Effects of Biodegradable Plastic Film on Carbon Footprint of Crop Production

Abstract

Polyethylene film mulch (PM) is a kind of widely used technology to improve crop yields worldwide; however, because of a problem related with plastic residual pollution, it has gradually been replaced by biodegradable plastic film mulch (BDP). Although BDP has helped to solve the plastic residual pollution, its consequences in terms of greenhouse gas (GHG) emissions have rarely been revealed. Related knowledge is important for forming low-carbon development strategies for the plastic industry and agriculture. The objective of this study is to evaluate the influence of BDP on GHG emissions at different stages of its life cycle, and determine whether replacing polyethylene (PE) film with BDP film is a helpful way to reduce national GHG emissions. The results of this study suggest that the application of BDP improved the GHG emissions associated with agricultural inputs, but induced lower GHG emissions at the growing stage and the waste disposal stage, and resulted in lower total area-scale GHG emissions. Compared to the no mulch (NM) cultivation system, the yield-scale carbon footprint was reduced in both the PM and BDP cultivation systems, which meant that both PM and BDP produced lower GHG emissions than NM for the production of the same amount of grain. It was concluded that BDP is not only a measure to control the problem of plastic residue pollution in agriculture, but it can also mitigate the GHG emissions.

1. Introduction

Polyethylene (PE) plastic mulch (PM) is a kind of technology used widely around the world to improve crop yields, control weeds and save water [1,2]. PE was chosen for agricultural film for a long time because of its excellent qualities in terms of mechanical strength, flexibility, transmittance, impermeability and non-toxicity. However, the use of a vast amount of PE film in agriculture resulted in serious plastic residue pollution [3,4]. If the PE film is not retrieved after usage, it accumulates in the soil and leads to several negative effects, such as retarding crop root growth, reducing the continuity of soil porosity and impeding the transport of soil water and nutrients, inhibiting soil microbial and enzyme activities [5,6,7]. These negative effects induce a high risk for crop yield reduction when the amount of plastic residue reaches a critical value [4].

Because of the negative effects of PE residue on crop production and the soil environment, PE film must be retrieved and disposed of after usage. Recycling is a common waste disposal method for plastic, but there is a lack of recyclers who can accept agriculture plastic film due to the large amount of soil contamination (up to 70% by weight) [8]. Landfilling is another method of plastic residue disposal; however, its cost is high and is difficult for agriculture producers to accept [9]. The advantage of biodegradable plastic film is that it can be directly tilled into soil after use, thus reducing the costs for its removal and disposal. In addition, biodegradable plastic film can fundamentally eliminate the environmental risk of plastic residues as it can be degraded to CO₂ and H₂O in a relatively short time [10], while the PE film was difficult to be removed completely and did not radically evade the risks of plastic residue.

Although BDP is a helpful way to solve the problem of plastic residue pollution, there have been few studies evaluating its influence on greenhouse gas (GHG) emissions. Over the last four decades, global plastics production has quadrupled, and GHG emissions from plastics will reach 15% of the global carbon budget by 2050 [11]. Identifying the influence of different types of plastic is helpful to produce a strategy to cut down the global GHG emissions caused by plastics. Different from other application scenarios, climate costs raised from the application of plastic in agriculture not only come from the production stage but are also influenced by its disturbance of soil carbon and nitrogen turnover during the usage stage. There is still a lack of knowledge about the consequences of replacing PE with BDP on the climate cost of crop production.

The objective of this study is to evaluate the influence of BDP on GHG emissions at different stages of its life cycle, and determine whether replacing PE film with BDP film is a helpful way to reduce the national GHG emissions. To achieve this objective, it was hypothesized that: (i) the application of BDP film will induce a higher carbon footprint associated with agricultural input than PE film because of its greater energy consumption during the production of BDP; and (ii) higher GHG emissions during BDP production can be offset by lower GHG emissions during the usage stage. A field experiment and carbon footprint evaluation were carried out to test these hypotheses.

