Effects of biodegradable plastics on soil properties and greenhouse gas production

ABSTRACT

Microplastics cause environmental problems. Biodegradable plastics have become popular because they aim to avoid such problems. However, their decomposition in the soil may have an impact. This study aims to investigate the effects of biodegradable plastics on the physicochemical properties of soil, specifically the production of CO₂ and N₂O in the soil and plant growth. Three kinds of biodegradable plastics in the forms of 1) nonwoven fabric sheets made of poly-lactic acid (PLA) and polybutylene-succinate (referred to hereafter as fabric), 2) laminate sheets made of polybutylene adipate terephthalate (PBAT) and pulp (hereafter laminate), and 3) drinking cups made of PLA (hereafter cup), were cut into small pieces (<5 mm), added to soil, then water-holding capacity was determined and incubated aerobically for 4 weeks at 30°C in the dark. Soil and gas samples were collected weekly to measure soil pH, nitrate-nitrogen content, CO₂, and N₂O productions. These plastics were also tested in a pot experiment with Komatsuna (Brassica napa var. perviridis). We tested for seed germination, plant growth, leaf color, and fresh weight at harvest. Results showed that the water retention capacity was higher in the fabric plastics as compared to the cup plastics and the control. Soil pH with the fabric plastics dropped during the initial 2 weeks of incubation, then recovered to a similar pH to the control (without plastic). Nitrate contents in the soil with laminate plastics were lower than those in the control, while CO₂ production in the soil with the laminate plastics was higher than that in the control and the other plastics during the incubation period, and even higher than the one of added plastic-C. N₂O was produced rapidly within 1 week of incubation in the soil with the laminate plastics, and cumulative N₂O production from incubation was more than that of the control. The seed germination and plant growth tended to be suppressed in the pot experiment with the fabric and laminate plastics. The results indicate that the influence of these biodegradable plastics on soil properties, greenhouse gas production, and plant growth on the kind of plastic and the timing.

KEYWORDS:

• Biodegradable plastics
• germination
• greenhouse gases
• microplastics
• pH

1. Introduction

Microplastics may cause environmental problems such as oceanic pollution and damage the health of marine animals (Gregory 1978). Biodegradable plastics have become popular, aiming to avoid such problems (Boots, Russell, and Green 2019). In agriculture, mulching sheets made of biodegradable plastics have become popular in Japan, and their rapid decomposition is possible because of microorganisms in the soil (NARO 2015). Some sanitary items such as nappies are also made of biodegradable plastics, aiming for faster decomposition in the soil after they become garbage and are dumped as land-fill or burnt. However, the decomposition of plastics in the soil may significantly impact its physical (de Souza Machado et al. 2018) and chemical properties (Lozano et al. 2021). Bi, He, and Chen (2020) reported that biodegradable plastics do not influence carbon (C) and nitrogen (N) metabolisms, while Iqbal et al. (2020) described a high potential risk of biodegradable plastics on nitrogen metabolisms in the soil that had been amended with them. However, little information is available for their impact on greenhouse gases such as carbon dioxide (CO₂) and nitrous oxide (N₂O) or their effect on plant growth (Boots, Russell, and Green 2019).

Amongst several greenhouse gases, CO₂ and N₂O are two major constituents. On the one hand, the terrestrial ecosystem is an important sink, with contributors such as forest soils that can sequester CO₂. Conversely, agricultural soils are sources of N₂O derived mainly from nitrogen fertilizer and manure. N₂O has about 300 times higher an effect on global warming potential in its molar basis than CO₂. N₂O also has a destructive impact on the stratospheric ozone layer (IPCC 2021). This study aims to investigate the effects of biodegradable plastics on soil physicochemical properties, the production of CO₂ and N₂O in soil, and their effects on plant growth.

2. Materials and methods

2.1. Soil and plastics

A sandy soil (Coase-Medium Texture Gray Lowland soil) was collected from an upland field in Yokoshiba-Hikari, in the Pacific coastal region of Chiba Prefecture, Japan, with the following properties; pH(H₂O): 6.78, electric conductivity (EC): 3.86 mS m⁻¹ measured as decribed below, total carbon and nitrogen (TC and TN, measured by CN Corder; Yanaco MT700) at 9.83 and 0.90 g kg⁻¹ NH₄⁺ and NO₃⁻, 0.31 and 2.75 mg N kg⁻¹, CEC at 1.90 cmol(+) kg⁻¹, glucosidase and urease activities at 32.7 mg p-nitrophenol kg⁻¹ h⁻¹ and 19.4 mg NH₄⁺-N kg⁻¹ h⁻¹, determined by Eivazi and Tabatabai (1988) and Tabatabai and Bremner (1972), respectively. Three kinds of biodegradable plastics were amended to the soil; 1) nonwoven fabric sheets (abbreviated hereafter as fabric) made of polylactic acid (PLA) and polybutylene-succinate (PBS), 2) laminate sheets (abbreviated hereafter as laminate) made of polybutylene adipate terephthalate (PBAT) and pulp, and 3) drinking cups (abbreviated hereafter as cup) made of PLA.

