Agricultural soils are prone to accumulate microplastics (MPs) due to the multiple sources of plastics used in agricultural practices (Bläsing and Amelung, 2018; Hurley and Nizzetto, 2018). It was reported that the abundance of MPs in agricultural soils was up to 320-12560 items/kg in Wuhan, central China (Chen et al., 2020). The MP contamination in agricultural soils will alter plant’s growth and developments, disrupt organisms’ digestive/root system, disperse toxic compounds, etc. exerting adverse impacts on soil ecosystems and human health through food chains (Huerta Lwanga et al., 2017; Zhu et al., 2018; Okeke et al., 2022).
Organic fertilizers, which are made of plant-derived materials, animal manures, and agricultural by-products by aerobic composting and/or anaerobic fermentation, are rich in plant essential nutrients and organic carbon (FAO, 2019). The application of organic fertilizers is a widespread management practice in agriculture for enhancing soil sustainability and crop production (Bläsing and Amelung, 2018; Li et al., 2021). However, it has been estimated that organic fertilizer application may lead to an annual microplastic (MP) input of at least 3.5 × 1010 − 2.2 × 1012 items into the environment in Germany (Weithmann et al., 2018). Yang et al. (2021) found the long-term repeated application of pig manures significantly increased MP contents in an agricultural soil by nearly 3 times compared with unamended soil. Thus, organic fertilizers are a non-negligible contamination source of MPs into agricultural soils (Bläsing and Amelung, 2018; Weithmann et al., 2018). However, detailed studies regarding MPs in organic fertilizers are currently lacking.
The abundance and characteristic of MPs in organic fertilizers varies greatly among source materials and their processing. Due to the higher trophic level and more complex food sources of animals in food chains, manures are speculated to have more MPs with varied properties than crop wastes (Huerta Lwanga et al., 2017; Pérez-Guevara et al., 2021). 129.8 ± 82.3 × 103 items/kg MPs and 997 ± 971 items/kg MPs were found in chicken feces from traditional Mayan home gardens in Southeast Mexico (Huerta Lwanga et al., 2017) and sheep feces from the intensive vegetable farming in Southeast Spain (Beriot et al., 2021), respectively. By contrast, Weithmann et al. (2018) only found negligible amounts of MPs (0–11 items/kg dry weight) in the end-of-process digestates from 11 agricultural biogas plants mainly processing regular energy crops and about 20 items per kilogram dry weight in the 8 mm and 15 mm sieved composts from a composting plant processing household waste together with green clippings in Germany, not taking < 1 mm MPs into account. Therefore, it is important to obtain the detailed information of MPs, such as the abundance, size, and polymer type, in both raw materials and end-product fertilizers for the qualified production and the safe application in agriculture (Bläsing and Amelung, 2018; Yang et al., 2021).
Although organic fertilizer has been believed to be an important vehicle for the entry of MPs into agricultural soils, there is limited data available on the presence of MPs in organic fertilizers to support this conclusion, partly due to the difficulties in effective analysis of MPs in the complex and organic-rich substrates (Hurley et al., 2018; Nguyen et al., 2019). Organic matter usually adheres to MP particles, hindering the identification of MPs during visual sorting and spectroscopic analysis (von Sperber et al., 2016; He et al., 2018). Besides, the densities of organic matter are close to those of some plastic types and it may impact the effectiveness of density separation for MPs (Bläsing and Amelung, 2018). Therefore, it is critical to remove organic matter before the density separation for extracting MPs from organic fertilizers. Hurley et al. (2018) compared 30% (v/v) H2O2, 1M and 10 M NaOH, 10% KOH, and Fenton’s reagent for the removal of organic material from soil and sludge samples, and validated Fenton’s reagent as the optimum protocol without significantly affecting the MP extraction efficiencies. While extracting MPs from feces sample, Yan et al. (2020) found Fenton’s reagent cannot fully digest feces solids, and then 65% HNO3 was incorporated to digest the remaining solids. However, it is still unclear if these protocols can be efficient for the MP extraction from organic fertilizers.
Giving that the extensive use of inorganic fertilizers has greatly decreased soil quality and deteriorated the environment, the use of nutrients from reused or recycled organic materials is widely encouraged (FAO, 2019). Especially in China, the government has launched soil organic matter enhancement project (MOA, 2012) and Organic-Substitute-Chemical-Fertilizer (OSCF) action (MOA, 2017) to promote the use of organic fertilizers in order to enhance soil health and fertility (Yi et al., 2021). From 2004 to 2014, the sales of organic fertilizer had increased more than 20 times in China (Du et al., 2020). Under such circumstances, the investigation of MPs in the organic fertilizers is quite necessary. In this study, MPs were extracted from 23 commercial organic fertilizers and 2 farm composts, which were processed differently from a variety of organic wastes that ranged from plant materials to animal manures and litters to agricultural by-products. The abundance, polymer size, type, and morphology of MPs were then investigated by visual sorting and Raman spectroscopy. Our objectives are to (1) compare the MP accumulation in different types of organic fertilizers (animal-derived versus plant-derived, naturally composted versus mechanically processed), thus providing suggestions on the choice of raw materials and processing technology for the manufacture of organic fertilizers; (2) evaluate the potential impact of applying organic fertilizers on the spread and fate of MPs in soils.