ACADEMIA Letters Understanding the sources of microplastics in agroecosystems Gerald John, Department of Civil and Environmental Engineering, Auburn University Ping Wang, Department of Agricultural Economics, Zhongnan University of Economics and Law Abstract Microplastics are plastic material that are less than 5 mm in diameter and are of great environmental concern. It is well established that microplastics are harmful to both terrestrial and aquatic organisms and can also serve as a vector for accumulating and transporting toxins. Several researchers have reported that biosolids from wastewater treatment plants (WWTP), which are used as fertilizers are the major source of microplastics to the agroecosystems. In this note, we make estimates from various plastics usage and disposal data to test a commonly accepted hypothesis that WWTP biosolids are the major source of microplastics in agroecosystems. Our results show that unrecovered agricultural plastics contributed to the microplastic load at a much higher level than WWTP biosolids, thus falsifying the hypothesis. We recommend that the use of biodegradable/compostable plastics, detention ponds to trap microplastics present in agricultural runoffs, and policy changes to reduce and recycle plastic usage to mitigate the impacts of microplastics in agroecosystems. Academia Letters, October 2021 ©2021 by the authors — Open Access — Distributed under CC BY 4.0 Corresponding Author: Ping Wang, z0004919@zuel.edu Citation: John, G., Wang, P. (2021). Understanding the sources of microplastics in agroecosystems. Academia Letters, Article 3668. https://doi.org/10.20935/AL3668. 1 Introduction Plastics are ubiquitous in the environment and their productions has increased exponentially since their first introduction at the end of World War II (Mason et al., 2016). In 2019, the global production of plastics exceeded 368 × 109 kg (PlasticsEurope, 2020). Plastics debris is increasing in the environment due to an exponential increase in production (Wilcox et al., 2015) and lower degradation rates (Frère et al., 2017). Plastic debris can be found in a wide range of sizes, varying from nanometers to meter scales (Hidalgo-Ruz et al., 2012). Plastic debris particles that are less than 5 mm, known as microplastics, have recently garnered much attention due to their harmful effects on various ecosystem (Fries et al., 2013; Law and Thompson, 2014). Microplastics comprises of two major types – 1) primary microplastics that are produced in microscopic sizes including industrial and pre-production pellets, scrubbers and cosmetic beads; and 2) secondary microplastics that are formed as a result of weathering of larger plastic debris through mechanical, photo-oxidative and other degradation pathways (Alimi et al., 2018; Hidalgo-Ruz et al., 2012). It is well known that microplastics are toxic to both terrestrial (Lu et al., 2018; Rodriguez-Seijo et al., 2017; Yang et al., 2014) and aquatic organisms (Jeong et al., 2016; Lu et al., 2016; Nobre et al., 2015; Ribeiro et al., 2017), and also serve as a vector for accumulating and transporting toxins (Brandts et al., 2018; Lin et al., 2019). Recently nanoplastics, which are particles that are 1 to 1000 nm in diameter, have also gained significant attention (Alimi et al., 2018; Gigault et al., 2018). Wastewater treatment plants (WWTPs) are considered as one of the major sources of microplastics to the ecosystem both through its effluent and through the disposal of biosolids on croplands. A well designed WWTPs can remove >90% of microplastics before the effluent is discharged into the receiving water body and hence most of the microplastics are retained in the biosolids (Bretas Alvim et al., 2020; Rolsky et al., 2020). These biosolids are disposed of either through incineration, buried in landfills, or applied as fertilizers on croplands (USEPA, 2019). The fraction of biosolids that are applied to croplands as fertilizers are considered as the major source of microplastics to the agroecosystems by many researchers (Crossman et al., 2020; Kumar et al., 2020; Ng et al., 2018). The objective of this note is to test the hypothesis that in the US the biosolids are the major source of microplastics to the agroecosystems. We used literature data to estimate the microplastics load to the agroecosystems through biosolids and other sources to test this hypothesis. Academia Letters, October 2021 ©2021 by the authors — Open Access — Distributed under CC BY 4.0 Corresponding Author: Ping Wang, z0004919@zuel.edu Citation: John, G., Wang, P. (2021). Understanding the sources of microplastics in agroecosystems. Academia Letters, Article 3668. https://doi.org/10.20935/AL3668. 