2.2. Incubation experiment

The plastics were cut into small pieces (<5 mm) by hand, then mixed well into the soil at 0.2% w/w for 1) fabric and 2) laminate, 2.0% w/w for 3) cup, and 0% (no amendment) as the control. These samples (50 g each) were incubated aerobically in containers (250 mL) for 4 weeks with 50% water-holding capacity at 30℃ in the dark with 3 replicates.

2.3. Soil analysis

Water holding capacity (WHC) of the plastic-soil mixture was determined by the Hilgard method (The University of Tokyo 1978); the plastic-soil mixture was placed on filter paper inside a Hilgard metal dish (diameter = 5.6 cm, depth = 1 cm, volume = 25 mL) and set inside a Petri dish with shallow distilled water to moisten the sample for 1 h, then calculated based on the weight before and after this period as:

WHC = Water added to the soil sample (mL) – Water unretained in the soil sample (mL) 1 hr after water addition]/weight of oven-dried soil (d.s.)

Soil pH (H₂O), and electric conductivity (EC) were measured periodically by pH/EC meter (Horiba, D-62/TOA, CM-31P), ammonium N and nitrate N contents by colorimetric methods (Anderson and Ingram 1989; Hayashi, Sakamoto, and Yoshida 1997, respectively), and cation exchange capacity (CEC) by batch extraction (Muramoto, Goto, and Ninaki 1992) with atomic absorption meter (Tokou Electric, ANA-182) for CEC. Another set of samples (10 g each) was incubated aerobically in a glass vial (120 mL) with a sponge stopper and was analyzed in the same way. The gas samples in the headspaces of the vials were collected on days 1, 5, 7, 14, and 28, 24 h after changing to butyl rubber stoppers to determine the gas production of CO₂ and N₂O concentrations by gas chromatography (GC) (Shimadzu, GC-14B), with a thermal conductivity detector (TCD) and an electron capture detector (ECD), respectively. Details of GC parameters are based on Inubushi, Wada, and Takai (1984) and Inubushi, Naganuma, and Kitahara (1996).

The C decomposition rate was calculated with:

[(CO₂ from the plastic amended soil) – (CO₂ from the control soil)]/Plastic-C

Plastic C was the assumed weight-based of the plastic C content at 51% for the fabric and laminate treatments and 48% for the cup treatment, based on the chemical formula of each compound as a raw material, respectively.

The N₂O proximal emission factor was calculated as

[(N₂O from the plastic amended soil) – (N₂O from the control soil)]/ [(nitrate N of the amended soil) – (nitrate N of the control soil)] * 100

Since the examined plastics contained no nitrogen, we calculated this as above to estimate their impact on N₂O emission, based on the influenced soil N dynamics caused by their addition.

2.4. Germination test and pot experiment

Germination test was conducted using 3 mL of plastic-water extract (1:10 ratio) and 15 seeds of Komatsuna (Brassica rapa var. perviridis) placed on filter paper inside a Petri dish with 4 replicates and incubated in a growth chamber at 20°C for 1 week under a continuously dark condition. The germination rate was calculated at the end of the incubation compared to control (distilled water) as 100%.

Another soil-plastic mixture set (200 g each) of 4 replicates was prepared for a pot experiment in the same way as the incubation experiment (2.2) and put into pots (350 mL each). Six Komatsuna seeds were planted in each pot and initially incubated for 10 days in a growth chamber at 25°C under light/dark conditions (16/8 hour cycle), watering to maintain the same moisture condition as the incubation experiment. Then the seedlings were thinned to 3 plants per pot and placed in a glass greenhouse at Chiba University’s Matsudo campus under natural light and temperature conditions during October-November, 2020 until harvest at the 4^th week.

Plant height and leaf color were measured by ruler and color meter (Konica-Minolta, SPAD-502) during greenhouse cultivation and on days 21 and 28, respectively. After harvest, ammonium and nitrate N contents remaining in the soil were measured in an incubation experiment. The fresh weight of the harvested plants was also measured after removing excess water by paper towel, and the dry weight was taken after drying in an oven at 80°C for 48 hours, respectively.

2.5. Statistical analysis

All statistical analyses comparing measured parameters among all the treatments and their temporal changes were carried out in IBM SPSS Statistics 24. Multi-reference was examined by Tukey test. Differences were considered significant at a p < 0.05 significance level.

