Microplastic emission by the use of plastic mulch films in agriculture

Microplastic emission by the use of plastic

mulch films in agriculture

Bachelor Project Future Planet Studies

Student: Carlijn Zeeberg

Supervisor: Dr. A. Praetorius

Daily supervisor: Dr. R.P.J. Hoondert

Date: 30-05-2021

Word count: 4154

Future Planet Studies

Abstract

This research is about the usage of plastic mulch films in agriculture and their role in emission of microplastics into surrounding streams. This study uses an emission model which compares the output of microplastics from day-to-day usage of plastic with the extra output from plastic mulch films. Five scenarios were created and in each scenario the emission was different. It was found that the emission of microplastics through the usage of plastic mulch did not account for a significant part of the total emission of microplastics. The only scenario where plastic mulch did cause a significant increase of emission was when the normal plastic

emission decreased with 82.5 percent. More research needs to be done in order to verify and improve results, however, this is a start for an area of research where no data is available yet.

Keywords

Index

SCENARIO 1: PLASTIC EMISSION 2017: ... 11

SCENARIO PLASTIC EMISSION 2025: ... 12

SCENARIO 3 PLASTIC EMISSION 2025 WITH BIODEGRADABLE MULCH: ... 13

SCENARIO 4 PLASTIC EMISSION 2050: ... 14

SCENARIO 5 PLASTIC EMISSION 2050 WITH BIODEGRADABLE MULCH: ... 15

DISCUSSION ... 16

ANALYZING RESULTS ... 16

RECOMMENDATION FOR FUTURE RESEARCH ... 16

CONCLUSION ... 17 REFERENCES ... 18 ACKNOWLEDGEMENTS ... 20 REPOSITORY ... 20 APPENDIX A ... 21 ... 21 APPENDIX B ... 22

Introduction

Plastics are a known problem in marine and freshwater ecosystems. Many fish and large sea animals are found with plastic in their stomach from ingesting floating plastics. These plastics cannot be broken down. However, they fragmentate into smaller plastic particles. Eventually plastics can become microplastics (MP). Microplastics are plastic particles smaller than 5mm (De Souza Machado et al., 2018). Around 88 percent of the ocean’s surface is contaminated with microplastics. They are ingested by marine organisms, and this can cause harm to these animals (Nerland et al., 2014).

There are two different types of microplastics namely primary and secondary microplastics. Primary microplastics are for the majority made for daily plastic products, such as cosmetic products. Secondary microplastics are formed by fragmentation of larger pieces of plastic (Rillig, 2012). This study will focus on secondary microplastics.

Microplastics end up in marine and freshwater ecosystems due to poor management of plastic waste in combination with high plastic usage (Peng et al., 2020). By poor management it is meant that plastic litter mostly ends up in nature, rivers, or on the streets. For the plastic that does end up on the tip there are no good alternatives to recycle the plastic, and it ends up in the incinerator. The production of plastic was 299Mt in 2013 (Van Wezel et al., 2015). While microplastics in fresh and marine water have been studied extensively, the sources and amounts of microplastics in terrestrial systems is underexposed (Rillig & Lehmann, 2020). In order to reduce the plastic in the marine and freshwater ecosystems, it is necessary to study the sources and reduce the use of plastic on land. The use of plastics in agriculture is

becoming an important contributor to the emission of microplastics into water bodies. One of the sources of microplastics on land is the use of plastic film in agriculture. Since 1948 plastic has been used in agriculture (Espí et al., 2006). The four main applications of plastic film in agriculture are walk-in tunnels, low tunnel covers, greenhouses and mulching (Espí et al., 2006). Plastic is used in agriculture to increase the soil temperature, consequently increasing crop yield and water use efficiency (Gao, et al., 2019). The microplastics that originate from this plastic use end up in the sediment or streams along the agricultural fields and will eventually be transported into large marine and freshwater streams. Fish and other organisms in the ocean can mistake plastics and microplastics for prey and as a consequence intentionally ingest them (Jovanoviç, 2017)

Next to the fact that the microplastics are harmful for marine organisms, the organisms in the sediment on land could also be affected by these microplastics, due to changes in their biophysical environment (De Souza Machado et al., 2018). For example, when ingested by small organisms, microplastics could grate gut tissues, causing health problems for these organisms (Zhu, et al., 2018). Furthermore, all the particles that end up in the soil or marine and freshwater ecosystems have the possibility to end up in humans and other organisms. Microplastics can fragmentate as well and become nano plastics. It has already been

points from above into consideration, it is important to establish how prominent the role of agriculture is in the emission of microplastics into the environment.