2. Materials and Methods

2.1. Carbon Footprint of Crop Production in Different Cultivation Systems

Three kinds of cultivation systems, namely, no mulch (NM), PE film mulch (PM) and BDP mulch (BDP), were selected for the comparison. The system boundaries for the carbon footprint assessment in NM, PM and BDP include GHG emissions during industrial, cropping and waste disposal processes, and mainly include the thorough following of GHG emissions through different pathways: (i) the energy consumption at the production stage of agricultural inputs (including seeds, fertilizer, herbicide and plastic film, etc.); (ii) the energy consumption of machinery operation; (iii) the GHG emissions during crop growing period; and (v) the GHG emissions at stage of plastic waste disposal. The area-scale GHG emissions (CF_area, kg CO₂-eq ha⁻¹) and yield-scale GHG emissions (CF_yield, kg CO₂-eq kg⁻¹ grain) were calculated as follows [12]:

CF_area = CF_input + CF_growing + CF_waste

(1)

CF_yield = CF_area/Y

(2)

in which CF_input, CF_growing and CF_waste were the indirect GHG emissions associated with agricultural input (kg CO₂-eq ha⁻¹), GHG emissions during growing period and GHG emissions at waste disposal stage, respectively. The Y was the crop yield in different cultivation systems (kg grain ha⁻¹).

2.2. Experiment Preparation to Characterize Carbon Footprint

To quantify influence of different types of plastic film on the area- and yield-scale GHG emissions at crop growing stage, an experiment was carried out at Shouyang rain-fed agricultural experimental station (37°45′N, 113°12′E, 1080 m altitude), Shanxi, China, in 2019–2020. The research area was characterized by a temperate semiarid continental monsoon climate, and annual average precipitation and temperature were 480 mm and 7.8 °C, respectively. The air temperature, precipitation and evaporation during experimental year is shown in Figure 1. The top 20 cm of soil had a pH of 7.8 ± 0.2, soil organic matter content 18.03 ± 0.72 g kg⁻¹ and total N of 0.85 ± 0.08 g kg⁻¹, and the soil type is Calcaric Cambisol. Planted crop was maize, and it was sown in late April to early May and harvested in late September to early October, with a fallow period which lasted from harvest to sowing next year. No crop was cultivated as a result of low temperature and shortage of water. Maize was cultivated under rain-fed condition, and no irrigation was applied during the whole growing season. In all three kinds of cultivation systems, rotary tillage was carried out to soil depth of 30 cm and all the required fertilizer was applied before sowing without topdressing, and application rates were 225 kg N ha⁻¹ (urea), 162 kg P₂O₅ ha⁻¹ (calcium superphosphate) and 45 kg K₂O ha⁻¹ (potassium chloride). The coverage rate was 67% in PM and BDP, and maize was sown in 60 cm row distance and 30 cm plant distance after fertilization by making hole in the surface of plastic film. The used PE and BDP had thickness of 0.01 mm and the BDP was produced with PBAT and PLA with ratio of 7:3. The PBAT is a kind of petro-based aliphatic–aromatic co-polyester, it was easily biodegradable as a result of the ester bonds in the soft chain of aliphatic polyesters being sensitive to hydrolysis [13,14]. While the PLA was bio-based material that was produced with plant starch, the degradation of PLA was basically caused by water molecules, which attacked the ester bond of PLA molecular chain, transforming it into lactic acid or lactide [15]. During growing period, seedling thinning was conducted manually to remove excess maize seedlings, and herbicide spraying was conducted manually to control weeds. After harvest, the grain was harvested by machine and maize stubble was removed for animal fodder.

