Microplastics altered soil microbiome and nitrogen cycling: The role of phthalate plasticizer

Highlights

• Changes in microbial community and essential nitrogen cycling processes were examined.

• PVC microplastics enriched pathogenic fungal taxa in the soil studied.

• PVC microplastics (0.5% w/w) resulted in increased NH₄⁺ and decreased NO₃^-.

• Plasticizer released from microplastics was the main driver of changes in soil microbiota and function.

Abstract

Microplastics are emerging contaminants that are increasingly detected in soil environment, but their impact on soil microbiota and related biogeochemical processes remains poorly understood. In particular, the mechanisms involved (e.g., the role of chemical additives) are still elusive. In this study, we found that plasticizer-containing polyvinyl chloride (PVC) microplastics at 0.5% (w/w) significantly increased soil NH₄⁺-N content and decreased NO₃^--N content by up to 91%, and shaped soil microbiota into a microbial system with more nitrogen-fixing microorganisms (as indicated by nifDHK gene abundance), urea decomposers (ureABC genes and urease activity) and nitrate reducers (nasA, NR, NIT-6 and napAB genes), and less nitrifiers (amoC gene and potential nitrification rate). Exposure to plasticizer alone had a similar effect on soil nitrogen parameters but microplastics of pure PVC polymer (either granule or film) had little effect over 60 days, indicating that phthalate plasticizer released from microplastics was the main driver of effects observed. Furthermore, a direct link between phthalate plasticizer, microbial taxonomic changes and altered nitrogen metabolism was established by the isolation of phthalate-degrading bacteria involved in nitrogen cycling. This study highlights the importance of chemical additives in determining the interplay of microplastics with microbes and nutrient cycling, which needs to be considered in future studies.

Introduction

There is increasing evidence that microplastics are ubiquitous in terrestrial environments (Li et al., 2021, Zhang and Liu, 2018), with concentrations of a few mg kg⁻¹ (Zhang et al., 2019) and up to 67 g kg⁻¹ (Fuller and Gautam, 2016) being reported in soil. Plastic mulching, compost/sludge/coated fertilizer application, wastewater irrigation and atmospheric deposition are the main sources of microplastics in soil (Katsumi et al., 2021, Zhu et al., 2019). Plastic production is predicted to increase in the next decades to meet human demand (Liu et al., 2020) and as a consequence microplastic accumulation in soil is also likely to increase.

Soil microbiota play a pivotal role in animal and plant health, and the cycling of nutrients such as nitrogen (Cavicchioli et al., 2019). Any compound(s) that impacts the microbiota or nutrient cycling may result in compromised ecosystem functioning (Rillig et al., 2019). Recently, a few studies have indicated that microplastics may alter soil microbial composition and either positively or negatively affect enzyme activities associated with nitrogen transformation (Fei et al., 2019, Gao et al., 2020b, Gao et al., 2020a, Rong et al., 2021). For example, polyvinyl chloride (PVC) microplastics (5% w/w) caused an increase in the relative abundance of Burkholderiaceae and urease activity in an acidic soil (Fei et al., 2019). In contrast, polystyrene (PS) particles (0.5% w/w) reduced urease and protease activities in rice rhizosphere soil (Dong et al., 2021).

Common microplastic types such as PVC and PS are stable in soil (Otake et al., 1995) and the mechanisms of their effect on soil microbiota are still elusive. The presence of microplastics may alter soil physical conditions such as water holding capacity and bulk density, which in turn may impact the soil microbiota (Boots et al., 2019, de Souza Machado et al., 2019, Iqbal et al., 2020). Furthermore, the release of monomers, oligomers, impurities and additive chemicals from microplastics may also alter soil chemical conditions, which may exert a selective pressure on soil microbes (Iqbal et al., 2020, Sun et al., 2022). However, little is currently known about how chemicals released from microplastics contribute to the risks associated with microplastics, and few has established a direct link between microbial changes/nutrient turnover and chemical release. Of particular concern is the fact that many commercial plastics contain a mixture of additive chemicals. The nature of these additives is often unknown, as companies tend not to share this information for business reasons. These additives are often more toxic than the plastics themselves and could have a greater effect on soil microorganisms (Rillig et al., 2021). Recently, some efforts have been made to assess the role of chemical additives in aquatic and sludge systems and results indicated that leaching of additives might contribute largely to the observed (micro)biological effects (Boyle et al., 2020, Wei et al., 2019). However, little is known about their impact on soil or sediment microbiome.

Phthalate esters are a group of widely used plasticizers, which are added to PVC and sometimes polyethylene (PE) and polypropylene (PP) products to improve their flexibility and durability (Fang et al., 2017, Paluselli et al., 2019). For example, flexible PVC can contain as much as 60% of phthalates (Bouma and Schakel, 2002; IHS Markit, 2021). The presence of phthalates in environmental microplastics has already been documented (Katsumi et al., 2021, Zhang et al., 2018) and although phthalates have been reported to alter microbial community and metabolic activity in a range of soils (Gao et al., 2020a, Gao et al., 2020b, Wang et al., 2016a, Zhu et al., 2018), the contribution of phthalates to the effects of microplastics on soil microbial communities remains unclear.

In summary, although microplastics have been reported to affect soil microbial community composition and activity, the role of plastic additives (e.g., phthalate plasticizer) remains unclear and solid evidence of released additives shaping community composition and function is lacking (de Souza Machado et al., 2019, Seeley et al., 2020). Therefore, the main objectives of this study were: 1) to investigate the role of phthalate plasticizer in any observed effect of microplastics on soil microbial community and nitrogen cycling, by comparing the impact of PVC microplastics prepared with and without dibutyl phthalate (DBP, also known as DnBP); 2) to reveal the relationships between phthalate release, microbial community changes and altered nitrogen metabolism using a combination of high throughput sequencing and pure culture methods. DBP was selected as it is one of the most widely used phthalate plasticizers (Paluselli et al., 2019, Wang et al., 2021a). PVC was selected as phthalates are predominantly used in PVC products (IHS Markit, 2021) and PVC is the third most commonly used polymer globally (Liu et al., 2020). Although PVC has long service life, and thus less abundant in soil than PE, up to 6.7% (w/w) PVC has been detected in hotspots (Fuller and Gautam, 2016). We hypothesized that DBP would play an important role in microplastic-induced effects as phthalates are easier to be utilized by microbes as a carbon source than PVC polymers. Findings from this study are beneficial for predicting how PVC microplastics affect soil health and for identifying key factors determining the risks of microplastics.