2 Estimation of biosolids applied on croplands Agroecosystems is one of the most microplastics contaminated terrestrial systems besides landfills, urban spaces, and beaches (Crossman et al., 2020). Several researchers have pointed out that biosolids from WWTPs that are applied as fertilizers on croplands are the major sources of microplastics in the agroecosystems (Alimi et al., 2018; Bretas Alvim et al., 2020; Kumar et al., 2020; Li et al., 2018). In US, there are approximately 320×106 acres of cropland that are used for growing a wide variety of crops including corn, soybeans, wheat, rice, vegetables, fruits, nuts, cotton, and livestock feed (USDA, 2019a; USDA, 2019b). In 2019, the US generated about 4.75×109 kg of dry biosolids. Of this mass, about 29% is applied on croplands, which is about 1.4×109 kg dry weight of biosolids being applied as fertilizers (USEPA, 2019). Since the acreage of land on which biosolids were applied are not available in public domain, an estimate was made using the amount of biosolids applied on croplands and the recommended application rate for two major US crops – corn and soybeans. The typical annual application rate of biosolids for corn and soybeans are 5 to 10 dry tons (or 4.5×103 to 9.1×103 kg) per acre and 5 to 20 dry tons (or 4.5×103 to 18.1×103 kg) per acre (USEPA, 2000), respectively. So, the maximum acreage of land that received biosolids is 309×103 acres. Though it has some degree of error, in case on non-availability of data, this the best possible approach to make an estimate. Since the total cropland area in the US is 320×106 acres (USDA, 2019a; USDA, 2019b), this estimate is about 0.1% of the total cropland, a relatively very small area. Estimation of plastics used in agroecosystems to aid crop productivity Plastics that are used in agricultural practices are termed as “agricultural plastics” and are used to increase crop productivity which ensures food safety (Scarascia-Mugnozza et al., 2012). Agricultural plastics are used in various shapes and forms – films as mulches, greenhouses, tunnels, and drip tapes; nets to protect crops from pests and birds; tubular forms as irrigation and drainage pipes; packaging for fertilizers, pesticides, and others (Jones, 2018; ScarasciaMugnozza et al., 2012). The US demand for plastics in 2017 was about 35.4×109 kg (USEPA, 2017). The demand for agricultural plastics in the US is not available in public domain. In Europe, the demand for agricultural plastics in 2017 was about 3.4% of the total plastics demand (PlasticsEurope, 2018). Since many European countries and US are developed economies, in this study we assumed that US would also have a similar demand for agricultural plastics compared to other sectors. Based on this assumption, the demand for agricultural plastics in Academia Letters, October 2021 ©2021 by the authors — Open Access — Distributed under CC BY 4.0 Corresponding Author: Ping Wang, z0004919@zuel.edu Citation: John, G., Wang, P. (2021). Understanding the sources of microplastics in agroecosystems. Academia Letters, Article 3668. https://doi.org/10.20935/AL3668. 3 the US in 2017 is estimated to be about 1.2×109 kg. Comparison of microplastics entering agroecosystems through biosolids vs. agricultural plastics The quantity of microplastics present in biosolids varies depending on the geographic location and the city the WWTP serves. The quantity of microplastics is also being reported in both numbers and mass per kg dry weight of biosolids. The estimated number of microplastics derived from biosolids is in the range of 1,000 to 170,000 particles per kg dry weight of biosolids (Li et al., 2018; Mahon et al., 2017; Mohajerani and Karabatak, 2020; Rolsky et al., 2020). The mass of microplastics is in the range of 471 to 6,600 mg per kg dry weight of biosolids (Crossman et al., 2020; Okoffo et al., 2020). Based on these reported values and 1.4×109 kg of dry biosolids applied on cropland, the microplastics that enter the agroecosystems annually through biosolids are in the range of 1.4×1012 to 2.4×1014 particles or 0.6×106 to 9.2×106 kg (4.9×106 kg average). Though this number is high, as the question is whether this is a major source of microplastics to agroecosystems when compared to other sources? Figure 1: Comparison of annual microplastics load onto agroecosystems in USA through WWTP biosolids and unrecovered agricultural plastics Academia Letters, October 2021 ©2021 by the authors — Open Access — Distributed under CC BY 4.0 Corresponding Author: Ping Wang, z0004919@zuel.edu Citation: John, G., Wang, P. (2021). Understanding the sources of microplastics in agroecosystems. Academia Letters, Article 3668. https://doi.org/10.20935/AL3668. 