Climate change is one of the most important burning questions of the today’s world. Climate change affects all kinds of ecosystems at multiple levels. Researchers found four key impacts of climate change: 1) Changes in species and populations; 2) Changes in the timing of natural events and cycles; 3) Changes in ecosystem interactions; 4) Altered/reduced ecosystem services [1] . According to Cox et al. [2] , fossil fuels—coal, oil and gas, are by far the largest contributor to global climate change, accounting for over 75% of global greenhouse gas emissions and approximately 90% of all carbon dioxide (CO₂) emissions. Other factors of climate change include power generation, industry, disruption of forest ecosystems, excessive consumption of power, production and use of chemical fertilizers for enhancing crop productivity and combustion of petroleum-based products. Not only the anthropogenic activities are responsible for climate change but climate change can be attributed to natural phenomena also. Volcanic eruptions, fluctuations in solar radiation, tectonic shifts, and changes in earth’s orbit have observable effects on planetary warming and cooling patterns leading to a shift in global climate [3] [4] .

Carbon dioxide (CO₂), carbon monoxide (CO), methane (CH₄), nitrogen oxides (NO_x) and volatile organic compounds (VOC_s) are important drivers of the anthropogenic greenhouse effects, which are released into the environment through various means [2] [3] . The sources of greenhouse gases emissions include electricity (31%), agricultural practices (11%), transportation (15%), burning of fossil and forestry (6%), as well as manufacturing (12%) [4] . Over time, the emissions have contributed to the overall effects of global warming. Every year the worldwide CO₂ emissions from energy needs increases, and by the year 2050 the global CO₂ emissions are forecast to increase to some 43.08 billion metric tons, in comparison to 35.30 billion metric tons of CO₂ in 2018 [5] .

It is well documented that emission from the agricultural soils contributes to environmental and human health problems [2] [4] . The greenhouse gases emission from the soil to the atmosphere is the primary mechanism of nutrient loss from the soil [4] which in turn contributes to the global climate change. For example, emission of CO₂ plays a significant role in the loss of carbon from rural and agricultural soils. Terrestrial ecosystems that are now considered as net sinks for CO₂ might become net CO₂ sources after about 2050, if the projected temperature rise becomes a reality [6] . Agricultural intensification is associated with a number of carbon and nitrogen containing compounds, including nitric oxide, nitrous oxide, nitrogen dioxide, methane, carbon dioxide, carbon monoxide and volatile organic compounds, which are emitted to the atmosphere through agricultural operations [7] .

Marine sediments, freshwater reservoirs and soils are important sources of greenhouse gases emissions [6] [8] . Most studies [9] [10] [11] to date mentioned the impacts of microplastics on the marine and freshwater ecosystems. However, the impacts of microplastics on the soil ecosystem is often overlooked due to the complex, heterogenous nature of the soil [9] [11] . Compared with marine sediments, soils are basically in an aerobic environment and exchange materials with the atmosphere through the surrounding air. Thus, immediate attention should be paid to that whether microplastics will affect the soil ecological environment and greenhouse gases emissions. Current concerns about the impact of microplastics on soil ecosystems are mainly concentrated on soil properties [12] , microbial diversity [13] , and crop productivity [9] [10] . Few studies [14] [15] focused on the impacts of microplastics on greenhouse gases emissions although the findings varied depending on the shape and size of the microplastic, soil inherent characteristics (pH, texture, structure, moisture content, etc.) and plant species. The impact of the microplastics on the greenhouse gases emissions from the soil flux will help us to explore impacts on the biogeochemical cycles particularly carbon and nitrogen cycles.

The aims of the present experiment were therefore to determine: 1) The impacts of high density polyethylene (HDPE) microplastic on the emission of greenhouse gases from the soil; 2) The impacts of low density polyethylene (LDPE) microplastic on the emission of greenhouse gases from the soil; 3) Whether UV weathered microplastic emits more greenhouse gases than the original microplastic; 4) Whether different doses of microplastic and days of exposure have impacts on the greenhouse gases emissions from the soil flux.

