The individual and combined effects of polystyrene and silver nanoparticles on nitrogen transformation and bacterial communities in an agricultural soil

Microplastics in heavy metal-contaminated soil drives bacterial community and metabolic changes

Microplastic (MP) and heavy metal pollution in soil are global issues. When MPs invade the soil, they combine with heavy metals and adversely affect soil organisms. Six common MPs-polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyethylene terephthalate, and polytetrafluoroethylene-were selected for this study to examine the effects of various concentrations and MP types on the physicochemical properties, bacterial community, and soil metabolism of heavy metal-contaminated soil. MP enhanced predation and competition among heavy metal-contaminated soil bacteria. Heavy metal-MPs alter metabolites in lipid metabolism, other pathways, and the bacterial community. MP treatment promotes energy production and oxidative stress of soil bacteria to resist the toxicity of heavy metals and degrade MP pollution. In conclusion, MP treatment changed the metabolism of the microbiome in heavy metal-contaminated soil and increased the abundance of Proteobacteria that responded to MPs and heavy metal pollution by 11.54 % on average. This study explored bacteria for the ecological regeneration and provided ideas for MPs and heavy metal-contaminated soil remediation.

Microplastic pollution promotes soil respiration: A global-scale meta-analysis

Microplastic (MP) pollution likely affects global soil carbon (C) dynamics, yet it remains uncertain how and to what extent MP influences soil respiration. Here, we report on a global meta-analysis to determine the effects of MP pollution on the soil microbiome and CO2 emission. We found that MP pollution significantly increased the contents of soil organic C (SOC) (21%) and dissolved organic C (DOC) (12%), the activity of fluorescein diacetate hydrolase (FDAse) (10%), and microbial biomass (17%), but led to a decrease in microbial diversity (3%). In particular, increases in soil C components and microbial biomass further promote CO2 emission (25%) from soil, but with a much higher effect of MPs on these emissions than on soil C components and microbial biomass. The effect could be attributed to the opposite effects of MPs on microbial biomass vs. diversity, as soil MP accumulation recruited some functionally important bacteria and provided additional C substrates for specific heterotrophic microorganisms, while inhibiting the growth of autotrophic taxa (e.g., Chloroflexi, Cyanobacteria). This study reveals that MP pollution can increase soil CO2 emission by causing shifts in the soil microbiome. These results underscore the potential importance of plastic pollution for terrestrial C fluxes, and thus climate feedbacks.

Effect of tire wear particle accumulation on nitrogen removal and greenhouse gases abatement in bioretention systems: Soil characteristics, microbial community, and functional genes

Tire wear particles (TWPs), as predominant microplastics (MPs) in road runoff, can be captured and retained by bioretention systems (BRS). This study aimed to investigate the effect of TWPs accumulation on nitrogen processes, focusing on soil characteristics, microbial community, and functional genes. Two groups of lab-scale bioretention columns containing TWPs (0 and 100 mg g-1) were established. The removal efficiencies of NH4+-N and TN in BRS significantly decreased by 7.60%-24.79% and 1.98%-11.09%, respectively, during the 101 days of TWPs exposure. Interestingly, the emission fluxes of N2O and CO2 were significantly decreased, while the emission flux of CH4 was substantially increased. Furthermore, prolonged TWPs exposure significantly influenced the contents of soil organic matter (increased by 27.07%) and NH4+-N (decreased by 42.15%) in the planting layer. TWPs exposure also significantly increased dehydrogenase activity and substrate-induced respiration rate, thereby promoting microbial metabolism. Microbial sequencing results revealed that TWPs decreased the relative abundance of nitrifying bacteria (Nitrospira and Nitrosomonas) and denitrifying bacteria (Dechloromonas and Thauera), reducing the nitrification rate by 42.24%. PICRUSt2 analysis further indicated that TWPs changed the relative abundance of functional genes related to nitrogen and enzyme-coding genes.

