Microplastics in agroecosystems-impacts on ecosystem functions and food chain

Micro- and nanoplastics (MNPs), particles smaller than a millimeter, pose a significant threat to global ecosystems and human health. While their impact on aquatic environments has been extensively studied, knowledge gaps remain regarding their effects on terrestrial and agroecosystems. The increasing global production of plastics (estimated at 335 million tons in 2016, with an annual growth rate of 8.6%), coupled with improper waste management, leads to a substantial accumulation of MNPs in soil and agricultural lands. These persistent pollutants are not easily degradable, and their small size allows them to be readily ingested by organisms, entering the food chain and posing significant risks to human health. This review addresses this knowledge gap by systematically examining the global literature on the ecological impact of MNPs in agroecosystems. The overarching goal is to provide a comprehensive understanding of the sources, occurrence, distribution, transport, and fate of MNPs in these crucial environments. Ultimately, this understanding is critical for developing effective strategies to mitigate the negative consequences of MNPs on food security and human well-being. The review will also explore potential remediation and management approaches to address the escalating challenge of MNP pollution in agroecosystems.

The review extensively examines existing literature on microplastics and nanoplastics in agroecosystems. It synthesizes findings from numerous studies that have investigated the impact of MNPs on various aspects of soil ecology and agriculture. This includes studies focusing on the sources of MNPs in agroecosystems (primary and secondary), their occurrence and distribution in different geographic locations, their fate and transport mechanisms within soil profiles, and their bioavailability and impacts on various soil organisms, microbial communities, and plant growth. Additionally, the review delves into the limited research on the uptake and accumulation of MNPs in plants and their consequences on the food chain. The literature review also incorporates studies on the role of MNPs in soil biogeochemical cycles (carbon and nutrient cycling) and their potential contribution to global warming. Finally, analytical methods employed in detecting and measuring MNPs in agroecosystems are reviewed and assessed for their strengths and weaknesses.

This review employed a systematic approach to analyze the global literature on the ecological impacts of micro- and nanoplastics (MNPs) in agroecosystems. The authors conducted a thorough search of relevant databases, including Web of Science, Scopus, and PubMed, using keywords related to microplastics, nanoplastics, agroecosystems, soil, plants, and ecological impacts. Inclusion criteria were established to ensure the selection of high-quality, peer-reviewed studies that directly addressed the research question. The search yielded a substantial number of publications, which were critically appraised for their methodology, data quality, and relevance to the review's objectives. The extracted data were then systematically synthesized and analyzed to identify patterns, trends, and knowledge gaps. The review focuses on presenting a comprehensive overview of the current state of knowledge, emphasizing the sources, occurrence, distribution, transport, and ecological impacts of MNPs in agroecosystems. The authors organized the review into several sections covering specific aspects of MNP pollution, including: major sources and occurrence, fate and transport, impacts on soil and agroecosystems (including effects on soil biota, plants, nutrient cycling, and the food chain), uptake and mechanisms of uptake by plants, and analytical methods for MNP detection and measurement. A detailed explanation of the analytical techniques, along with their limitations, is provided. Furthermore, the review suggests potential remediation and management strategies to address this growing environmental concern. This included a discussion of the benefits and feasibility of different approaches.

