Organic carbon loss from agricultural soil is accelerated by biodegradable microplastics and mitigated by mineral nitrogen addition

Microplastics derived from agricultural plastic films accumulate in soils, potentially impacting ecosystem functions such as soil organic carbon (SOC) storage. Microbial degradation of biodegradable plastics, which are intentionally tilled into soil, may accelerate or inhibit the mineralization of native SOC, known as priming effects. Moreover, the interaction between microplastics and nitrogen (N) on SOC dynamics remains poorly understood, despite their concurrent presence in agroecosystems. We used a 193-day incubation experiment to investigate the degradation and priming effects of three biodegradable microplastics (polybutylene succinate [PBS], polylactic acid [PLA], and a PLA-polyhydroxyalkanoate blend [PLA/PHA]) compared to a conventional microplastic (low-density polyethylene [LDPE]) in agricultural soil under low and high N conditions. Isotope (¹³C) tracing allowed us to determine the cumulative loss of plastic- versus SOC-derived C as CO₂-C. Biodegradable microplastics varied in biodegradation rates and priming effects, with PLA/PHA losing the most plastic-C (17.88%) and inducing the greatest positive priming effects (371 µg C g dry soil⁻¹). In contrast, PLA, PBS, and LDPE showed <1% plastic-C loss and weaker priming effects ranging from positive to negative (73.1, 8.81, and −45.4 µg C g dry soil⁻¹, respectively). Although N addition decreased total C loss from both native SOC and microplastics, it did not alter priming effects. Priming effects were positively associated with dissolved organic and microbial biomass C, enzyme activities, and pH. We conclude that biodegradable microplastics may threaten native SOC pools, and higher N availability may promote persistence of biodegradable plastics in soils.