The impacts of microplastics on the cycling of carbon and nitrogen in terrestrial soil ecosystems: Progress and prospects

As contaminants of emerging concern, microplastics (MPs) are ubiquitously present in almost all environmental compartments of the earth, with terrestrial soil ecosystems as the major sink for these contaminants. The accumulation of MPs in the soil can trigger a wide range of effects on soil physical, chemical, and microbial properties, which may in turn cause alterations in the biogeochemical processes of some key elements, such as carbon and nitrogen. Until recently, the effects of MPs on the cycling of carbon and nitrogen in terrestrial soil ecosystems have yet to be fully understood, which necessitates a review to summarize the current research progress and propose suggestions for future studies. The presence of MPs can affect the contents and forms of soil carbon and nitrogen nutrients (e.g., total and dissolved organic carbon, dissolved organic nitrogen, NH4+-N, and NO3--N) and the emissions of CH4, CO2, and N2O by altering soil microbial communities, functional gene expressions, and enzyme activities. Exposure to MPs can also affect plant growth and physiological processes, consequently influencing carbon fixation and nitrogen uptake. Specific effects of MPs on carbon and nitrogen cycling and the associated microbial parameters can vary considerably with MP properties (e.g., dose, polymer type, size, shape, and aging status) and soil types, while the mechanisms of interaction between MPs and soil microbes remain unclear. More comprehensive studies are needed to narrow the current knowledge gaps.

Machine learning prediction and interpretation of the impact of microplastics on soil properties

The annual microplastic (MP) release into soils is 4-23 times higher than that into oceans, significantly impacting soil quality. However, the mechanisms underlying how MPs impact soil properties remain largely unknown. Soil-MP interactions are complex because of soil heterogeneity and varying MP properties. This lack of understanding was exacerbated by the diverse experimental conditions and soil types used in this study. Predicting changes in soil properties in the presence of MPs is challenging, laborious, and time-consuming. To address these issues, machine learning was applied to fit datasets from peer-reviewed publications to predict and interpret how MPs influence soil properties, including pH, dissolved organic carbon (DOC), total P, NO3--N, NH4+-N, and acid phosphatase enzyme activity (acid P). Among the developed models, the gradient boost regression (GBR) model showed the highest R2 (0.86-0.99) compared to the decision tree and random forest models. The GBR model interpretation showed that MP properties contributed more than 50% to altering the acid P and NO3--N concentrations in soils, whereas they had a negligible impact on total P and 10-20% impact on soil pH, DOC, and NH4+-N. Specifically, the size of MPs was the dominant factor influencing acid P (89.3%), pH (71.6%), and DOC (44.5%) in soils. NO3--N was mainly affected by the MP type (52.0%). The NH4+-N was mainly affected by the MP dose (46.8%). The quantitative insights into the impact of MPs on soil properties of this study could aid in understanding the roles of MPs in soil systems.

Co-exposure effects of butyl benzyl phthalate and TiO2 nanomaterials (anatase) on Metaphire guillelmi gut health

Phthalic acid esters (PAEs) and TiO2 nanomaterials (nTiO2) are commonly used as plastic additives, nano-fertilizers or nano-pesticides. Their excessive co-applications led to the co-occurrence, which can induce damage to soil organisms such as Metaphire guillelmi (an earthworm widespread in farmland). However, the co-exposure effects of butyl benzyl phthalate (BBP, a typical PAEs) and nTiO2 on Metaphire guillelmi at environmental-relevant concentrations remain unclear. In this study, 1 mg kg-1 BBP and 1 mg kg-1 nTiO2 (anatase) were added into the soil to assess: (1) their effects on oxidative damage, digestive system, and neurotoxicity in Metaphire guillelmi gut on days 14 and 28; and (2) whether BBP and nTiO2 affected Metaphire guillelmi gut health by disrupting intestinal microorganisms. The results demonstrated that BBP and nTiO2 had the potential to inhibit the activity of superoxide dismutase, cellulase, protease, Na+K+-ATPase, and Ca2+-ATPase, as well as cause oxidative damage by altering intestinal bacteria such as Marmoricola and Microvirga at genus levels after 28 d-exposure. However, the exposure did not cause disorders of the intestinal bacteria. The present study provides more evidence for the sustainable application and scientific management of BBP and nTiO2, thus providing better guidance for PAEs and engineered nanomaterials regulations in agroecosystems.