3. Results

3.1. Water holding capacity (WHC)

WHC was not significantly different among all the treatments, although it tended to be slightly higher (6%) in the fabric treatment compared to the control (Figure 1).

Figure 1. Changes in water holding capacity by plastics. Error bar = S.D. (n = 3), different letters indicate significant difference (p < 0.05, Tukey).

3.2. Soil pH and EC

Soil pH with the fabric dropped about 0.5 points during the initial 2 weeks of the incubation, then recovered to almost the same pH as control (without plastic) by the end of the incubation (Figure 2). Similarly, soil pH in the laminate treatment dropped about 0.4 points in the 2nd week of incubation, then recovered to a similar pH as the control by the end of incubation.

Figure 2. Changes in soil pH during incubation Error bar = S.D. (n = 3), different letters indicate significant difference (p < 0.05, Tukey).

Soil EC increased gradually but similarly among the treatments during the incubation period (Figure S1).

3.3. Soil nitrate and ammonium

Nitrate content in the soil with the laminate treatment decreased to about half the level of the initial content within the first week of incubation. It remained lower than the control and other treatments until the end of the incubation period, while that of the fabric treatment increased to almost double in the 1^st week and then decreased to the initial level afterward (Figure 3). On the other hand, the nitrate content in the cup treatment increased to 8.7 mg N kg⁻¹, almost 3-times higher than the initial level, and similarly to the control, throughout the incubation.

Figure 3. Changes in nitrate contents during incubation. Error bar = S.D. (n = 3), different letters indicate significant difference (p < 0.05, Tukey).

The ammonium content remained about one order of magnitude lower than the nitrate content, decreasing from about 0.31 to 0.17–0.20 mg N kg⁻¹ during the 1^st week of incubation and gradually increasing to its initial level of 0.28–0.36 mg N kg⁻¹ by the end of the incubation period (Figure S2).

3.4. CO₂ and N₂O production

CO₂ production increased almost constantly, that in the laminate treatment being more than 3-times higher than that of the other treatments and control during the incubation period (Figure 4). There was no significant difference among the other treatments and the control.

Figure 4. Changes in cumulative CO₂ production during incubation. Error bar = S.D. (n = 3), different letters indicate significant difference (p < 0.05, Tukey).

N₂O increased rapidly in the laminate-treated soil within the first week of the incubation, even with no measurement on day 1. On the other hand, gradual increases were observed in the fabric treatment in the 2^nd week of the incubation and in the soil containing the cup treatment and the control after the 2^nd week. Cumulative N₂O production by the end of the incubation was more than double in the fabric treatment compared to that of the other plastics and the control (Figure 5).

Figure 5. Changes in cumulative N₂O production during incubation. Error bar = S.D. (n = 3), different letters indicate significant difference (p < 0.05, Tukey).

3.5. Seed germination and plant growth

Seed germination tended to be less in the fabric and laminate treatments, although these had a greater variation (Figure 6). Plant height increased almost constantly at the same growth rate, except with the laminate treatment, in which it stopped after the 2^nd week (Figure 7).

Figure 6. Germination rate.

Figure 7. Changes in plant height. Error bar = S.D. (n = 3), different letters indicate significant difference (p < 0.05, Tukey).

Fresh weight and dry weight were lowest with the laminate treatment, followed by the fabric treatment, while there was no significant difference between the cup treatment and the control (Figure 8). There was no significance among the amount of remaining mineral N (sum of ammonium N and nitrate N) in the soil after harvest (Figure S3). At the same time, root length tended to be shorter with the laminate-treated soil (Figure S4), although no significant difference was observed in leaf color, except in the laminate treatment, which had insufficient leaf area to measure color (Figures S5, S6).

Figure 8. Dry weight and moisture weight at harvest.Error bar = S.D. (n = 3), different letters indicate significant difference (p < 0.05, Tukey).

4. Discussion

4.1. Effects of plastics on soil properties

Higher water holding capacity with the fabric plastics than the cup plastics, even 10 times higher application rate than other plastics, and the control was probably due to physical properties of the fabric materials and sheet structure, similar to the laminate, although not significantly different (Figure 1). This finding was similar to de Souza Machado et al. (2018), who found no change with plastics. In contrast, Lozano et al. (2021) reported that micro-plastics increased soil aggregation by ~18% and soil pH by ~4%, so further study is needed with more materials.

