Accumulation of microplastics in soil after long-term application of biosolids and atmospheric deposition

Could soil microplastic pollution exacerbate climate change? A meta-analysis of greenhouse gas emissions and global warming potential

Microplastics pollution and climate change are primarily investigated in isolation, despite their joint threat to the environment. Greenhouse gases (GHGs) are emitted during: the production of plastic and rubber, the use and degradation of plastic, and after contamination of environment. This is the first meta-analysis to assess underlying causal relationships and the influence of likely mediators. We included 60 peer-reviewed empirical studies; estimating GHGs emissions effect size and global warming potential (GWP), according to key microplastics properties and soil conditions. We investigated interrelationships with microbe functional gene expression. Overall, microplastics contamination was associated with increased GHGs emissions, with the strongest effect (60%) on CH4 emissions. Polylactic-acid caused 32% higher CO2 emissions, but only 1% of total GWP. Phenol-formaldehyde had the greatest (175%) GWP via 182% increased N2O emissions. Only polystyrene resulted in reduced GWP by 50%, due to N2O mitigation. Polyethylene caused the maximum (60%) CH4 emissions. Shapes of microplastics differed in GWP: fiber had the greatest GWP (66%) whereas beads reduced GWP by 53%. Films substantially increased emissions of all GHGs: 14% CO2, 10% N2O and 60% CH4. Larger-sized microplastics had higher GWP (125%) due to their 9% CO2 and 63% N2O emissions. GWP rose sharply if soil microplastics content exceeded 0.5%. Higher CO2 emissions, ranging from 4% to 20%, arose from soil which was either fine, saturated or had high-carbon content. Higher N2O emissions, ranging from 10% to 95%, arose from soils that had either medium texture, saturated water content or low-carbon content. Both CO2 and N2O emissions were 43%-56% higher from soils with neutral pH. We conclude that microplastics contamination can cause raised GHGs emissions, posing a risk of exacerbating climate-change. We show clear links between GHGs emissions, microplastics properties, soil characteristics and soil microbe functional gene expression. Further research is needed regarding underlying mechanisms and processes.

Ecological effect of microplastics on soil microbe-driven carbon circulation and greenhouse gas emission: A review

Soil organic carbon (SOC) pool, the largest part of terrestrial ecosystem, controls global terrestrial carbon balance and consequently presented carbon cycle-climate feedback in climate projections. Microplastics, (MPs, <5 mm) as common pollutants in soil ecosystems, have an obvious impact on soil-borne carbon circulation by affecting soil microbial processes, which play a central role in regulating SOC conversion. In this review, we initially presented the sources, properties and ecological risks of MPs in soil ecosystem, and then the differentiated effects of MPs on the component of SOC, including dissolved organic carbon, soil microbial biomass carbon and easily oxidized organic carbon varying with the types and concentrations of MPs, the soil types, etc. As research turns into a broader perspective, greenhouse gas emissions dominated by the mineralization of SOC coming into view since it can be significantly affected by MPs and is closely associated with soil microbial respiration. The pathways of MPs impacting soil microbes-driven carbon conversion include changing microbial community structure and composition, the functional enzyme's activity and the abundance and expression of functional genes. However, numerous uncertainties still exist regarding the microbial mechanisms in the deeper biochemical process. More comprehensive studies are necessary to explore the affected footprint and provide guidance for finding the evaluation criterion of MPs affecting climate change.

The microplastics distribution characteristics and their impact on soil physicochemical properties and bacterial communities in food legumes farmland in northern China

Microplastics (MPs) pose a threat to farmland soil quality and crop safety. MPs exist widely in food legumes farmland soil due to the extensive use of agricultural film and organic fertilizer, but their distribution characteristics and their impact on soil environment have not been reported. The abundance and characteristics of MPs, soil physical and chemical properties, and bacterial community composition were investigated in 76 soil samples from five provinces in northern China. The results showed that the abundance of MPs ranged from 1600 to 36,200 items/kg. MPs in soil were mostly fibrous, less than 0.2 mm, and white. Rayon, polyester and polyethylene were the main types of MPs. The influences of MPs on soil physicochemical properties and bacterial communities mainly depended on the type of MPs. Notably, polyethylene significantly decreased the proportion of silt particles, and increased the nitrate nitrogen content as well as the abundance of MPs-degrading bacteria Paenibacillus (p < 0.05). Moreover, bacteria were more sensitive to polyesters in soil with low concentration of organic matter. This study indicated that MPs in food legumes farmland soil presented a higher-level. And, they partially altered soil physicochemical properties, and soil bacteria especially in soil with low organic matter.

Effective removal of nitrate and phosphate using graphene nanosheets synthesized from waste plastics

In this work, waste plastics have been used with bentonite clay to produce silica-containing graphene nanosheets (GNs) for adsorption of nitrate and phosphate from synthetic water. The GNs were obtained by the two steps process, namely (1) pyrolysis at 750 °C and (2) ball milling. Then, GNs were characterized by Raman spectroscopy, FTIR, XRD, FESEM, HRTEM and EDX spectroscopy, which provided the details of material's morphology, surface properties, and composition. From Raman spectroscopy, D and G bands were found at 1342 cm-1 and 1594 cm-1, respectively, which confirmed the presence of nanosheets on the graphene surface. Furthermore, the layers of nanosheets were confirmed by the HRTEM analysis and XRD peaks. In analytical study, the batch experiment was conducted to investigate the influence of operational parameters such as pH (03-12), contact time (05-120 min), adsorbent dosage (0.01-0.06 g), and initial concentrations of adsorbates (10-50 mg/L for nitrate and 03-15 mg/L for phosphate) on adsorption process. The removal percentage of nitrate and phosphate at optimum dosage = 0.05 g, pH = 6.5, contact time = 60 min, nitrate concentration = 30 mg/L, and phosphate concentration = 09 mg/L were found to be 85 and 91, respectively. The highest adsorption capacity of nitrate and phosphate was found to be 53 mg/g and 16.4 mg/g, respectively. The adsorption behaviour of both nitrate and phosphate showed chemisorption as the experimental data were well fitted by the pseudo-2nd-order kinetic and Langmuir isotherm model. Life cycle cost analysis (LCCA) of the synthesis process was conducted to evaluate the cost-benefit analysis for commercial feasibility. The estimated price for the synthesis of GNs using 1 kg of waste plastics and bentonite clay as precursor was $4.21, suggesting commercialization.

Microplastics in soils: Production, behavior process, impact on soil organisms, and related toxicity mechanisms

In recent years, microplastics (MPs) pollution has become a hot ecological issue of global concern and MP pollution in soil is becoming increasingly serious. Studies have shown that MPs have adverse effects on soil biology and ecological functions. Although MPs are evident in soils, identifying their source, abundance, and types is difficult because of the complexity and variability of soil components. In addition, the effects of MPs on soil physicochemical properties (PCP), including direct effects such as direct interaction with soil particles and indirect effects such as the impact on soil organisms, have not been reported in a differentiated manner. Furthermore, at present, the soil ecological effects of MPs are mostly based on biological toxicity reports of their exudate or size effects, whereas the impact of their surface-specific properties (such as environmentally persistent free radicals, surface functional groups, charge, and curvature) on soil ecological functions is not fully understood. Considering this, this paper reviews the latest research findings on the production and behavioral processes of MPs in soil, the effects on soil PCP, the impacts on different soil organisms, and the related toxic mechanisms. The above discussion will enhance further understanding of the behavioral characteristics and risks of MPs in soil ecosystems and provide some theoretical basis for further clarification of the molecular mechanisms of the effects of MPs on soil organisms.