Reveal molecular mechanism on the effects of silver nanoparticles on nitrogen transformation and related functional microorganisms in an agricultural soil

Silver nanoparticles (AgNPs) are widely present in aquatic and soil environment, raising significant concerns about their impacts on creatures in ecosystem. While the toxicity of AgNPs on microorganisms has been reported, their effects on biogeochemical processes and specific functional microorganisms remain relatively unexplored. In this study, a 28-day microcosmic experiment was conducted to investigate the dose-dependent effects of AgNPs (10 mg and 100 mg Ag kg-1 soil) on nitrogen transformation and functional microorganisms in agricultural soils. The molecular mechanisms were uncovered by examining change in functional microorganisms and metabolic pathways. To enable comparison, the toxicity of positive control with an equivalent Ag+ dose from CH3COOAg was also included. The results indicated that both AgNPs and CH3COOAg enhanced nitrogen fixation and nitrification, corresponding to increased relative abundances of associated functional genes. However, they inhibited denitrification via downregulating nirS, nirK, and nosZ genes as well as reducing nitrate and nitrite reductase activities. In contrast to high dose of AgNPs, low levels increased bacterial diversity. AgNPs and CH3COOAg altered the activities of associated metabolic pathways, resulting in the enrichment of specific taxa that demonstrated tolerance to Ag. At genus level, AgNPs increased the relative abundances of nitrogen-fixing Microvirga and Bacillus by 0.02 %-629.39 % and 14.44 %-30.10 %, respectively, compared with control group (CK). The abundances of denitrifying bacteria, such as Rhodoplanes, Pseudomonas, and Micromonospora, decreased by 19.03 % to 32.55 %, 24.73 % to 50.05 %, and 15.66 % to 76.06 %, respectively, compared to CK. CH3COOAg reduced bacterial network complexity, diminished the symbiosis mode compared to AgNPs. The prediction of genes involved in metabolic pathways related to membrane transporter and cell motility showed sensitive to AgNPs exposure in the soil. Further studies involving metabolomics are necessary to reveal the essential effects of AgNPs and CH3COOAg on biogeochemical cycle of elements in agricultural soil.

Effects of microplastics exposure on soil inorganic nitrogen: A comprehensive synthesis

Microplastics, a growing environmental concern, impact soil inorganic nitrogen (N) transformation, specifically affecting water-extractable nitrate N (NO3--N) and ammonium N (NH4+-N). However, inconsistencies among relevant findings necessitate a systematic analysis. Accordingly, the present meta-analysis addresses these discrepancies by evaluating the effects of microplastics on soil inorganic N and identifying key influencing factors. Our meta-analysis of 216 paired observations from 47 studies demonstrates microplastics exposure causes an overall significant reduction of 7.89% in soil NO3--N concentration, but has no significant impact on NH4+-N concentration. Subgroup analysis further revealed effects of microplastics on soil inorganic N were modulated by microplastics characteristics, experimental conditions (exposure time, experimental temperature, plant effects), and soil properties (soil texture, initial soil pH, initial soil organic carbon, soil total N concentration). We found that microplastics exposure above 27 ℃ enhances soil NO3--N concentration, a finding linked to specific soil properties and conditions, underscoring the impacts of global warming. Importantly, the microplastics polymer type was the most influential predictor of effects on soil NO3--N concentration, while soil NH4+-N concentration was primarily affected by soil texture and microplastics type. These findings illuminate the complex effects of microplastics on soil inorganic N, informing soil management amid increasing microplastics pollution.