As mentioned before, there are many different ways in which plastic can spill into the soil with agriculture. The one that will be studied in this research is the use of plastic mulch films. This is because it is found that plastic mulch films are a large contributor to microplastic emission in agriculture (Serrano-Ruiz, 2020). These are large plastic sheets that cover the soil and contribute to more favorable circumstances for plant growth in agriculture, such as higher temperature and improved water efficiency. However, these large plastic sheets fragmentate over time and form microplastics (De Souza Machado et al., 2018). According to the Agrobiofilm Consortium (2013), the usage of plastic mulch was around 136.000 tons in the European Union in 2011. There are already alternatives to this conventional plastic mulch, such as biodegradable mulch. Only 1 or 2 percent of all mulch showed to be biodegradable, this means that between 1360 and 2720 tons of mulch used in European agriculture is biodegradable (Agrobiofilm Consortium, 2013). When this is the case, about 135.000 tons of the mulch used in Europe is conventional, thus non-biodegradable. Biodegradable means that the mulch sheets can be broken down by nature and eventually disappear. This is why

biodegradable plastic mulch is preferred over conventional plastic mulch. The goal of this research is to investigate to which extent the use of conventional plastic mulch sheets in agriculture contributes to the emission of microplastics into the aquatic ecosystem.

Therefore, the research question will be: How does the application of plastic mulch films in agriculture increase the release of microplastics into the aquatic ecosystem in Hoorn? This study has been done in an area near the city of Hoorn, because it is part of a larger research, for which sediment samples have been taken in this area of the Netherlands. The agricultural fields which have been studied have various crops. Some grow cabbages and other flowers and bulbs, however, there are also fields which are grasslands or fallow lands. As opposed to cabbage and flower fields, grasslands and fallow lands do not add plastic mulch on the soil. The difference of plastic emission between these types of agricultural fields will be examined in this research. The plastic emitted to the environment on a day-to-day base will be assigned to the fields without plastic mulch addition. This will account for the baseline measurement. The normal and constant emission of plastics will be assigned to the fields with plastic mulch as well, because of this, it is possible to determine the effects of plastic mulch usage on the emission of plastics into sediments.

One of the stakeholders for this issue are policy makers, nationally and internationally. For Europe this can be internationally as most policies made for agriculture are European

arrangements. For other countries these policies are national matters. Another stakeholder is the group of farmers which use the plastic mulch on their land. The plastic mulch is a

relatively cheap way to improve growing conditions on the agricultural fields. When policies change, it is possible that these farmers must transition to more expensive ways to improve the conditions such as biodegradable plastic mulch.

Methods

This study is largely based on literature research, in order to determine how much and when the plastic mulch is used. Furthermore, literature research is used to determine the possibly constant release of plastics into the environment as a baseline measurement. This was done by comparing the amount of plastic waste produced in the Netherlands in 2017 by the total land surface of the Netherlands in ha. The amount of plastic mulch used on agricultural land was found through literature research as well. Not much research has been done for the emission of microplastics in terrestrial areas and in agriculture. Therefore, in order to obtain results, a model was built that represents the circumstances of microplastic emission in the Netherlands.

This research is part of a larger research project done by Bernou Boven from the University of Amsterdam. In this project, Bernou has taken sediment samples of streams surrounding agricultural fields near the city of Hoorn in the province North-Holland (figure 1A). Bernou hypothesizes that plastic used in the agricultural fields, release microplastics into the streams and water bodies that surround these agricultural fields. Eventually, these microplastics might accumulate in the sediments. The samples were taken from different agricultural fields, where plastic mulch was used, as well as fields where no mulch was used. Every few years, the crops are rotated, meaning that different crops are cultivated on the same fields. In consultation with Bernou, the choice was made that this research will be based on the way the fields are distributed in the spring of 2020. This is because Bernou took her sediment samples at the end of 2020. Figure 1B shows the sampling sites from Bernou which have been used for the calculations of this research. There is a possibility that for future research, the distribution and crops of the fields will be different and therefore, the outcomes could change. This must be considered when considering follow up research.