2.3. GHG Emissions Associated with Agricultural Input

The indirect GHG emissions associated with agricultural input (CF_inputs, kg CO₂-eq ha⁻¹) was calculated as [16]:

$$\mathrm{C}{\mathrm{F}}_{\mathrm{inputs}}=\sum _i({\mathrm{Q}}_{\mathrm{u}\mathrm{s}\mathrm{e}\mathrm{d}\mathrm{i}}\times {\delta }_i)$$

(3)

in which Q_usedi is the used amount for the ith type of individual agricultural input (kg year⁻¹); δ_i is the emissions factor for the ith type of individual agricultural input (kg CO₂-eq kg⁻¹). The used amounts of seeds, urea, calcium superphosphate, potassium chloride and herbicide were 30, 489, 953, 82 and 2 kg ha⁻¹, respectively, and diesel oil amounts for tillage, sowing, harvest were 48, 29 and 39 kg ha⁻¹. The used amount of plastic film (Q_film) in PM and BDP was calculated as follows [16]:

Q_film = F_mulch × Thick_film × ρ_film × 10,000

(4)

where F_mulch stands for the ratio of area covered by plastic film (67% for PM and BDP) and Thick_film and ρ_film are, respectively, the thickness and density of the plastic film (0.01 mm for PM and BDP; 0.93 and 1.26 kg cm⁻³ for PE and BDP). The BDP was produced with PBAT and PLA plastic resin, and they were transformed to agricultural film after extrusion process [17].

The GHG emissions during production of PBAT and PLA basin were obtained through previous report [11,17], and were 12.2 and 1.82 kg CO₂-eq kg⁻¹, respectively, and the GHG emissions during extrusion were obtained through Ecoinvent 3.1 database; (www.ecoinvent.org, accessed on 5 June 2022), it was 0.59 kg k⁻¹. Finally, the GHG emissions during production stage for BDP were calculated as the total of GHG emissions during production of plastic basin and extrusion and were 9.7 kg CO₂-eq kg⁻¹. For PE film, the GHG emissions during production stage were 2.5 kg CO₂-eq kg⁻¹ [18]. The emissions factors for other types of agricultural input used in this study were summarized in our previous research [16]. For the seeds, urea, calcium superphosphate, potassium chloride, diesel oil, herbicide, the emission factors were 0.58, 2.30, 0.30, 0.13, 0.89, 10.15 kg CO₂-eq kg⁻¹, respectiv2.4. GHG Emissions at Growing and Waste Disposal Stage

At crop growing stage, the GHG emissions were caused by the change in soil carbon stock and N₂O emissions. CH₄ was not taken into account in this study, because of its small percentage in GHG emissions in dry land [19,20]. The change in soil carbon stock was estimated with net ecosystem carbon budget (NECB) methods [21,22], and it was calculated using following equation:

$$\mathrm{NECB}={\displaystyle \sum {\mathrm{C}}_{\mathrm{input}}-\sum {\mathrm{C}}_{\mathrm{output}}=(\mathrm{NPP}+{\mathrm{C}}_{\mathrm{Urea}})}-({\mathrm{C}}_{\mathrm{respired}}+{\mathrm{C}}_{\mathrm{harvest}})$$

(5)

in which the carbon input (C_input) includes the net primary production (NPP) of maize and carbon input from urea (C_urea), and the carbon output sources include respired carbon (C_respired) and removed carbon by harvest (C_harvest). The NPP of maize was calculated as the summary of carbon of aboveground biomass, belowground biomass, litter and rhizodeposit. At the end of growing season, aboveground biomass of maize and its carbon content was measured and the productivity of belowground biomass, litter and rhizodeposited components were calculated using allometric relationship. The root/aboveground biomass, litter/aboveground biomass and rhizodeposits/above and root biomass were estimated as 0.16, 0.05 and 0.08, respectively [22].

The soil CO₂ and N₂O emissions were measured with static closed methods [23]. During measurement, gas was sampled at 0, 20 and 40 min after chamber closure, and then gas samples were analyzed with gas chromatograph. CO₂ and N₂O emission rates were calculated from the linear increase in CO₂ and N₂O concentrations per unit of surface area of the chamber for a specific time interval, and cumulative CO₂ and N₂O emissions were estimated using linear interpolation [24]. Global warming potential of N₂O relative to CO₂ over 100 years was assumed to be 265 [25]. The contributions of soil carbon stock change (CF_SOC, kg CO₂-eq ha⁻¹) and N₂O emissions (CF_N2O, kg CO₂-eq ha⁻¹) to total carbon footprint of maize production were calculated as [12]:

CF_SOC = NECB × 44/12

(6)

CF_N2O = N₂O × 44/28 × 265

(7)

At disposal stage, incineration, landfill and recycling were three typical waste treatments methods for agriculture film, and the emission factors for them were 4.65, 2.95 and 3.74 kg CO₂-eq kg⁻¹ plastic residual [22]. In this study, we assumed 90% of PE film was removed as waste and BDP film did not need to be removed.

