Polypropylene microplastics exacerbate microbial phosphorus limitation and disrupt organ-specific nutrient homeostasis in maize-soil systems

This study examined how pristine polypropylene microplastics (PP-MPs) affect carbon (C), nitrogen (N), and phosphorus (P) stoichiometry across the soil-maize continuum. Under PP-MPs stress, soil organic C increased, soil total N first increased and then decreased with increasing PP-MPs concentration, while soil total P did not change significantly, leading to increased soil C:N and decreased N:P, indicating microbial P limitation. Microbial biomass C increased and biomass P decreased, while biomass N was unchanged, raising the microbial C:N:P ratio and reinforcing exacerbated microbial P limitation. Enzyme activities shifted from C- to N- and P-acquisition, and vector analysis confirmed that PP-MPs alleviated microbial C limitation but sustained P limitation. Maize growth exhibited a concentration-dependent biphasic response, low PP-MPs levels (≤2%) promoted root biomass and root-to-shoot ratio, whereas high levels (≥5%) strongly inhibited both shoot and root growth. Notably, the R/S ratio remained elevated even at inhibitory concentrations, suggesting a preferential allocation of biomass to root under PP-MPs stress. Root showed sensitive homeostasis for N and N:P, whereas shoot maintained strict homeostasis for P and N:P, indicating a strong regulatory capacity of photosynthetic tissues to buffer against microbially mediated soil P limitation under PP-MPs stress. Structural equation modeling identified that PP-MPs affect maize growth through integrated pathways, directly via phytotoxicity and indirectly by modifying soil physical properties, microbial biomass, and enzyme activities. These findings establish a chemometric framework linking microplastic pollution to nutrient remodeling and seedling adaptability in maize, providing mechanistic insights into the risks posed by microplastics to soil-plant systems during early growth stages.