(A)

(B)

Study site

The study site is a country site where different types of agriculture are taking place. Throughout the sampling location there is a small stream next to the agricultural fields.

Bernou has taken several sediment samples from this stream. Five of these sampling locations are used in this research to calculate the amount of microplastics present. The sampling locations which have been chosen are upstream in order to minimize the effect of inflow of microplastic from other points of the stream and can be found in figure 1B. For each sampling site, the size of agricultural fields 1 km upstream has been calculated with the use of Google Earth (table 1). The type of agriculture performed was already noted by Bernou, therefore it was possible to determine whether plastic mulch was used. The types of agriculture per location are found in Appendix A. These findings where then used in the model.

Location 1

(2.2) Location 2 (2.3) Location 3 (2.4) Location 4 (2.5) Location 5 (2.6) Surface with mulch (ha) 15.9 15.0 3.2 19.7 32.3 Surface without mulch (ha) 5.2 16.5 33.2 19.7 49.4 Total surface (ha) 21.1 31.5 36.4 39.4 81.7

Figure 1A. Chosen area, red polygon marks the study site from Bernou Boven and the yellow arrow marks the lower stream for this research

Figure 1B. Exact sampling locations used for research

Table 1. Location with corresponding sampling sites from figure 1B and corresponding measured surfaces

Model description

The model has been made with Python, using Jupyter Notebook as an editor. The model has been made around two main formulas and 2 sub-formulas. The first main formula calculates the constant plastic emission in the Netherlands per ha. This is the baseline measurement and represents the plastic emission on plots along the river where no mulch sheets are used. These plots are for example grasslands or fallow lands. Little is known about the presence of microplastic in soils, therefore an estimation was made. The estimation is based on the amount of plastic waste processed in the Netherlands in 2017 and the total terrestrial area of the Netherlands, compared with the estimated remnant and runoff rate of microplastics (CE Delft, 2019; Centraal Bureau Statistiek, 2020; Cerdan et al., 2010). For the runoff rate of microplastics, the erosion rate of soil in the Netherlands was used. The second main formula represents the amount of plastic mulch sheets used on the plots that can theoretically enter the soil in the Netherlands. This is calculated with the weight of plastic mulch sheets in comparison with the surface it covers (Bondt et al., 2010). In table 2 there is an overview of the parameters used in the formulas. Firstly, the mass of microplastics that can end up in the sediment, due to everyday plastic use was calculated (msed). This was done by comparing the

constant plastic emission (mconst) with the remnant on the soil and the runoff rate.

m

=

(1)

frem is the fraction of plastics that fragments into microplastics and remain in the soil. For this

research frem was set for three different scenarios. The first scenario being that 100 percent of

the plastic waste ends up in the soil, the second scenario is 50 percent of the plastic end up in the soil and the last scenario is that five percent of the plastic ends up in the soil. The

scenarios for frem are the same for microplastics from plastic waste and microplastics from

plastic mulch. These different scenarios were made because it is not clear how much of the plastic waste actually ends up in soils in the Netherlands. Vrunoff is the runoff rate of

microplastics towards the surrounding streams. mconst is the plasticthat can end up in the soil

due to everyday plastic usage. mconst is calculated with the use of the plastic waste produced

(mwaste) in the Netherlands. mwaste was divided by the surface of the terrestrial area of the

Netherlands (ANL) and multiplied by the surface of the agricultural land (Aland). Aland was

determined via Google Earth, by measuring the size agricultural plots 1 km upstream of the sediment samples of which Bernou Boven had taken.

Secondly, the mass of microplastics emitted by plastic mulch sheets into the sediment (Msed)

was calculated. This was done by multiplying the weight of plastic mulch sheets on the agricultural soil (Mmulch) with frem and dividing it by vrunoff.

M

(2)

Mmulch is the weight of the plastic mulch sheets that are used on the chosen agricultural plots.

