investigate the interactions between microbial activity, environmental factors and compostable plastics, this work focused on an ISO 14855 biodegradability test led on a corn starch based compostable plastic bag and the influence of two moisture contents - ∼30% and ∼50% - on microbial structure, enzymatic activities of cellulase and ligninase, and biodegradability efficiency. In the condition maintained at ∼30% moisture content, the tested compostable plastic emitted less CO₂ , along with higher pH, electroconductivity and relative abundances of Actinobacteriota (Actinomarinales) and Longimicrobiaceae. At ∼50% moisture content, the tested compostable plastic reached ∼82% biodegradability, with higher relative abundances of Deinococcota - Truepera and SBR1031. No noticeable impact was observed on cellulase and ligninase activities while comparing the two moisture conditions. This work provided insights into the multiple interactions ruling composting of compostable plastics and recommendations for optimizing their treatment, including revision of relevant standards.