Our data indicated that the emissions of various greenhouse gases (CO₂, CO, VOC_s, CH₄, NO, NO₂ and N₂O (Figures 1-4; Table 2, Table 3) from the soil flux

were significantly affected by the microplastic treatments. Emissions were increased with the increases in microplastic treatments which were consistent with previous studies [11] [24] although the authors used different type of microplastics. Emission of CO₂ (Figure 1) is linked to microbial respiration; higher emission could be due to higher microbial respiration in the soil. Microplastic contains carbon (typically around 80%) and the carbon acts as a food source for the soil microbes [9] . The microplastics could feed the soil microbes causing a priming effect which led to increased microbial activity with potential increases in microbial respiration leading to the higher emission of CO₂ from the soil. Higher emissions of greenhouse gases with the application of high levels of microplastic treatments as suggested by [10] [25] was the leaching of additives and monomers (atoms/molecules that bond together to form polymer) that could provide a food source for the soil microbes.

[26] observed increased respiration with the increases in microplastic doses which supported our findings. Weathered microplastics treated soils had higher emissions of greenhouse gases compared to that of the soils treated with the original form of microplastics which might be due to the increased surface area of the weathered microplastics. By increasing the surface area, chemical processes acted more easily upon the microplastic surface [27] . [28] and [29] observed increased adsorption of metals by the UV weathered microplastics. They suggested that that the increased adsorption was due to the increases in surface area and greater heterogeneity [29] .

The increases in the emissions of CH₄ and NO_x (Figure 4; Table 2, Table 3) from the microplastic treated soils were not in line with previous literatures [7] [10] [24] . However, the authors conducted their experiments in marine sediments where ocean currents, marine plants and marine microbes played a significant role in the emission of greenhouse gases from the sediment. The physiology of marine plant and microbes are completely different from the terrestrial ones. UV weathering is also pronounced in the marine environments rather than the terrestrial ecosystems [30] . Microplastics could affect microbial community while entering the marine sediment leading to a change in the ecological environment. This could, in turn, lead to a change in the emissions of greenhouse gases and ultimately changes on the global climate. Authors [24] mentioned that the presence of polyethylene terephthalate microplastics reduced the bacterial diversity in paddy soils which resulted in a reduction of cumulative CH₄ emissions by 53%. Polyethylene (PE) microplastics in fertilized soils reduced the global warming by 2% by decreasing N₂O emissions. However, this PE microplastics accelerated the formation of aromatic functional groups which had a detrimental effect on microorganisms [15] . Presence of LDPE microplastics reduced the emissions of CO₂ and N₂O along with the microbial biomass carbon content in the soil [31] . Microplastics significantly affected NO_x emissions by affecting microbial diversity and community structure related to inorganic nitrogen content in overlying water. Studies showed that microplastics increased activities of oxidizing bacteria (e.g. Betaproteobacteria, Gammaproteobacteria and Proteobacteria) which resulted in increased microbial biomass [32] . These findings supported our present study. Betaproteobacteria and Gammaproteobacteria can oxidize ammonia ( NH 4 + ) to nitrate (NO₂) whereas Proteobacteria have the ability to reduce NO₂ to nitric oxide (NO). Thus, the emissions of NO₂ and NO gases from the soil were significantly higher from the microplastic treated soils (Table 2, Table 3).

The addition of microplastics affected both nitrification and denitrification processes at different times in freshwater sediments [15] . With the increase of exposure time of microplastics, the denitrification was gradually enhanced, followed by an increased content of N₂O. Thus the presence of microplastics could affect nitrogen cycle in freshwater sediment by emissions of NO_x. However, due to different particle sizes, the peak values of N₂O and NO in water measured in various treatment groups were different. It becomes imperative to determine the peak and timing of the effects of microplastics with different particle sizes in the long-term exposure experiment.