4 About 80% of the agricultural plastics market is dominated by linear low density polyethylene (LLDPE) plastic films (Muise et al., 2016). These plastic films are of single use and there are less incentives for reuse because the removal and reuse steps are labor intensive. In addition, disposal is costly and options for reusing and recycling are difficult due to soiling and mechanical damage. These plastic films are either intentionally left to be tilled (Steinmetz et al., 2016) or illegally burnt (Brodhagen et al., 2017). In US states like Florida, agricultural plastic film wastes can be burnt legally (FloridaStatutes, 2019). The usage of plastics in agriculture has also increased the plastic residue content of the soil (Liu et al., 2014). The recovery rate of agricultural plastics is only about 50% (González-Sánchez et al., 2014; PlasticsEurope, 2012). Since we estimated that 1.2×109 kg of plastics would have been used in agriculture, at this 50% recovery rate, it can be estimated that annually about 0.6×109 kg of plastics will be left in agroecosystems. These plastics that are not recovered from the croplands eventually lose their integrity and breakdown into smaller particles forming microplastics (Steinmetz et al., 2016). If all the unrecovered agricultural plastics transform into microplastics, the microplastics annual load entering agroecosystems is about 1,000 times more than the microplastics introduced through WWTP biosolids. Concluding remarks A comparison of the annual microplastics load released into agroecosystems through biosolids application and from the use of agricultural plastics has been completed. This is the first time such a quantitative comparison has been made. Based on this analysis, we conclude that the agricultural plastics that are not recovered from the croplands after the growing season is the major source of microplastics in agroecosystems when compared to the microplastics released from biosolids application. We further estimated that the amount of microplastics derived from biosolids are at least 1,000 times less than unrecovered agricultural plastics and thus falsifying our research hypothesis that the biosolids are the major source of microplastics to the agroecosystems. We make the following recommendations to mitigate the impacts of microplastics contamination on agroecosystems. 1. Promote the usage of compostable/biodegradable plastics (Brodhagen et al., 2015; Li et al., 2014); further prioritize the production of these types of materials to reduce the load of microplastics into agroecosystems. 2. Use detention ponds (Liu et al., 2019; Piñon-Colin et al., 2020) that can trap and prevent the transport of microplastics present in the agricultural runoffs to receiving water bodies. Academia Letters, October 2021 ©2021 by the authors — Open Access — Distributed under CC BY 4.0 Corresponding Author: Ping Wang, z0004919@zuel.edu Citation: John, G., Wang, P. (2021). Understanding the sources of microplastics in agroecosystems. Academia Letters, Article 3668. https://doi.org/10.20935/AL3668. 5 3. Implement policies that can help increase the recovery of agricultural plastics (Brodhagen et al., 2017), provide economic incentives for recycling, and prioritize the research and increased application of biodegradable/compostable plastics. Acknowledgements The author likes to thank Dr. Prabhakar Clement at the University of Alabama for help in providing critical review comments. The authors also like to thank the reviewers of Science of the Total Environment for their valuable comments. Funding This research note was supported by the Fundamental Research Funds for the Central Universities, Zhongnan University of Economics and Law (Program No. 31512110805). Conflict of interest The authors declare there are no conflict of interest. Academia Letters, October 2021 ©2021 by the authors — Open Access — Distributed under CC BY 4.0 Corresponding Author: Ping Wang, z0004919@zuel.edu Citation: John, G., Wang, P. (2021). Understanding the sources of microplastics in agroecosystems. Academia Letters, Article 3668. https://doi.org/10.20935/AL3668. 6 References • Alimi OS, Farner Budarz J, Hernandez LM, Tufenkji N. Microplastics and Nanoplastics in Aquatic Environments: Aggregation, Deposition, and Enhanced Contaminant Transport. 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Academia Letters, October 2021 ©2021 by the authors — Open Access — Distributed under CC BY 4.0 Corresponding Author: Ping Wang, z0004919@zuel.edu Citation: John, G., Wang, P. (2021). Understanding the sources of microplastics in agroecosystems. Academia Letters, Article 3668. https://doi.org/10.20935/AL3668. 11