Long‐Term Plastic Mulching Reshapes Soil Biogeochemistry and Microbial Assembly: Implications for Carbon–Nitrogen Coupling and Ecosystem Resilience

ABSTRACT Plastic film mulching is a cornerstone of intensive agriculture, yet its long‐term impact on the coupling of soil carbon–nitrogen (C–N) cycles and microbial assembly—particularly in humid subtropical regions—remains poorly understood. Here, we investigated the biogeochemical and microbiological trajectories of vegetable soils in South China following 10 and 20 years of continuous polyethylene mulching. We show that long‐term mulching fundamentally reshapes the soil micro‐environment, driving a deterministic shift toward a high‐nitrification, carbon‐enriched state. Mulching elevated soil organic carbon (SOC) and nitrate levels, creating a specific ‘incubator effect’ that tightened C–N coupling. This environmental selection favoured stress‐tolerant taxa, significantly enriching the nitrite‐oxidizing genus Nitrospira and the spore‐forming Neobacillus within the ‘plastisphere’, while competitively suppressing plant‐growth‐promoting guilds such as Sphingomonas . This community succession underpins a self‐reinforcing feedback loop: carbon accumulation fuels accelerated nitrification, which in turn exacerbates moderate soil acidification and nitrate accumulation, reducing functional redundancy and ecosystem stability. Our findings highlight the ‘double‐edged’ nature of plastic mulching—enhancing agronomic fertility at the expense of micro‐ecological resilience. We propose that sustainable intensification in subtropical systems must transition toward adaptive management strategies, integrating biodegradable alternatives and microbiome‐targeted regulation to mitigate these latent ecological risks.