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Conventional low-density polyethylene microplastic induce stronger adverse effects on maize–soil–bacteria system than polylactic acid microplastic
Summary
A comparison of conventional low-density polyethylene (LDPE) and biodegradable polylactic acid (PLA) microplastics from mulch film found that LDPE consistently inhibited maize growth while low concentrations of PLA actually promoted it. Both plastic types entered maize root tissues but followed different pathways, and LDPE caused greater harm to soil bacterial communities, suggesting that switching from conventional to biodegradable mulch is genuinely beneficial for agricultural soil health.
Mulch residues are the most direct microplastics (MPs) source in agricultural soils. This study explored the impacts of MPs derived from conventional low-density polyethylene (LDPE) and biodegradable polylactic acid (PLA) mulch films on maize-soil-bacteria community system (both common and glutinous maize), and the accumulation of MP in maize roots. PLA MPs were found in vascular tissues (xylem/phloem) in common maize but only in cortical layers in glutinous maize, while LDPE MPs showed phloem-specific entry. In common maize, low-concentrations of PLA MPs (0.1 and 1 %) promoted maize growth and chlorophyll synthesis, whereas LDPE MPs consistently inhibited maize growth. PLA MPs increased soil organic carbon (SOC), total nitrogen (TN), and enzyme activity, whereas LDPE MPs decreased TN and catalase activity. Low-concentration PLA MPs increased bacterial diversity and altered bacterial community composition and pathways, while LDPE MPs exhibited inhibitory effects. PLA MP stimulated bacterial diversity at low concentrations and inhibited it at high concentrations, whereas LDPE MP showed consistent suppression. High LDPE MP concentrations significantly reduced the abundance of Chloroflexi, Proteobacteria, and Deinococcota, while increasing that of Firmicutes. Low LDPE MP concentrations impaired Carbon metabolism and Glycolysis/Gluconeogenesis (-3.80 and -7.23 %, respectively. Glutinous maize exhibited greater resistance than common maize. Structural equation modeling revealed that MP types and concentrations directly altered soil properties (-0.37, P < 0.05) and bacteria communities (0.65, P < 0.001) and indirectly inhibited maize growth. Our findings show that the potential hazards from MPs cannot be generalized and highlight the need to focus on variability among crop varieties.
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