Macroplastics in soybean cultivation: Neutral on plant growth but disruptive to nitrogen-fixing microbiome

Abstract

Macroplastics are an emerging yet underexplored pollutant in agricultural soils, with the potential to disrupt nitrogen (N) cycling through physical interference and microbial community shifts. While extensive studies have focused on microplastics, the effects of larger plastic debris (>2 cm) on soil-plant systems in legume cropping systems remain poorly understood. We conducted a 71-d mesocosm study utilizing ¹⁵N isotopic tracing and metagenomic sequencing to demonstrate how macroplastics influence soybean growth and soil-soybean continuum N cycling. Soybean growth was not affected under macroplastics exposure (up to 200 kg ha⁻¹). However, macroplastics increased soil NO₃⁻ and NH₄⁺ concentrations, and elevated urease and ammonia monooxygenase activities, suggesting enhanced N availability. Paradoxically, macroplastics significantly disrupted the N-fixing microbial community, reducing the abundance of key bacteria such as Azorhizobium and Bradyrhizobium. Nitrogen fixation pathways (in log10-transformed TPM+1) were markedly suppressed in soils treated with 200 kg ha⁻¹ macroplastics compared to untreated soils (p < 0.001). Our findings highlight the potential risks of macroplastics posing to N cycling and microbial health in agricultural soils. This study addresses a critical knowledge gap by shifting the focus from micro- to macroplastic impacts on biogeochemical cycling.