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Microplastics As Modulators Of Soil Nutrient Cycling Under Hydrological Connectivity In Earth's Critical Zone
Summary
Tiny plastic bits from mulch, sludge, and tire wear are building up in farm soils and messing with how the soil handles carbon and nitrogen — this review of existing studies found effects like increased greenhouse gas emissions and disrupted nutrient cycles that plants need to grow. This matters because healthier soil means safer, more reliable food production, and the chemicals released from tire particles (like a toxic compound called 6PPD-quinone) are showing up in concerning amounts near roads, raising questions about their broader environmental and health impact that scientists are still working to understand.
Microplastics have emerged as pervasive contaminants in terrestrial ecosystems, with agricultural soils serving as major sinks through sources including plastic mulch, sewage sludge, and atmospheric deposition. This comprehensive review synthesizes current knowledge on how microplastics modulate soil nutrient cycling within the Earth's Critical Zone, with particular emphasis on the mediating role of hydrological connectivity. The physical presence of microplastics fundamentally alters soil structure, porosity, aggregate stability, and hydraulic properties in polymer- and concentration-dependent manners, creating preferential flow pathways that govern both microplastic transport and nutrient dynamics. Biodegradable microplastics generally exert more pronounced effects than conventional polymers, increasing CO₂ emissions while simultaneously altering microbial carbon use efficiency. Nitrogen cycling is consistently disrupted, with reduced nitrate concentrations, enhanced N₂O emissions (up to 140.6% increase), and shifts in nitrogen-transforming microbial guilds. Tire wear particles and their associated chemicals, particularly the highly toxic transformation product 6PPD-quinone, represent emerging contaminants of high concern with substantial emissions (estimated 79.5 kt annually in the U.S.), accumulation in roadside soils (up to 26,400 mg/kg), and demonstrated toxicity to soil biota. The effects of microplastics on nutrient cycling are profoundly context-dependent, modulated by soil properties, microplastic characteristics, environmental conditions, and biological factors. Significant knowledge gaps remain regarding deep Critical Zone processes, temporal dynamics, and interactive effects with global change factors. As plastic production continues to increase, understanding and mitigating the effects of microplastics on soil nutrient cycling under varying hydrological regimes represents a pressing environmental imperative with implications for soil health, agricultural productivity, and climate regulation.