3. Result and Discussion

3.1. GHG Emissions Associated with Agricultural Input

In NM, PM and BDP treatments, the CF_input was 1562, 1717 and 2381 kg CO₂-eq ha⁻¹, respectively (Figure 2). The application of plastic film increased the CF_input in PM and BDP, and compared to NM, CF_input was improved by 10% and 52% by PM and BDP. Replacing PM with BDP increased the CF_input by 39%. This result indicated that replacing PM with BDP increased the total GHG emissions at the production stage of agricultural inputs. The main reason for that is the production of BDP film is more complex than PE film. In this study, the materials used for BDP were produced with PBAT and PLA basin. A previous study suggested that the processing chain of PBAT basin was longer than PE basin [17]. The PBAT was produced with a 1,4 -butanediol (BDO), adipic acid (AA) and terephthalic acid (PTA) thorough esterification and polymerization process, and the BDO, AA and PTA were produced with high energy consumption to complete the complex reactions [17]. PLA is made of bio-based resins, and the base materials for their production are widely used materials for crops such as maize and sugarcane. It was produced by the extraction of dextrose from starch, the modification from dextrose to lactic acid, the modification from lactic acid to lactide and then the production of PLA pellets [26]. Compared to PBAT, it has a relatively lower energy consumption during production, but its application ratio during the production of agricultural film is limited because of its brittleness and low toughness. As such, the production of the biodegradable plastic film induced higher GHG emissions than the PE film.

3.2. GHG Emissions during Growing Season

The experiment results suggested that both PM and BDP can improve the crop yield (Figure 3). In 2019, compared to NM, the crop yields in PM and BDP were improved by 33% and 25%, and in 2020, the crop yields were improved by 25% and 15%, respectively (p < 0.05). There was no significant difference for the crop yields between PM and BDP in 2019 or 2020. In previous researches, it was also found that BDP can achieve a similar yield to PM [27,28]. Plastic mulching induced a higher crop yield because it accelerated the seedlings’ growth with improved soil microclimate conditions. The degradation of the biodegradable plastic film occurred during the middle and later stages of maize growth, and had little influence on the maize growth at the early stage.

BDP induced lower soil CO₂ and N₂O emissions during the growing periods than PM. Compared to NM, PM significantly increased soil CO₂ emissions by 15% and 22%, and N₂O emissions by 20% and 31% in 2019 and 2020, respectively. There were no significant differences for CO₂ and N₂O emissions between BDP and NM in 2019, but they were increased by 10% and 14%, respectively, in BDP in 2020. These results are in accordance with previous researches that compared the GHG emissions between NM and PM. In meta-analysis reports, it was reported that PM improved the CO₂ emissions by 27% [29] and the N₂O by 24% compared to those from non-mulched fields [30]. Higher CO₂ emissions under the plastic mulch were attributed to the higher microbial activity and higher rate of soil organic matter decomposition [6,31,32], and higher N₂O emissions were caused by the higher inorganic nitrogen contents, the higher activity of denitrification bacteria and the lower aeration in the soil [33,34]. The reason for the difference between BDP and PM was caused by their differences in the materials’ properties. The PE film is completely impermeable, while a high susceptibility for oxygen and water vapor permeation is a common problem for BDP film under the current technology levels [35]. The higher permeability and cracking of the biodegradable film decreased the warming and moisture conservation effects of the plastic mulch, and then the microbial-driven soil CO₂ and N₂O emissions [36,37].

Improvements in CO₂ emissions lead to lower NECBs in PM and BDP, and the NECBs were ranked in the order of NM>BDP>PM for the two experimental years (

Table 1. Characteristics of net ecosystem carbon budget (NECB), greenhouse gas emissions (GHG), and greenhouse gas intensity (GHGIs) in no mulch, PE mulch (PM) and biodegradable plastic mulch (BDP) at growing stage.

