EFFECT OF MICROPLASTICS ON SOIL PHYSIOCHEMICAL PROPERTIES


The plastic industry has grown drastically since 1950s, reporting 348 million tons of global plastic production in 2017 and by 2050 it is likely to reach a production level of 33 billion tons.

By Farwa Akhtar1, Zahoor Ahmad1* and Orooj Surriya2

The extensive production, disposal and degradation of plastics have contributed an adverse effect on the environment, which may persist for centuries. The plastic debris ranges in size from meters to nanoparticles, however microplastics are the emerging contaminants that pose potential global environmental threats to terrestrial and aquatic ecosystems.

Microplastics are very small pieces of plastic which are less than 5 mm in size. They can be derived from primary or secondary sources of plastics. The primary sources include microplastics which are used in medical and cosmetics products. Secondary sources involve large plastic fragments which are degraded by chemical, physical and biological processes into microplastics. Microplastics are a broad category of polymer-based plastics such as include polyproplylene (PP), polystyrene (PS) and polyethylene (PE), that have emerged enduring depictions of human waste and environmental degradation. It is understood that microplastics contribute to global warming by contaminating natural ecosystems. Microplastics pose a potential risk to both the environment and health of living organisms. The identification, prevalence, characterization, and toxicity of microplastics have been the subject of recent studies. Microplastics existence in soil ecosystems has increasingly attracted a lot of attention. Compared to aquatic habitats, our knowledge of the ecological consequences of microplastics in soil ecosystems is very restricted. Rubber wear is a potentially significant source of microplastics in soils, however it is yet unclear how common these particles are in comparison to other particle kinds of microplastic polymers.

Microplastics pollution is now viewed as a new danger to ecosystem health and biodiversity. The breakdown of microplastics in soil aggregates can change the physicochemical properties of soil, including its capacity to store water, bulk density, topologies, and pH. Although it has been recognized that soil habitats, particularly agricultural land are a significant sink for microplastics; little is known about the effects of microplastic pollution on ecosystem functioning (e.g. above and below ground). Microplastics in soils can alter the physicochemical characteristics of soil, the composition of the soil microbial community, rooting capacity and photosynthesis of plants. Microplastics alter the composition and structure of the soil, resulting in a series of changes to the soil environment. The shape, size, or content of plastic particles had visible effects significantly different from that of the organic soil’s particles. The physicochemical characteristics of the soil, such as its organic materials, total nitrogen concentrations, electrical conductivity, water holding capacity and microbial population, were also influenced by microplastics. The sediments normally screened out of wastewater to be utilized as fertilizer for agricultural fields, known as sewage sludge; can also introduce microplastics into agricultural soils as a result of wastewater contamination due to detergent, personal care items and urban runoff products that are primary sources of microplastics in wastewater.

Enzymatic activity and soil physicochemical characteristics combine to form the primary markers of soil quality. Increase in pH is observed due to the increase in availability of oxygen and porosity brought on by the addition of microplastic foams and fragments. Microplastics have been found to directly interfere with the equilibrium of plant chlorophyll a/chlorophyll b ratios, modification of soil bulk density, water-holding capacity and reduction in photosynthetic rate in plants. Additionally, microplastics interfere with biotic or abiotic variables, such as sesquioxide and exchangeable cations, to impact the stability of aggregation (e.g., soil organic matter and organism activities in the soil). The loss in soil bulk density, results in changed soil pore structure and water movement is one of the main direct impacts of microplastic. Additionally, fibers in particular can have a significant impact on soil structure, namely on the stability and size distribution of soil aggregates. Although the ability of microplastic fibers to alter soil characteristics has already been studied but nothing is known about how these fibers would affect various soil types. Additionally, nothing is known about how microplastics presence may alter soil water erosion processes, which are critical problems in many settings.

Microplastics effects the ecosystem and functionality of soil. In bulk soils and the rhizosphere, the presence of microplastics had a substantial impact on soil bulk density, water-stable aggregates and soil structure. In the absence or presence of plants, these changes in soil structure had a major impact on water evaporation, water availability and soil microbial activity. By both direct and indirect means, microplastics have the capacity to change how organisms develop and go through their life cycles, with the potential to eventually enter food chains. For example, microplastics may have an impact on cycling soil nutrients by changing the dominant bacterial phyla in the soil or the genes and enzymes involved in the carbon, nitrogen, and phosphorus cycle. Microplastics adhered to the root surface physically prevent pollutants from making contact with the roots when they have combined effects on plants with other pollutants, but they are more likely to increase the harm that pollutants do to plants. The soil-plant system is affected differently by microplastics of various types, sizes and concentrations. Microfibers, small-sized microplastics and biodegradable plastic particles have greater noticeable effects than microplastics that resemble soil particles in size and shape.

Microplastic pollution has a deleterious impact on ecological processes in terrestrial habitats by altering microbial activity and soil characteristics (elemental composition and organic matter). However, depending on the type of polymer, changes in soil characteristics may be very different. The ability of plants to absorb macro and micronutrients (such as potassium) is altered as a result of biophysical and chemical changes in the soil, which also have an impact on plant growth. However, a variety of variations in the impact of microplastics on plants depend on the species under consideration, the amount of exposure time used during treatments, concentration, size, shape and chemical makeup of the plastic particles. These microplastics in plants may break down and produce harmful substances (like benzene), which would interfere with the regular metabolism of plants. Although microplastics with a comparatively higher molecular weight cannot be ingested by plant roots, they may still have an adverse influence on the growth of the targeted plants through a variety of mechanisms, including blocking cell wall pores, obstructing sunlight, harming root tissue and changing the physicochemical characteristics of soils.

 In mycorrhizal associations PES pathogens with an average length of 5 mm greatly boosted the amount of soil microorganisms colonizing A. fistulosum roots. Mycorrhizal relationships can aid in the uptake of nutrients, which could enhance plant biomass. The conventional plastic treatments lowers WFPS (Water Filled Pore Space), which could be explained by the fact that conventional PE microplastic is more hydrophobic than biodegradable PHBV. This suggests that water was repelled from the plastic particles, allowing more oxygen to pass between the microplastic and soil micro aggregates, limiting de-nitrification and subsequently allowing N2O emissions. In contrast to typical plastic mulch films, biodegradable polymers such as polylactic acid (PLA) mulch films are expected to totally dissolve into water and carbon dioxide. However, their breakdown proved unexpected in different soil settings, which may also yield microplastics. Their effects on plant biomass, tissue elemental composition, root characteristics, and seed germination should be studied.

Biodegradable plastics may be the potential future savior for controlling microplastics production. However, with regards to already existing microplastics in the environment a promising approach could be used by biodegradation of microplastics using microorganisms employed for the breakdown of synthetic plastic polymers. For example Pseudomonas sp., Staphylococcus sp. and Bacillus sp. isolated from soil can degrade polyethylene, Rhodococcus ruber can break down polystyrene and Pseudomonas putida can degrade polyvinyl chloride etc. Thus biodegradation is an environmentally safe option which can be used in future to control microplastics contamination. 

Authors:

Farwa Akhtar1, Zahoor Ahmad1* and Orooj Surriya2

1Department of Botany, Punjab Group of Colleges, University Campus, Bahawalpur 63100, Pakistan

2Department of Zoology, Punjab Group of Colleges, University Campus, Bahawalpur 63100, Pakistan



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