Correlation of contamination/pollution from microplastics with physical properties of inceptisols

Microplastic contamination of soil is increasingly impacting soil performance in agriculture. This research studied the relationship between microplastic contamination with the physical properties of Inceptisols at different slopes in the horticultural production center of Banuhampu, Agam, West Sumatra. The determination of soil samples was carried out by purposive random sampling with a quantitative survey method. Sample testing using randomized completely block design (RCBD) with five treatments and three replications: 25-45% in forests and 0-8%, 8-15%, 15-25%, and 25-45% in fields at a depth of 0-20 cm. Characteristics of physical properties of Inceptisols contaminated with microplastics have a moisture content of 31.69%, a bulk density of 0.78 g cm−3, a total pore space of 70.67% (without considering the importance of soil organic matter), and a soil texture class of sandy loam (68.19% sand, 19.55% silt, and 12.26% clay). The value of microplastic concentration in Inceptisols was 438.33 particles kg−1. The slope does not significantly affect the physical properties of Inceptisols contaminated with microplastics. The effect of microplastic contamination has a significant relationship with the physical properties of Inceptisols, namely clay with a value of r = 0.650** and a linear equation: Microplastic = 15.474 (Clay) + 248.57; R2 = 0.4228.


Introduction
Plastic was a polymer material that became popular and is now used worldwide.Since their origin, plastics in the environment have proven challenging to regulate.Plastic manufacturing is expanding at an alarming rate, owing to the fact that the amount of plastic recycled is tiny in comparison to the amount produced [1].1315 (2024) 012034 IOP Publishing doi:10.1088/1755-1315/1315/1/012034 2 Plastic degrades gradually, resulting in a large number of microscopic particles entering the environment [2].Microplastics (MPs) are defined as polymer particles smaller than 5 mm in size [3].MPs and their toxicity to organisms.However, knowledge of MPs pollution in soil ecosystems, especially agriculture, is still very limited, because soil as a huge natural sink for the use of potentially polluting materials, especially microplastics, is extensive with increasingly intensive human activities.MPs in soil are estimated to be mostly derived from human activities through waste disposal and wastewater irrigation [4].Especially on agricultural land due to the use of plastic mulch which is used intensively to reduce weeds.
Soils are natural sinks that delay or prevent MPs from entering the environment.However, the effectiveness of soil retention has not been thoroughly explored [5].MPs accumulation in agricultural areas is also linked to unknown effects on soil production and quality.There are various distinct microplastic breakdown mechanisms, all of which have slow speeds dependent on the environment.Degradation mechanisms might be chemical, physical, biological, or a mix of these activities.Polymer degradation activities that contribute to microplastic breakdown through thermal, mechanochemical, ozone, biological, photolysis, and catalytic degradation mechanisms can all have an influence on the toxicity of MPs to the environment [6].MPs breakdown via ozone induction, biodegradation, and photo-oxidative reactions all contribute significantly to the decomposition of "macro" and "meso" plastics, and hence to the creation of microplastics in the soil [7].The impact of MPs is very interesting to study further on agricultural land, especially on soil physical properties.In general, the main soil physical properties such as water content, Bulk density (BD), texture, and others.While the cultivation processes involved are tillage, compaction, percolation, leaching, runoff, and erosion [8].Water and nutrient flow, aeration, soil temperature, nutrient cycling and root development are components of the physical properties of soil quality that impact crop yields and the quality of the agricultural environment.This is also reinforced by the use of plastic mulch in agricultural fields, which is one of the sources of microplastic contamination [9], where plastic mulch was used continuously, with concentrations of 80.3; 308, and 1075.6 particle kg -1 soil in fields that used plastic mulch for 5, 15, and 24 years [10].Plastic mulch is considered a major source of microplastic contamination in agricultural soils.However, due to the presence of various other causes of pollution (Figure 1).However, due to the presence of various other causes of pollution, the impact of MPs with considerable human activity remains unclear [11].The presence of microplastic contamination in soil is feared to have a further impact on soil performance in agriculture.This research studied the relationship between the impact of microplastic contamination and the physical properties of Inceptisol at different slope levels in the horticultural production center of Banuhampu, Agam, West Sumatra.

Methods
This research was conducted from February to July 2023.The location of field observations on several types of land use and slope in the central horticultural production in Banuhampu Agam, West Sumatra.Samples were tested at the Soil Department Laboratory, Faculty of Agriculture, and Basic and Central Laboratory, University of Andalas.

Soil sampling and analysis
Soil sampling includes: (a) Observing the physical condition of the land, where observations will be made in the form of physiographic observation around the research location, and (b) Soil sampling, where the research sample will be in the form of soil samples.Determination of soil samples will be carried out by purposive random sampling.Soil samples to be taken are disturbed soil samples with composites and undisturbed soil samples with rings at a depth of 0-20 cm.Sample testing using randomized completely block design (RCBD) with five treatments and three replications: 25-45% in forests and 0-8%, 8-15%, 15-25%, and 25-45% in fields, so that 15 soil samples were obtained.Survey activities are guided by the Technical Guidelines for Field Observations by the Soil Research Centers [12].The process of analyzing soil samples focused on physical properties such as water content, bulk density, total pore space, and soil texture, as well as data analysis through the Geography Information System (GIS) (Figure 2).

