Biodegradable and conventional microplastics differentially affect soil nutrient availability and stress responses in spinach (Spinacia oleracea L.)

The widespread use and mismanagement of agricultural mulches have increased microplastic (MP) contamination in soils, potentially altering soil–plant interactions. This study compared the effects of biodegradable (B-MPs) and conventional polyethylene (P-MPs) microplastics on soil nutrient dynamics, microbial activity, and physiological responses of spinach ( Spinacia oleracea L.) grown in mesocosms. This study provides a holistic and integrated comparison of biodegradable and conventional microplastics across soil, microbial, and plant dimensions, contributing to a better understanding of how these materials influence soil–plant systems. Soils were amended with 0.5, 1, and 2% ( w /w) of each MP type and compared with unamended controls over a four-month period. Soil samples were characterized by pH, N, Corg, NH4+, SO42−, elements availability microbial abundances and respiration (Resp), DNA yield, and eubacterial and fungal abundances. Spinach performance was assessed by element content in roots and leaves, photosynthetic activity, weight, length, leaf relative water content, specific leaf area, chlorophyll and carotenoid contents, and several stress responses (GAE, FRAP, HSP70, APX, GR, CAT). B-MPs significantly reduced microbial respiration by 20–25% and decreased Fe, K, Na, Zn, and V availability by up to 15-fold compared to controls, whereas 2%-P-MP soils showed 1.3–2.3-fold higher Fe and Zn availability. Spinach grown in B-MP soils exhibited lower biomass (−48%), carbon (−20%), and nitrogen (−25%) contents, while P-MP exposure mainly induced oxidative stress, with enhanced antioxidant activity (+40% FRAP) and increased expression of HSP70, APX, and CAT proteins. Bioaccumulation factors exceeded 1 for most elements, confirming active uptake, but translocation patterns differed between MP types. In conclusion, both conventional and biodegradable MPs compromise soil fertility and plant growth by reducing nutrient availability and acting as physiological stressors, highlighting the need for critical assessment and management of both biodegradable and conventional mulches.