Mmulch is calculated with the weight of plastic mulch sheets per ha (mmulch)and the size of the

agricultural field (Aland)

Mmulch =

Symbol Description Unit Values

mconst Constant plastic emission t/ha Outcome

formula 1.1

mwaste Plastic waste NL t 1.65x10^6

mmulch Weight of mulch sheets t/ha 0.2

Mmulch Weight of used mulch

sheets t Outcome formula 2.2

msed Microplastic in sediment

through constant emission t Outcome formula 1 Msed Microplastic in sediment

through plastic mulch use t Outcome formula 2

vrunoff Runoff speed t/ha/year 0.4

ANL Surface Netherlands ha 3.4x10^6

Aland Surface agricultural land ha Table 1

frem Fraction of plastic remnant NULL 1, 0.5, 0.05

Next to the fact that there were different scenarios set for frem, a longer time span was added

as well. The difference between 2017 and 2025 was calculated, as the government of the Netherlands were set to reduce plastic use with 20 percent by 2025 (Rijksinstituut

Volksgezondheid en Milieu, 2019). Using the rate of reduction of plastic waste from 2017-2025, an estimation was made how much plastic waste would have been reduced by 2050. This is a scenario in the results as well. Furthermore, a reduction of conventional, non-biodegradable plastic mulch use was used in the scenarios. Scenarios within the different timespans were made where 20 percent of the plastic mulch used, would be biodegradable in

2025 and 40 percent in 2050. These reductions are purely estimations, as there are no official targets set for the reduction of conventional plastic mulch use. An overview of these different scenarios per year can be found in table 3. In order to determine whether de difference between two scenarios significant, a t-test was performed.

2017 2025 2025 Biodegradable mulch 2050 2050 Biodegradable mulch Plastic waste 100% 80% 80% 17.5% 17.5% Plastic mulch 100% 100% 80% 100% 60%

Little is known about the amount of waste in terrestrial ecosystems in the Netherlands. Estimations run from 50 million kilos to 300 million kilos of waste in nature (Staatsbosbeheer, n.d.). Therefore, an estimation of the percentage of waste that ends up in nature was made. The amount of waste generated per person in the Netherlands is 550 kilos in 2017 (Centraal Bureau Statistiek, 2017). This means that about five percent of waste in the Netherlands ends up in nature.

Results

Five scenarios are run in the model over five different sediment sample points. At every location there are three calculations that have been made. One with only plastic mulch (mulch), one with only constant plastic (not mulch) and one with both, called total. These three calculations are shown per location from left to right, first showing the mulch, then the area without mulch and lastly in the darker color the total amount of plastic emission into the sediment. Every color in the graphs show different locations, these are explained as well on the x-axis where the numbers represent the location. The amount of plastic in the sediment is shown for five percent remnant (frem), the 50 and 100 percent remnant are show in Appendix

B. The y-axis shows the amount of plastic in t/year. The five percent remnant means that 5 percent of the percentage plastic waste and plastic mulch films are transformed into microplastics.

Scenario 1: plastic emission 2017:

In this scenario all factors are as they were stated in 2017. This is the baseline measurement. When comparing total with not mulch, the p-value is 0.55. Meaning there is no significant increase when mulch is added to the soil. However, when considering the percentages of increase with plastic mulch, there is an average increase of 26.8 percent.

Figure 2: Microplastic emission in t\year, where each color palette represents a sampling location. This is the baseline scenario; thus, plastic waste and plastic mulch are 100 percent.

Scenario plastic emission 2025:

In this scenario the plastic waste was reduced by 20 percent, while the weight of plastic mulch remained the same. The p-value was 0.46, therefore, plastic mulch does not account for a significant amount of plastic on the sediment. With the reduction of plastic waste, the share of mulch within the total increases. In this scenario, there is an average increase of 31.3 percent when mulch is used.

Figure 3: Microplastic emission in t\year, where each color palette represents a sampling location. In this scenario the plastic waste is 80 percent relative to 2017 and the plastic mulch remains 100 percent.

Scenario 3 plastic emission 2025 with biodegradable mulch:

In this scenario 20 percent of biodegradable plastic mulch was added and the plastic waste is still set at 80 percent relative to 2017. The P-value here was 0.55, therefore comparable with scenario number 1 and the plastic mulch does not account for a significant amount of total plastic emission. This can also be seen in the percentages. In scenario 1 plastic mulch

accounted for 26.8 percent of the total and in scenario 3 this is 27.5. Thus, the share of plastic mulch is actually higher than in scenario 1.