In our study, a temporal drop of soil pH by 8 ~ 10% occurred with the fabric and laminate plastic treatments (Figure 2). We deduce that the decomposition of the amended plastics into things such as organic acids may have caused it, but those components were not tested. Both of those plastics had a much wider surface area than the cup plastics, despite the higher application rate of cups, and their effect on soil pH was not detected for in this incubation experiment. The temporal drop in soil pH may have inhibited N transformations such as N mineralization, nitrification, and immobilization. We did not measure nitrite, however, as the ammonium levels did not change among the treatments; nitrite oxidation may have been inhibited by the laminate and fabric treatments during incubation.

4.2. Effects of plastics on CO₂ and N₂O production

CO₂ production in soil with the laminate plastics was much higher than that in the control and the other treatments during the incubation period, which may have been due to the effect of the laminate plastics on soil pH.

The C decomposition rate of plastics in the soil was calculated at 49%, 102%, and 4.5% for the fabric, laminate, and cup treatments, respectively. This difference may have been due to their composition, particularly that of laminate plastics, which contain pulp, mainly cellulose. Lozano et al. (2021) reported that microplastic fibers impact soil enzymes (β-glucosidase, β-glucosaminidase, β-D-cellobiosidase, and phosphatase) and respiration. However, we measured soil enzyme activities such as glucosidase and urease activities only before the experiment; therefore, further investigation is necessary to measure enzymatic activity in the soil-plastic mixtures. Another possibility may be due to the effect of the amended C and their decomposition products in enhancing soil C decomposition, in a so-called “Priming effect” (Fontaine and Abbadie 2003). This may relate to the initial drop of soil pH in the disturbed soil, affecting the decomposition of the soil organic matter, especially with the laminate plastics.

N₂O production was also high in the soil with the laminate plastics in the initial incubation stage, probably also due to the initial drop of soil pH. Since the examined plastic did not contain N, the side-effects of the amended plastic on N was estimated to be an N₂O proximal emission factor, based on the 4.5–17% changes in nitrate for the fabric and laminate, respectively.

In general, N₂O is produced in soil mainly through nitrification or denitrification and nitrification is a dominant process in aerobic soils (Yoshida and Alexander 1970). In this experiment, N₂O is asumeed to be produced mostly through nitrification under aerobic conditions, although denitrification may also be possible due to high incubation temperature with C-rich plastics. Nitrification is carried out by specific soil microorganisms, namely ammonium oxidizers and nitrite oxidizers, while denitrification by rather general heterotphic bacteria and some fungi. N₂O is produced in the balance and activities of these microorganisms (Inubushi and Yashima 2021). If nitrification is inhibited under stressed conditions such as high salinity, then N₂O production increases (Inubushi, Barahona, and Yamakawa 1999). Amended plastic might create stress conditions during N metabolism, as observed by the drop of soil pH or by other intermediate products. Further investigation including the corresponding microorganisms is needed for more detailed examinations. In this study, methane (CH₄) was not measured, as its production is dominant under anaerobic conditions. However, the future examination of CH₄ and N₂O production through denitrification could be considered, owing to the C-rich amendments.

4.3. Effects of plastics on seed germination and plant growth

Seed germination tended to be partially inhibited by the fabric and laminate plastics, which also suppressed the initial growth. Lower pH and other intermediates can influence germination and initial plant growth, especially the roots’ sensitive development and activities. Tanaka (2002) reported that germination of rice (Oryza sativa L.) seedlings was inhibited by aliphatic acids and aromatic acids during the decomposition of wheat straw in flooded paddy soil. Therefore, the cause of changes in soil pH, such as organic acids or other compounds, should be investigated in further studies.

Boots, Russell, and Green (2019) reported that microplastics derived from biodegradable PLA decreased soil pH by 0.6 points, similar to our results, but only measured once at the end of the 30-day experiment with no data about temporal changes. They also showed the suppression of biodegradable PLA on the germination and growth of perennial ryegrass (Lolium perenne), again similar to our results, and divulged other effects on ecosystem functions such as earthworm activity in the soil. Generally, plant growth is also influenced by nutrient supplies from the soil, such as soil N, but so far we could not detect significant changes of mineral N remained after harvest (Figure S3). However, there is no other long-term study so far with various plants. Thus, further long-term studies with plants other than Komatsuna are necessary.

5. Conclusion

These results from our incubation and pot experiments indicate marked influences of biodegradable plastics on soil chemical and biological properties depending on the kind of plastic and the period and timing during the investigation. These effects included greenhouse gas production, related microorganisms and changes in plant growth – even temporal, such as with an initial pH drop in the soil, N₂O production, and inhibitions in plant growth. For healthy soil and ecosystems, further long-term experiments with these aspects, other forms of plastic, and different plants are required.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed here.

Additional information

Funding

This work was part of a collaboration with Havix Co., Japan. Jasso provided the Tanken Scholarship to SYU.