Effects of compostable plastic residues in municipal organic waste compost on soil quality and lettuce growth

Biodegradable and compostable plastics (BCPs) are increasingly present in municipal organic waste streams, yet their effects on compost quality and subsequent soil–plant interactions remain insufficiently understood. This study evaluated municipal organic waste composts containing 0% (MOWC0), 1.43% (MOWC1; national average), and 2.86% (MOWC2; twice the national average) certified BCPs. Compost quality was assessed before application, followed by a six-week soil incubation and a four-week pot experiment with lettuce (Lactuca sativa L.). The presence of BCPs did not compromise compost maturity, stability, regulatory compliance, or trace-element quality. During soil incubation, total organic carbon and nitrogen were only slightly affected, whereas microbial indicators showed clearer responses. MOWC2 increased cumulative soil respira tion by 81% and 63% compared with MOWC0 and MOWC1, respectively, and increased the metabolic quotient by 97% relative to the unamended control. Dehydrogenase, laccase, tyrosinase, and N-acetyl-b-glucosaminidase activities were significantly affected by treatment, indicating that compost addition influenced both microbial oxidative metabolism and enzymatic processes related to N cycling, while microbial biomass nitrogen in MOWC2 was 21% higher than that of the control and more than fivefold higher than that of MOWC0. After lettuce cultivation, compost application increased soil total organic carbon by 16–20%, total nitrogen by 26–31%, dissolved organic carbon by 45–72%, total dissolved nitrogen by 48–83%, and Olsen phosphorus by 18–22% relative to the control. Lettuce shoot biomass increased in all amended soils, reaching a maximum increase of 26% in MOWC2, whereas root biomass and biomass allocation were unaffected. Leaf potassium increased by 28–44%, and no accumulation of potentially toxic elements was observed. Overall, certified BCP residues at realistic and moderately increased concentrations were compatible with industrial com post production and soil application. Composts retained their agronomic bene fits, although the stronger microbial responses observed at the highest BCP content suggest that soil biological indicators may detect early BCP-related effects.