Effects of complex pollution by microplastics and heavy metals on soil physicochemical properties and microbial communities under alternate wetting and drying conditions

Microplastics (MPs) broadly coexist with heavy metals (HMs) in soil, Cd and Cu are the main types of soil HMs contamination, in addition to polystyrene (PS), which is also widely present in the environment and prone to aging. However, differences in the effects of MPs and HMs on soil properties and microbial characteristics under alternating wetting and drying (AWD) remain unclear. Thus, this study investigated the effects of four conventional (0.2% (w/w)) and aged MPs in indoor incubation experiments on soil properties under desiccation (Dry) and AWD. We found that with the influence of the "enzyme lock" theory, the coexistence of MPs and HMs under Dry had a more pronounced effect on soil physicochemical properties, whereas the effects on soil enzyme activity under AWD were more significant. In addition, MPs decreased the available Cu by 4.27% and, conversely, increased the available Cd by 8.55%. Under Dry, MPs affected microbial function mainly through physicochemical properties, with a contribution of approximately 72.4%, whereas under AWD enzyme activity and HMs were significantly greater, with increases of 28.2% and 7.9%, respectively. These results indicate that the effects of MPs on environmental variation and microbial profiles under AWD conditions differed significantly from those under Dry.

Polyethylene and polyvinyl chloride microplastics promote soil nitrification and alter the composition of key nitrogen functional bacterial groups

Microplastics (MPs) contamination in soils seriously threatens agroecosystems globally. However, very few studies have been done on the effects of MPs on the soil nitrogen cycle and related functional microorganisms. To assess MP's impact on the soil nitrogen cycle and related functional bacteria, we carried out a one-month soil incubation experiment using typical acidic soil. The soil was amended with alfalfa meal and was spiked with 1% and 5% (mass percentage) of low-density polyethylene (LDPE) and polyvinyl chloride (PVC) MPs. Our results showed that both LDPE and PVC addition significantly increased soil nitrification rate and nitrate reductase activity, which could further promote soil denitrification. The relative abundance of diazotrophs, ammonium oxidizing, and denitrifying bacterial groups were significantly altered with MPs addition. Moreover, the MPs treatments greatly enhanced denitrifying bacteria richness. Redundancy analysis showed that nitrate reductase activity was the most significant factor affecting the soil functional bacterial community. Correlation analysis shows that Nitrosospira genus might be for the improvement of soil nitrification rate. Our results implied that MPs exposure could significantly affect the soil nitrogen cycling in farmland ecosystems by influencing essential nitrogen functional microorganisms and related enzymatic activities.

Microplastics stimulated nitrous oxide emissions primarily through denitrification: A meta-analysis

Microplastics can profoundly alter nitrogen cycling. However, it remains poorly understood how microplastics impact soil nitrogen processes and generate N₂O. A meta-analysis was conducted for this investigation based on 60 published studies to elucidate the effects of microplastics on soil nitrogen cycling, from genes to processes. Under microplastic exposure, the emissions of soil N₂O was significantly increased (140.6%), while the nitrate reductase activities increased by 4.8%. The denitrification rate and number of denitrifier genes were increased by 17.8% and 10.6%, respectively. Meanwhile, the nitrification rate and nitrifier genes were not significantly altered, so did the nitrogen immobilization and mineralization rates. The additional emission of soil N₂O might primarily from stimulated denitrification. Soil N₂O emission and denitrification genes were always increased, regardless of the concentrations of microplastic or experiment duration. As a result, the nitrite was increased by 38.8% and nitrate was decreased by 22.4%, respectively. Interestingly, the N₂O emission increments and copy number of denitrifiers genes diminished over time. This study revealed divergent changes in soil nitrogen processes and highlighted N₂O emissions with a greater denitrification rate under microplastic exposure. The negative impacts of microplastics on soil health were revealed from the perspective of soil nitrogen availability and N₂O emissions.

