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Crumb Rubber Microplastics Alter Soil Water Dynamics and Plant Biomass Allocation in Soybeans
Summary
Recycled tire crumbs (used in artificial turf and sometimes mixed into soil) don't stunt soybean plants on the surface, but they change what's happening underground: plants grew more roots, held onto soil moisture differently, and absorbed noticeably more zinc into their leaves — enough to exceed normal healthy levels. Since soybeans and other crops can take up these metals from contaminated soil, this raises questions about whether recycled tire materials near farmland or gardens could quietly work their way into the food we eat, even when plants look perfectly normal above ground.
Crumb rubber, a recycled tire product used in artificial turf fields, alters soil physical properties and releases chemical contaminants, but few studies have examined its effects on plant physiology and resource allocation. We conducted a greenhouse experiment to evaluate the responses of soybeans (Glycine max) grown in soils containing increasing proportions of crumb rubber (0%, 16.6%, 33.3%, and 50% by weight). Germination, plant growth, chlorophyll content, soil respiration, biomass production, water loss, root allocation, and elemental composition of soils and plant tissues were measured over a 21-day period. Most indicators of plant performance (germination, plant height, chlorophyll content, soil respiration, and total biomass) were not significantly affected by crumb rubber additions. However, crumb rubber significantly reduced cumulative water loss and increased both root biomass and root-to-shoot ratios at moderate and high concentrations. Soil and plant tissue analyses revealed substantial increases in zinc concentrations across the crumb rubber gradient, with leaf zinc concentrations exceeding sufficiency ranges at higher treatments. These results demonstrate that crumb rubber contamination can alter soil water dynamics, plant resource allocation, and trace metal accumulation even when aboveground growth responses remain limited.