Figure 4: Microplastic emission in t\year, where each color palette represents a sampling location. In this scenario the plastic waste is 80 percent relative to 2017 and 20 percent biodegradable mulch is used, leaving conventional plastic mulch at 80 percent.

Scenario 4 plastic emission 2050:

In this scenario, the output of plastic waste has been reduced by 82.5 percent. The usage of plastic mulch sheets has not changed, in comparison with the scenario from 2017. Under these circumstances the P-value is 0.048 and the percentage of mulch regarding the total is 61.1 percent. This means that plastic waste output must be reduced with more than 80 percent in order to make the addition of plastic mulch films to the sediment significant.

Figure 5: Microplastic emission in t\year, where each color palette represents a sampling location. In this scenario the plastic waste is 17.5 percent relative to 2017 and plastic mulch remains at 100 percent conventional mulch.

Scenario 5 plastic emission 2050 with biodegradable mulch:

For this scenario the weight of plastic mulch was reduced by 40 percent, meaning that 40 percent of the conventional plastic mulch was replaced by biodegradable mulch, and the amount of plastic waste was reduced by 82,5 relative to 2017. In this scenario the p-value is 0.12. This indicates that the addition of plastic mulch into the sediment is not significant, however the mulch accounts for 51.5 percent of the total. This indicates that even though it is not a significant amount, mulch output is more than half of the total output.

Figure 6: Microplastic emission in t\year, where each color palette represents a sampling location. In this scenario the plastic waste is 17.5 percent relative to 2017 and 40 percent of conventional mulch was replaced by biodegradable mulch. Therefore, conventional plastic mulch was set at 60 percent relative to 2017.

Discussion

In this study, the amount of microplastics emitted by the use of plastic mulch sheets in agriculture was studied. This was done by creating a model which compares the output of microplastics from day-to-day usage of plastic with the extra output from plastic mulch films. In table 4 there is an overview of the results found. Results show that the usage of plastic mulch films does not account for a significant part of the total microplastic emission. Only when the normal and constant plastic waste was reduced by 82.2 percent, and the plastic mulch films remained at 100 percent conventional plastic mulch, was the addition of plastic mulch films significant to the total. This indicates that the normal plastic waste is so high that the addition of plastic mulch has no significant role in the total microplastic emission.

P-value Percentage Scenario 1 (100, 100) 0.55 26.8 Scenario 2 (80, 100) 0.46 31.3 Scenario 3 (80, 80) 0.55 27.5 Scenario 4 (17.5, 100) 0.048 61.1 Scenario 5 (17.5, 60) 0.12 51.5

Recommendation for future research

With the results there are many uncertainties. The most important is that it is not clear how accurate the created model is. This is mainly because there are no standardized models for these emission data (De Souza Machado et al., 2018). Therefore, the difference between expected concentrations and actual measured concentrations differs a lot (Van Wezel et al., 2015). These standardized models cannot be created due to a lack of general data such as behavior or presence in the soil. It is difficult to perform soil samples in order to determine the amount of microplastics in soil and sediments. When soil samples are taken, it is possible that microplastics stick to soil particles and are overlooked. This causes an underestimation of the amount of microplastics in soil (De Souza Machado et al., 2018). A possibility is that when the research from Bernou Boven is finished, results can be compared, and the model can possibly be improved in order to make it more realistic. Next to that, not much data is available on the amount of microplastics in the terrestrial soil or on the emission of

microplastics into the soil for the long term. When plastic particles are taken up by the soil, it is not clear what their vertical mobility is (O’Connor et al., 2019). Finally, many studies use different measurements for microplastic presence in soils. Some articles calculate

microplastics in particles per km2_{, while others calculate it in t/ha. This way these findings}

cannot be compared. Therefore, a standard measurement should be announced so that all research can be compared.

Table 4: Overview of the results per scenario, where the numbers behind the scenario represent respectively percentages of plastic waste and plastic mulch films.

Next to the fact that the accuracy of the model is unclear, this research has not distinguished different types of plastic. Plastics are made from different types of polymers and all these polymers have different characteristics, such as density and if they can be retained in sediments. These differences in characteristics were not considered for this research, however, even when these characteristics are known and considered, it can be difficult to predict what the polymer does in soil and water (Horton et al., 2017).

