Microplastics and nanoplastics in soil: Sources, impacts, and solutions for soil health and environmental sustainability

Tracking the source of microplastics in soil-an exploratory case study in peach orchards from east-central Portugal

In the last 20 years, world plastic production has increased rapidly, reaching 4.00 × 1011 kg in 2022. However, less than 10% was recycled. Moreover, most conventional plastics are persistent and, therefore, remain in the environment long after their release. Although most studies on microplastic contamination focus on a single environmental compartment, an integrated and multicompartment approach is highly recommended considering the multitude of interactions between those compartments. This study addresses this knowledge gap, investigating the presence and potential sources of microplastics (MP) in agricultural soils under typical conditions of the Mediterranean region, characterised by dry summers and relatively moist and mild winters (Csa according to the Köppen-Geiger classification). For this, 19 orchards from east-central Portugal were used as case study sites, and a total of 111 samples were analysed. Soil content in MP was assessed in 3 soil layers (0-5, 5-15 and 15-25 cm). To quantify potential sources to soil, information from farmers was used, and samples from irrigation water, atmospheric deposition and manure were analysed. Optical assessment with the assistance of a stereomicroscope was used for MP quantification. The 0-5 cm soil layer showed a lower content (average of 2.2 particles·g-1) and higher particle sizes (average of 168 µm) than deeper soil layers. The identified sources contributed with 1.02 × 105 particles·m-2·year-1. Irrigation water was the main source, representing 55.9% of the aforementioned input rate encountered for the 3 identified MP sources. Additional knowledge is needed regarding the possible variation in MP content throughout the year and between years and the polymer identification, not only in samples collected from soil but also from its main contamination sources. Furthermore, this study should be extended to other crops and regions as part of broader soil health monitoring.

Mechanisms for iron oxide nanoparticle alleviation of nanoplastic-induced stress in Perilla frutescens revealed by integrated physiological and transcriptomic analysis

Nanoplastics, infiltrating soil ecosystems through diverse pathways such as agricultural practices, sludge application, and atmospheric deposition, present significant potential risks to global ecological systems. Through adsorption, iron oxide nanoparticles (IONPs) could reduce toxicity and bioavailability of nanoplastics in polluted soil ecosystems. However, little is known about how interactions between IONPs and polystyrene nanoplastics (PSNPs) affect plant growth. This study revealed that iron oxide nanoparticles (IONPs) effectively mitigated the uptake of polystyrene nanoplastics (PSNPs) in Perilla frutescens, demonstrating a substantial reduction of PSNPs accumulation by 46.15% in roots and 24.83% in stems. Furthermore, IONPs application significantly improved plant growth parameters, with notable increases of 20.40% in plant height and 34.22% in biomass compared to plants exposed solely to PSNPs.Compared with PSNPs alone, application of PSNPs + IONPs enhanced plant photosynthetic parameters, reduced the quantity of osmotic substances and reduced the activity of antioxidant enzymes. KEGG analysis was concentrated on photosynthetic metabolism and flavonoid synthesis. Further analysis combined with metabolic pathways revealed that IONPs treatment improved plant growth by up-regulating photosystem genes (PsbP, Psak, and PetC) and flavonoid synthesis genes (CHS, CHI, and F3H). Overall, IONPs enhance Perilla frutescens growth by upregulating photosystem-related genes and mitigate PSNPs-induced oxidative stress through flavonoid biosynthesis pathway activation. The present study provides new insights that will aid development of nano iron fertilizers capable of improving the adverse effects of nano plastics on agricultural production.