Microplastic analysis
Soil samples have been collected from agricultural fields to analyze microplastic concentrations.Following existing and modified methods [13][14] [15], 1 kg of composited soil samples were collected and placed in transparent glass bottles with sample labels on the outside.In the laboratory, soil samples were prepared based on the technical guidelines for soil analysis.40g of soil samples ≤ 2 mm in size were weighed and put into a 500 ml beaker, 20 ml of H2O2 30% and 20 ml of 0.05 M FeSO4 were added, heated for 30 min to dry, and cooled.The sample was added 120 ml of 5M NaCl, stirred well, and filtered using filter paper.The filtered sediment was analyzed using a photomicroscope to find the number of microplastic particles in the soil.The concentration of microplastics (CMPs) in the soil using formula 1.

Statistic analysis
The results of all analyses were statistically analyzed using SPSS 16.0, Statistic 8, and Excel 2016 software.R 2 coefficient and p-values [** = Significant correlation at the 0.01 (2-tailed) level and * = Significant correlation at the 0.05 (2-tailed) level] were calculated using bivariate correlation and two-sided significance tests.

Characteristics of physical properties of Inceptisols
Characteristics of soil physical properties in the horticultural production center Banuhampu Agam West Sumatra has an average value of bulk density and total pore space is 0.78 g cm -3 and 70.67%.The type of soil found in the horticultural production centers of West Sumatra, classified as Inceptisol (Figure 2B), where the BD of Inceptisol soil ranges from 1.0-1.3g cm -3 [16] and the total pore space of Inceptisol soil ranges from 41.17-42.5% [17].Soil BD is largely determined by naturally formed pores.If the soil pores are large and well-developed, the soil density value is certainly low.If the BD value is close to 1, and if it gets additional organic matter, the BD tends to decrease.The average soil water content is 31.69%,whereas if the bulk density is low or classified as clay, then the water content stored by the soil will also be smaller because the macro pores will be less so it is difficult to store water [18].Soil texture in horticultural production centers Banuhampu Agam West Sumatra is dominated by sandy loam (68.19% sand, 19.55% silt, and 12.26% clay).The ability of soil to retain water is influenced by soil texture because clay-textured soil has a better ability to retain water than sand-textured soil [19].The average concentration of MPs in the soil at the horticultural production center was 43.83 particles kg -1 (Table 1).The concentration of MPs is highly dependent on the shape, type, size, and amount that can reduce or increase soil aggregates to water.Changes in soil structure are important for soil ecosystem function, especially soil physical properties [9].Therefore, MPs strongly influence soil physical properties and as a consequence, MPs can interact with soil components which can be seen from the degree of relationship between them.Soil water content is very important in agricultural ecosystems.The moisture content of Inceptisols in horticultural production centers fluctuated at the observed slope levels.The highest moisture content in the forest (25-45%) was 38.66% and the lowest in the field (25-45%) was 24.84% (Figure 3).The forest has a different topography level from the field, where the steeper the slope, the lower the soil moisture content (Figure 3).On skid trails, soil water content is low due to reduced pore space available for infiltration and water retention at bulk density levels.Soil compaction can reduce moisture content by up to 11% after topsoil removal, but has no effect when topsoil is still present in the forest.The relationship between bulk density and soil moisture content was shown to have a negative relationship, with soil moisture content decreasing with increasing bulk density [20].The BD identification on different slopes in the horticultural production center Banuhampu Agam West Sumatra looks different due to processing factors, cropping patterns, and the level of the slope of each region.It is also seen that the highest BD of both land use and slope is the field which is 0.90 g cm -3 on a slope of 0-8%, followed by a slope of 25-45% (0.87 g cm -3 ) (Figure 4A).The soil bulk density causes soil density to increase, aeration and drainage are disturbed, so that root development becomes abnormal [21].Bulk density values can vary from one point to another in the field due to various factors such as topography and cultivation systems.This is due to variations in soil organic matter, texture, structure, soil clay mineral type, rooting depth, and crop cultivation.Slope steepness has a strong influence on soil physical properties [22].Increasing the slope of the skid trajectory from 10% to 20% corresponded to a 15%, 22%, and 67% decrease in total porosity, water content, and forest floor mass, respectively.Bulk density of sandy loam soil upon increasing the concentration of polyester MPs fibers [23], while on clay loam soil (field and greenhouse), found no difference in soil bulk density [24].
Soil texture is the composition of soil particles with relative proportions of sand, dust, and clay.The soil texture in the horticultural production center Banuhampu Agam West Sumatra is sandy loam which has a rather coarse texture and the soil in general also has a shallow solum (Figure 4B).This soil is included in the Inceptisols order, which is a soil that is undergoing development so that soil minerals have not been fully weathered.This is evidenced by the many rocks found at a depth of 0-20 cm during the field survey.Elevation and slope were discovered to be the most important parameters influencing the geographical distribution of soil types [25].The challenge of identifying the geographical distribution of soil qualities, including texture, in farmlands located at different slopes and elevation classes is difficult due to variations in area complexity, parent material, and microclimate.Clay content is higher in areas with higher altitudes.This might be attributed to erosion and sedimentation processes.

