Polystyrene micro- and nano-plastics alter tropical soil nutrients without affecting lettuce (Lactuca sativa L.) productivity

Polystyrene-derived microplastics (PS-MPs) and nanoplastics (NPs) are emerging contaminants in agricultural soils, yet their effects on soil properties and crop growth in tropical agroecosystems remain poorly understood. We synthesized polystyrene microplastics (PS-MPs, 5–10 μm) and nanoplastics (PS-NPs, 100–150 nm) and confirmed their morphology and size via SEM and FTIR analyses. These particles were applied to a tropical Typic Rhodustalf soil at concentrations ranging from typical agricultural contamination to industrial pollution scenarios (0.002–0.2% w/w) in a controlled greenhouse pot experiment with lettuce (Lactuca sativa) over 45 days. Soil and plant quality indicators were monitored, including soil pH, electrical conductivity (EC), available nutrient levels, plant biomass, height, and relative chlorophyll content. Factorial ANOVA revealed that PS-MPs and PS-NPs significantly altered soil nutrient dynamics at the highest concentrations, increasing ammonium, nitrate, and available phosphorus concentrations, with PS-MPs inducing stronger effects than PS-NPs. Conversely, they had no detectable impact on soil pH, EC, plant growth or biomass during the 45-day exposure period. These findings indicate that changes in soil nutrient availability were detectable only at the highest PS concentrations, corresponding to heavily contaminated soils, whereas no measurable effects on lettuce growth were observed during the 45-day experiment. Whether changes in soil nutrient availability precede plant responses under longer-term exposure or field conditions remains to be determined. This study provides baseline evidence that the environmental impacts of PS particles in tropical agricultural soils are strongly concentration-dependent and that soil nutrient responses are more sensitive indicators of contamination than short-term plant growth under controlled conditions. At high concentrations, micro and nano-polystyrene (PS) altered soil nutrient dynamics. PS particles increased soil ammonium, nitrate, and available phosphorus concentrations. Short-term (45-day) exposure to 0.002–0.2% PS particles did not significantly affect lettuce growth or biomass. Nutrient cycling disruption occurred before observable plant effects.