Furthermore, the plastic sheets used in these models are estimated on the same weight. Meaning that in this research, a distinction was only made on surface area. This is because this area has mainly horticulture, and the weight of these sheets is approximately the same. Only asparagus have heavier sheets, however, in this area there are no asparagus fields registered (Bondt et al., 2010). Therefore, for future research, it should be studied whether the types of mulch used in the fields have significant differences in weight.

Lastly, this research is uncertain due to an absence of data on normal plastic emission. For the purpose of this research an estimation for plastic emission into the terrestrial environment was made. However, there is no data available on the mismanagement of plastics in the Netherlands and how much plastic waste ends up in the environment in the Netherlands. Therefore, it is possible that the amounts of plastic waste in the Netherlands in this research are either overestimated or underestimated, and the Netherlands should investigate the rate of mismanaged plastics for a more valid conclusion.

Conclusion

It can be concluded that for most of the scenario’s plastic mulch does not increase plastic output significantly. Only when the normal plastic emission is decreased by more than 80%, there is a significant difference. However, while the increase might not be significant, with every scenario the increase is more than 25%. Therefore, it can be concluded that the usage of plastic mulch films does contribute to the output of microplastics in agricultural areas.

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Acknowledgements

I would like to thank dr. A. Praetorius and dr. R.P.J Hoondert for supervising this research. Without their help and expertise I would not have been able to finish this project.

Repository

The model can be found in GitHub through the following link.

Appendix A

Location 1 Grasslands

(3.5 ha) Flower/bulb fields (6.0 ha) Orchard (10 ha) Glass greenhouse (1.7)

Location 2 Grasslands

(14.8 ha) Flower/bulb fields (3.6 ha) Orchard (11.4 ha) Glass greenhouse (1.7 ha)

Location 3 Grasslands

(30.2 ha) Cabbage fields (3.2 ha) Fallow lands (2.7 ha) Glass greenhouse (0.3 ha)

Location 4 Grasslands

(17.0 ha) Cabbage fields (3.3 ha) Orchard (16.4 ha) Fallow lands (2.7 ha)

Location 5 Grasslands

(47.5 ha) Cabbage fields (1.3 ha) Orchard (9.6 ha) Fallow lands (1.9 ha) General agriculture (21.4 ha)

Table 5: Overview of the agricultural fields present in each location, with size in ha. Glass greenhouse, grasslands and fallow lands were marked as fields with no plastic mulch. All the other fields were marked with plastic mulch

Appendix B

Scenario 1: Plastic emission 2017

Scenario 2: Plastic emission 2025

Figure 7: Microplastic emission in t\year, where each color palette represents a sampling location. This is the baseline scenario; thus, plastic waste and plastic mulch are 100 percent. Figure A represents a 50 percent emission and figure B represents a 100 percent emission.

A B

Figure 8: Microplastic emission in t\year, where each color palette represents a sampling location. In this scenario the plastic waste is 80 percent relative to 2017 and the plastic mulch remains 100 percent. Figure A represents a 50 percent emission and figure B represents a 100 percent emission.

Scenario 3: Plastic emission 2025 biodegradable

Scenario 4: Plastic emission 2050

Figure 9: Microplastic emission in t\year, where each color palette represents a sampling location. In this scenario the plastic waste is 80 percent relative to 2017 and 20 percent biodegradable mulch is used, leaving conventional plastic mulch at 80 percent. Figure A represents a 50 percent emission and figure B represents a 100 percent emission.

A B

Figure 5: Microplastic emission in t\year, where each color palette represents a sampling location. In this scenario the plastic waste is 17.5 percent relative to 2017 and plastic mulch remains at 100 percent conventional mulch. Figure A represents a 50 percent emission and figure B represents a 100 percent emission.

Scenario 5: Plastic emission 2050 biodegradable

Figure 6: Microplastic emission in t\year, where each color palette represents a sampling location. In this scenario the plastic waste is 17.5 percent relative to 2017 and 40 percent of conventional mulch was replaced by biodegradable mulch. Therefore, conventional plastic mulch was set at 60 percent relative to 2017. Figure A represents a 50 percent emission and figure B represents a 100 percent emission.