Correlation of microplastics with physical properties in Inceptisols
The identification, 263 microplastics, or an average of 88 particles of slope level were found in Inceptisols in the horticultural production center of Banuhampu Agam West Sumatra at different slope levels.The highest microplastics were found in 0-8% slope (19 particles) and the lowest in the forest (16 particles).This

25-45%
Sandy loam will affect the concentration of microplastics in the soil (Table 2).Slope had no effect on microplastic concentrations.However, microplastic concentrations were higher in fields than in forests.When compared to other slopes, fields with 0-8% slope had the highest microplastic content (475 kg particles).Remarks: ns = non-significant; SE = standard error The microplastics found were assumed to be a result of the continued use of plastic mulch.Plastic film is commonly used in agricultural areas because it can increase soil temperature, keep soil moisture levels stable, and promote seed germination and seedling growth.Plastic film is insoluble and accumulates over time, the longer farmland is covered with plastic film, the greater the amount of microplastics in the soil [26].Microplastics had a significant positive correlation to the clay fraction of Inceptisols (r = 0.650**), with a linear equation y = 15.474x+ 248.57;R² = 0.4228 (Table 3 and Figure 5).Clay has a low density, and MPs are carried through the soil system at greater current velocities from high-energy to low-energy settings.Microplastic contamination or pollution may follow the same pattern as clay fraction values in the soil.Microplastics are more abundant in sediment samples with a higher percentage of clay, suggesting that the distribution and fate of microplastics in estuarine systems are comparable to those of clay particles [27].Soft sediments containing a high percentage of clay and silt can contain more debris simultaneously than hard sediments such as gravel and rock.Therefore, the presence of clay and silt causes the sediment to be a stronger trap for MPs [28].Microplastics had a greater impact on water retention in fine and coarse-textured soils, demonstrating that foreign organic particles change water transport in different soils in various ways [29].Fine-textured clay soils have excellent soil structure, with more fine particles and larger soil micropores.Low microplastic concentrations can enhance the porosity and water permeability of such soils, making them more favorable for plant development.However, they discovered that when the microplastic level grew, the favorable benefits faded, potentially harming plant development.By continually increasing the diameter of the mixed particles, increasing the quantity of microplastic particles adhering to soil particles modifies the pore size distribution in the soil.Microplastics have a positive correlation with clay, assumedly because the degraded plastic particles are retained by the clay, as clay particles are denser than sand or silt.In the event of runoff or erosion, the microplastics have been retained by the clay and are not carried downslope.Therefore, many microplastic samples were found on land uses with 0-8% slope, due to farmers using plastic mulch continuously, over some time.

Conclusion
Characteristics of physical properties of Inceptisol, Banuhampu Agam West Sumatra contaminated with microplastics have a moisture content of 31.69%, a bulk density of 0.78 g cm -3 , a total pore space of 70.67% (without considering the importance of soil organic matter), and a soil texture class of sandy loam (68.19% sand, 19.55% silt, and 12.26% clay).The value of microplastic concentration in Inceptisols was 438.33 particles kg -1 .The slope does not significantly affect the physical properties of Inceptisols contaminated with microplastics.The effect of microplastic contamination has a significant relationship with the physical properties of Inceptisols, namely clay with a value of r = 0.650** and a linear equation: Microplastic = 15.474(Clay) + 248.57;R 2 = 0.4228.Therefore, farmers need to implement the use of organic material-based mulches such as rice straw instead of plastic mulch in the horticultural production center Banuhampu Agam West Sumatra. )

Figure 1 .
Figure 1.The use of plastic mulch as a source of microplastic contamination in the horticultural production center of Banuhampu, Agam, West Sumatra.

Figure 3 .
Figure 3.Effect of slope on the water content in Inceptisol, Banuhampu Agam West Sumatra

Figure 4 .
Figure 4. Effect of slope on bulk density and total pore space (A) and texture (B) in Inceptisol, Banuhampu Agam West Sumatra

Figure 5 .
Figure 5. Linear regression between microplastics with the clay of Inceptisol, Banuhampu Agam West Sumatra

Table 1 .
Descriptive Statistics of microplastics with physical properties in Inceptisol, Banuhampu Agam West Sumatra

Table 2 .
Number and concentration of microplastics in Inceptisol, Banuhampu Agam West Sumatra

Table 3 .
Person correlation coefficient (r) between microplastics with physical properties in Inceptisol, Banuhampu Agam West Sumatra