The review highlights several key findings regarding MNP pollution in agroecosystems: 1. **Sources and Occurrence:** MNPs originate from both primary (directly introduced as micro/nanoparticles) and secondary (breakdown of larger plastics) sources. Their occurrence varies geographically, with higher concentrations typically found in areas with high population density and intensive agricultural practices. While data on global MNP distribution in agroecosystems is still limited, available estimates indicate substantial amounts in various regions (e.g., Europe, North America, China, Mexico, Australia). 2. **Fate and Transport:** MNPs in soil undergo both vertical (leaching through soil pores) and horizontal (water runoff, wind) transport. Bioturbation (activities of soil organisms) also plays a role in their distribution. MNPs can reach groundwater through downward migration. 3. **Impacts on Soil and Agroecosystems:** MNPs have various direct and indirect effects. Directly, they can inhibit seed germination and plant growth by physically blocking seed pores and affecting nutrient uptake. Indirectly, they alter soil physical structure, microbial communities, and nutrient cycling. Studies show mixed effects on microbial activity, with some reporting enhanced activity while others show inhibition, possibly depending on MNP type and concentration. Changes in soil properties can alter the composition and functional diversity of soil microbial communities and ecosystem processes. 4. **Uptake by Plants:** While larger MNPs may be unable to directly enter plant tissues, smaller particles can be taken up through various pathways, including endocytosis (by plant cells), stomatal uptake (through leaf stomata), and via the vascular system through root uptake, where transpiration pull is a driving force for particle movement within the plant. The uptake and translocation of MNPs within plants vary based on factors like particle size, shape, and surface charge. 5. **Effects on the Food Chain:** MNPs bioaccumulate in organisms and transfer through trophic levels, presenting potential risks to higher organisms, including humans. Evidence suggests trophic transfer from soil to lower invertebrates (earthworms), then to higher animals (poultry), and finally to humans through consumption of contaminated food. Limited studies highlight potential health risks from MNP ingestion, including oxidative stress, inflammation, and other forms of toxicity. 6. **Effects on Nutrient Cycling:** MNPs can alter soil nutrient cycling. Some studies indicate that they reduce the activity of enzymes involved in nitrogen cycling, affecting nitrogen availability. Conversely, others show enhanced nutrient release, depending on MNP type and concentration. 7. **Effects on Global Warming:** MNPs may impact greenhouse gas emissions, although the specific mechanisms require further investigation. Some studies suggest they can reduce N2O emissions but may also alter CO2 fluxes depending on soil conditions and MNP characteristics. 8. **Analytical Methods:** The review details various analytical methods for detecting and quantifying MNPs in soil, including visual microscopy, FTIR spectroscopy, Raman spectroscopy, GC-MS, and near-infrared spectroscopy (NIR). Each technique has strengths and limitations regarding sensitivity, specificity, and applicability to different MNP types and sizes.

The findings of this review highlight the significant, yet still partially understood, impacts of MNPs on agroecosystems. The variability in reported effects on microbial communities and nutrient cycling emphasizes the need for more research to elucidate the complex interactions between MNP properties (size, type, concentration), soil characteristics, and biological processes. The observation of MNP uptake by plants and transfer through the food chain underscores the potential for MNPs to enter the human food chain, posing a significant health concern. The limited data on long-term effects and the lack of standardized analytical methods remain significant challenges. Future research should focus on developing reliable, efficient, and standardized methods for MNP detection in soils, and on understanding their long-term fate and transformation in the environment. Further research is also needed to better understand the mechanisms behind the observed effects of MNPs on soil biogeochemical cycles and the health risks of MNP accumulation in the food chain.

MNPs are emerging pollutants with far-reaching consequences for agroecosystems and human health. This review synthesizes current knowledge on the sources, fate, transport, and impacts of MNPs in soil and agricultural systems. Addressing this pervasive issue necessitates improved waste management, development of biodegradable alternatives to synthetic plastics, and further investigation into the complex interplay between MNPs and ecological processes. Standardization of MNP detection and quantification methods is crucial for effective monitoring and mitigation strategies. Future research should prioritize long-term studies in diverse agroecosystems, studies focusing on the cumulative impact of multiple pollutants, and a deeper understanding of the health risks associated with MNP exposure.

This review is limited by the existing body of research on MNPs in agroecosystems. Many studies are still in their early stages, and there is a considerable lack of long-term data and standardized methodologies. Geographic variations in MNP occurrence and their impacts also highlight the need for more geographically diverse studies. A significant portion of the studies are laboratory-based; field studies are necessary for a better understanding of the real-world conditions and their complexities. The limited understanding of the specific mechanisms of MNP toxicity to soil organisms, plants, and humans is another significant limitation.