Contextual Recalcitrance of Polylactic Acid (PLA) in Soils: Degradation Mechanisms, Microbial Traits, and Fate-Relevant Endpoints

Polylactic acid (PLA) is among the most widely used biodegradable, biobased polymers, yet it can persist in soils and waters, where accumulation may affect plants and agroecosystems. This narrative, mechanistic review asks why PLA behaves differently under industrial composting and ambient soil conditions, drawing on original studies and using soils as the interpretive lens, with aquatic and waste management systems for contrast. We examine hydrolysis, Norrish photochemistry, biofilm formation, enzymatic cleavage, environmental controls, polymer crystallinity, physical accessibility, and microbial community interactions. Elevated temperature and moisture promote rapid degradation during industrial composting, whereas ambient conditions slow hydrolysis and restrict microbial access. We propose contextual recalcitrance as an organizing framework, in which persistence arises from interactions among polymer structure, environmental conditions, physical accessibility, and the local availability of depolymerizing traits rather than intrinsic chemical resistance. Within it, we advance trait gating by primary depolymerizers and cross-feeding of soluble lactate as a testable conceptual model rather than a demonstrated soil succession because community-resolved field evidence is lacking. We conclude that environmental biodegradability should be assessed using endpoints beyond CO2 evolution, including dissolved organic carbon, microbial assimilation, and organic matter partitioning, to distinguish fragmentation, depolymerization, assimilation, mineralization, and stabilization of PLA-derived carbon in soils.