		NECB	N2O	GHG	GHGIs
		kg CO2-eq ha−1			kg CO2-eq kg−1 Grain
2019	NM	−4253 (155)	645 (67)	4898 (166)	0.64 (0.03)
	PM	−4437 (188)	775 (77)	5211 (228)	0.51 (0.02)
	BDP	−4327 (116)	712 (57)	5038 (158)	0.53 (0.02)
2020	NM	−4693 (109)	757 (49)	5451 (74)	0.62 (0.04)
	PM	−5757 (213)	989 (56)	6746 (235)	0.61 (0.02)
	BDP	−4913 (234)	866 (47)	5780 (356)	0.57(0.03)
Average	NM	−4473 (123)	701 (58)	5174 (110)	0.63 (0.03)
	PM	−5097 (200)	882 (67)	5979 (230)	0.56 (0.02)
	BDP	−4620 (239)	789 (52)	5409 (226)	0.55 (0.03)

). Because of the lower NECB and high N₂O emissions, the area-scale GHG emissions at the growing stage were increased by 313 and 140 kg CO₂-eq ha⁻¹ by PM and BDP in 2019, and by 1295 and 329 kg CO₂-eq ha⁻¹ by PM and BDP in 2020, with the average change values of 804 and 235 kg CO₂-eq ha⁻¹ in PM and BDP, respectively. However, because of the improvements in yield, PM and BDP resulted in lower GHG emission intensities (GHGIs). PM and BDP reduced the GHGIs by 0.13 and 0.11 kg CO₂-eq kg⁻¹ grain, respectively in 2019, and reduced it by 0.01 and 0.05 CO₂-eq kg⁻¹ grain in 2020, with an average reduction of 0.07 and 0.08 kg CO₂-eq kg⁻¹ grain, respectively.

3.3. Effects on Total GHG Emissions

BDP did not require waste disposal processes, while for the PE film, it required 261, 165 and 210 kg CO₂-eq ha⁻¹ for incineration, landfill or recycling treatments, respectively. The CF_area was 7868–7964 kg CO₂-eq ha⁻¹ for PM, and was 7790 kg CO₂-eq ha⁻¹ for BDP. There was no significant difference for CF_area between PM and BDP (Figure 4). The yield-scale GHG emissions (CF_yield) were 0.82 kg CO₂-eq kg⁻¹ grain in NM, and it was reduced to 0.74–0.75 kg CO₂-eq kg⁻¹ grain by PM and 0.79 kg CO₂-eq kg⁻¹ grain by BDP. It was suggested by previous researches that there are offset effects between GHG emissions and the crop yield in fields with plastic mulching [22,38]. Plastic mulching improves the CF_area as a results of the improvement in CF_input and higher GHG emissions during the growing stage, but it was offset by yield improvement. Compared to PM, although CF_input was higher in BDP, it had lower GHG emissions at the use and disposal stages; this is the reason why BDP had a similar CF_area to PM. These results indicate that replacing PM with BDP will not increase the GHG emissions from farm land, and production with BDP produces lower GHG emissions than NM for the production of the same amount of grain.

4. Conclusions

The influence of BDP on the carbon footprint of maize production was evaluated with a life cycle assessment, and the results suggested that replacing PM with BDP has different effects at different life cycle stages. At the manufacturing stage, the biodegradable plastic film required more energy and produced higher GHG emissions than the PE film, and this resulted in higher CF_input in the maize cultivation system. However, during the maize growing stage, the application of the biodegradable plastic film induced lower N₂O and CO₂ emissions but had no significant effect on crop yield, and the biodegradable plastic film did not require waste disposal. As such, it had lower GHG emissions at the maize growing and waste disposal stages. Overall, there was no significant difference in CF_area between PM and BDP, and compared to NM, both PM and BDP reduced the CF_yield. The results of this study suggest that the application of BDP is a helpful way to reduce the GHG emissions of crop production, and it should be popularized as it can solve the problem of plastic pollution and alleviate climate change at the same time.