Carbon monoxide (CO) is produced from the decomposition of organic matters present in the soil. The microplastics could enhance the activity of decomposing microbes in the soil which facilitated the decomposition of organic matters leading to the higher emissions of CO. Studies [24] [32] hypothesized that the application of microplastics to soils resulted in greater soil carbon sequestration potential that would reduce CO emission which contradicted our study. It is well documented that the microplastics have cracks on their surfaces [33] which might enable a wide range of volatile organic compounds (VOC_s) to be retained in the cracks of microplastics. This was in agreement with our present observations; we found comparatively higher emissions of VOC_s from the microplastic treated soils than the controls.

Global climate change is not a future problem; it is affecting our world at present. Changes to the earth’s climate driven by increased emissions of heat-trapping greenhouse gases are already having widespread effects on the environment, and consequently glaciers are shrinking, river and lake ice sheets are breaking up earlier, geographic ranges of plants and animals are shifting, as well as plants are blooming sooner. The Intergovernmental Panel on Climate Change (IPCC) [34] stated that “the magnitude and rate of climate change and associated risks depend strongly on near-term mitigation and adaptation actions, and projected adverse impacts and related losses and damages escalate with every increment of global warming”. Cheng [35] and Yadvinder [36] mentioned that “the scientific evidence is unequivocal; climate change is a threat to human wellbeing and the health of the planet”.

At present, some changes in our global climate (such as droughts, wildfires, and extreme rainfall) are happening faster than scientists previously assessed [35] [36] [37] . According to the report of IPCC [34] , human emissions of heat-trapping gases have already warmed the climate by nearly 2˚F (1.1˚C) since 1850-1900. The global average temperature is expected to reach or exceed 1.5˚C (about 3˚F) within the next few decades. These changes will affect all regions of Earth. The magnitude of climate change effects will depend on the human activities [37] . More greenhouse gas emissions will lead to more climate extremes and widespread damaging effects across our planet [38] [39] .

This study examined how the microplastics impact on greenhouse gases emissions from the soil, and whether these impacts were varied depending on the microplastic type, microplastic concentration and days of exposure. The experiment was designed with two types of microplastics (HDPE and LDPE) and four levels of microplastic treatments (0%, 0.01%, 0.1% and 1.0%). We also used original (untreated) and weathered microplastics (UV treated) of the same HDPE and LDPE to investigate the impacts of original and weathered microplastics on the emissions of greenhouse gases. This study showed that both HDPE and LDPE microplastics significantly increased the emissions of greenhouse gases from the soil than that of the control soils. The higher the concentration of microplastic, the greater was the emission of greenhouse gas. The weathered microplastic emitted greater quantity of gases compared to that of the original microplastics which could be due to the higher surface area and increased heterogeneity of the weathered microplastic. In contrast to a low initial emission quantity, the emissions were gradually increased at the termination of the experiment. Given the higher emission quantities of greenhouse gases from the soil flux due to the presence of microplastics in soil it seems likely to impact global climate change. A more detailed investigation into the greenhouse gases emissions would be required coupled with climatic parameters (temperature, precipitation, relative humidity, etc.) to cast further light on this. The natural progression for future work from this experiment would be field scale studies that would aim to understand how the microplastic alters the greenhouse gases emissions in the natural environment. Overall our results showed that microplastic impacts had consequences for greenhouse gases emissions in the terrestrial ecosystems. These impacts depend on the type and concentration of the microplastic as well as the days of exposure to the microplastic.

The authors declare no conflicts of interest regarding the publication of this paper.

Khan, T.F. and Rabbani, M.M.G. (2024) Microplastic Impacts on Greenhouse Gases Emissions in Terrestrial Ecosystems. Open Journal of Soil Science, 14, 64-80. https://doi.org/10.4236/ojss.2024.141004