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Biodegradable microplastics-induced free-living nitrogen fixation enhancement and diazotrophic community differentiation in soils
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Scientists found that tiny pieces of biodegradable plastic in farm soil actually boost the activity of helpful bacteria that add nitrogen to the soil, which plants need to grow. However, these same plastic pieces also reduce other important nutrients in the soil and change which types of bacteria live there. This matters because as farmers use more biodegradable plastics, we need to understand how the tiny plastic pieces left behind might affect our food production and soil health.
Rising global use of biodegradable plastics (BPs) in agriculture raises concerns about the impacts of microplastics (MPs) derived from these materials on soil ecological functioning. Free-living nitrogen fixation (FLNF) serves as a major contributor of new nitrogen to soil ecosystems, playing a critical role in sustaining global ecosystem productivity. Yet, little is known about the biodegradable microplastics (BMPs) effects on FLNF. Here, we conducted microcosm experiments by adding biodegradable poly(butylene adipate-co-terephthalate) (PBAT) and conventional polyethylene (PE) MPs to three agricultural soils. Results showed that PBAT MPs enhanced FLNF in a dose-dependent manner, with significant increases in N fixation rate by 170.88% in SD, 108.85% in SX, and 171.41% in XJ under 2% PBAT treatment, whereas PE MPs exhibited no significant effects except potential low-dose stimulation. High-dose PBAT MPs significantly increased soil dissolved organic carbon (DOC) content but reduced the availability of key nutrients, specifically decreasing available nitrogen (AN), available phosphorus (AP), and available potassium (AK) contents by 7.90-25.63%, 7.53-18.21%, and 3.49-7.00%, respectively. Moreover, compared to PE MPs, PBAT MPs inputs triggered more pronounced changes in diazotrophic communities. In particular, diazotrophic communities exhibited significant niche differentiation under PBAT MPs exposure, characterized by a shift towards specialists and a reduction in generalists. Despite divergent pathways through which PBAT MPs affect FLNF in different soils, DOC content and specialists relative abundance were consistent factors regulating N fixation rate across all soils. Collectively, our findings demonstrated the distinct impacts of biodegradable and conventional MPs on FLNF, highlighting the stimulatory effects of BMPs on FLNF through regulation of soil carbon availability and diazotrophic community specialization. This work provides novel insights into the ecological consequences of BMPs functioning as labile C substrates on microbially mediated nitrogen cycling.
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