Silver nanoparticles inhibit nitrogen fixation in soybean (Glycine max) root nodules

Antimicrobial silver nanoparticles (AgNPs) are popular in consumer and industrial products, leading to increasing concentrations in the environment. We tested whether exposure to AgNPs could be detrimental to a microbe, its host plant, and their symbiotic relationship. When subjected to 10 µg/mL AgNPs, growth of Bradyrhizobium japonicum USDA 110 was halted. Axenic nitrogen-fertilized Glycine max seedlings were unaffected by 2.5 µg/mL of 30 nm AgNPs, but growth was inhibited with the same dose of 16 nm AgNPs. With 2.5 µg/mL AgNPs, biomass of inoculated plants was 50% of the control. Bacteroids were not found in nodules on plants treated with 2.5 µg/mL AgNPs and plants given 0.5-2.5 µg/mL AgNPs had 40-65% decreased nitrogen fixation. In conclusion, AgNPs not only interfere with general plant and bacterial growth but also inhibit nodule development and bacterial nitrogen fixation. We should be mindful of not releasing AgNPs to the environment or to agricultural land.

Improving the removal efficiency of nitrogen and organics in vertical-flow constructed wetlands: the correlation of substrate, aeration and microbial activity

Four vertical-flow CWs (VFCWs) with different substrates and aeration conditions were studied on nutrient-removal capacity from synthetic wastewater. Zeolite substrate VFCWs (none-aerated: VFCW-1, aerated: VFCW-3) paralleled with ceramsite (none-aerated:VFCW-2, aerated: VFCW-4) were used to study the removal efficiencies of N and organics, the bacterial community, and the related functional genes. The results indicated that the pollutant removal efficiency was significantly enhanced by intermittent aeration. VFCW-4 (ceramsite with aeration) demonstrated a significant potential to remove NH₄⁺-N (89%), NO₃^--N (78%), TN (71%), and COD (65%). VFCW-3 and VFCW-4 had high abundances of Amx, amoA, and nirK genes, which was related to NH₄⁺-N and NO₂^--N removal. The microbial diversity and structure varied with aeration and substrate conditions. Proteobacteria, Actinobacteria, Candidatus, and Acidobacteria were the main bacteria phyla, with the average proportion of 38%, 21%, 19%, and 7% in the VFCWs. Intermittent aeration increased the abundance of Acidobacteria, which was conducive to the removal of organic matters. Overall, ceramsite substrate combined with intermittent aeration has a great potential in removing pollutants in VFCWs.

Interactions between bacteria and nano (micro)-sized polystyrene particles by bacterial responses and microscopy

Microorganisms play an important role in biogeochemical cycles, and are inevitably found associated with plastic debris. The interplay between microbes and plastics may change the characteristics of certain plastics over time and drive the environmental fate of plastics. In this study, we evaluated interactions of bacteria with nano- and microplastics. Here, polystyrene (PS) polymer particles of various diameters, specifically 60, 220, 430, 700, 1040, 1700, and 2260 nm, were used as the plastics. Escherichia coli (E. coli, gram-negative) and Bacillus sp. (gram-positive) were chosen as model bacteria. The effects of nano- and microPS particles on E. coli and Bacillus sp. cells were investigated by measuring the growth and viability of the cells in laboratory-scale flasks and their generation of reactive oxygen species (ROS) upon their exposure to these particles of 100 mg/L. The particles inhibited the growth and viability of both types of bacterial cells, but their inhibitory effects varied depending on the diameter of PS particle. The 60-nm-diameter PS particles were visually observed to enter the cells as well as accumulate on their surfaces and enhanced ROS generation of the cells. Unexpectedly, the 1040-nm-diameter PS particles, similar in size to the bacterial cells, inhibited the growth of both E. coli and Bacillus sp. cells the most. The E. coli and Bacillus sp. cells formed microPS-biofilm complex by secreting an extracellular polymeric substance (EPS) in response to their exposure to the ∼ 1-μm-diameter PS particles. A positive correlation between relative ROS levels and specific growth rates of the E. coli cells were observed with a Pearson correlation coefficient r value of 0.676 (p < 0.05).