Biodegradable microplastics decreased plant-derived and increased microbial-derived carbon formation in soil: a two-year field trial

Microplastic pollution in agricultural soils threatens ecosystem services, yet its impacts on soil organic carbon (SOC) stabilization remain unresolved. In a two-year field experiment, conventional (polypropylene, PP) and biodegradable (polylactic acid, PLA) microplastics (0.2% w/w) were applied to assess their effects on SOC composition (i.e., plant lignin and microbial necromass) in topsoils (0–20 cm) as compared to an unamended control (without microplastic addition). While neither plastic type altered total SOC, PLA reduced lignin phenol content by 32% and reduced its contribution to SOC relative to the controls and PP. We ascribe this to the dominance of K-strategists that prioritize decomposition of relatively recalcitrant C-rich substrates and subsequent production of oxidases. This was supported by the negative correlation between the contribution of plant-derived C to SOC and the abundance of K-strategists as well as oxidases. Simultaneously, PLA increased microbial necromass contributions to SOC by 35%, linked to increased microbial diversity (+ 5.3%) and network complexity (+ 11%). Fungal necromass further dominated SOC contributions in PLA-added soils (24% vs. 11% under PP), driven by fungal-mediated macroaggregate formation. Due to the N restriction in PLA-added soils, however, the presence of PLA promoted microbial N-limitation. This in turn, triggered preferential depletion of bacterial necromass (− 19%) to meet their N growth demand, as evidenced by the negative correlation between bacterial necromass and N-acquiring enzymes. In contrast, PP suppressed necromass synthesis via C deprivation and toxic additive leaching, reducing its role in SOC persistence. Our findings reveal that biodegradable microplastics restructure SOC composition without altering total C stocks—highlighting necromass accrual as a critical yet overlooked stabilization pathway. Graphical Abstract: