In situ degradation of biodegradable plastic mulch films in compost and agricultural soils -

In situ degradation of biodegradable plastic mulch films in compost and agricultural soils

In this study quantified the degradation of biodegradable plastic mulches in compost and in soil at warm and cool climates (Tennessee and Washington). Mulch degradation was assessed by Fourier transformed infrared (FTIR) spectroscopy, molecular weight analysis, thermo gravimetric analysis (TGA), nuclear-magnetic resonance (NMR), and mulch surface-area quantification. Biodegradable plastic mulches degraded faster in compost than in soil: degradation, as assessed by surface-area reduction, in compost ranged from 85 to 99% after 18 weeks, and in soil from 61 to 83% in Knoxville and 26 to 63% in Mount Vernon after 36 months. This study results indicate that biodegradable plastic mulches degrade in soil, but at different rates in different climates and that degradation occurs over several years. Faster degradation occurred in compost, making composting a viable disposal method, especially in cool climates.

Materials and methods

The overall design of this study involves the investigation of the in situ degradation of biodegradable plastic mulches in compost and soil. Here used biodegradable plastic mulch films under standard agronomic practices and then buried the mulches in compost and soil. The experiment was carried out in two different climatic regions: a humid subtropical climate in Tennessee and a cool mediterranean climate in Washington. And also tested the mulches for biodegradation by CO₂ emission with a standard laboratory ASTM D5988 test.

Three commercial biodegradable plastic mulches and one experimental biodegradable plastic mulch (PLA/PHA, manufactured by Metabolix Inc., Cambridge, MA) were used. In addition, cellulosic-paper mulch (manufactured by Sunshine Paper Co., Aurora, CO) and non-biodegradable polyethylene mulch (manufactured by Filmtech, Allentown, PA) were used as control treatments. BioAgri was produced with N-type Mater-Bi, a product made from polybutylene co-adipate coterephthalate (PBAT) blended with starch. Naturecycle was produced with a blend of starch and polyesters, and Organix was produced with BASF ecovio grade M2351, which consists of PLA and PBAT. The PLA/ PHA (polylactic acid/polyhydroxy alkanoate) experimental mulch consisted of 88.4% MD05-1501 (56% Ingeo PLA from Natureworks (Blair, NE), 24% Mirel™ amorphous PHA, 15% calcium carbonate and 5% plasticizer and processing additives), 10.0% of a master batch (20% carbon black in PLA 3052) prepared by Techmer PM (Clinton, TN), and 1.6% PLA. Biobased content of the mulches (BioAgri, 20–25%; Naturecycle, ~20%; Organix, 10%; PLA/PHA, 86%; paper, 100%; and polyethylene, b1%) was provided by the manufacturers. BioAgri, Organix, PLA/PHA, and the cellulosic paper satisfy the criteria of biodegradation in compost as specified in ASTM D6400, which prescribes that N90% of the organic carbon be converted into CO₂ within 180 days of simulated composting under controlled temperature conditions. The ASTM D5988 test also showed biodegradability of cellulosic paper, BioAgri, and PLA/PHA in soil. Thus, all studied mulches, except polyethylene, are biodegradable in compost and soil based on CO₂ release tests.

The mulches, as received from the manufacturers, were used in field trials with pie pumpkin (Cucurbita pepo), green pepper (Capsicum annuum L.), and sweet corn (Zea mays convar.).  Knoxville has a sandy loam soil (59.9% sand, 23.5% silt, and 16.6% clay), classified as fine kaolinitic thermic Typic Paleudults, whereas Mount Vernon has a silt loam soil (14.2% sand, 69.8% silt, and 16.0% clay), classified as fine-silty, mesic Fluvaquentic Endoaquepts. A no mulch treatment was also included to check for soil changes induced by mulches. Each raised bed was 0.2-m high, 0.8-m wide, and 9-m long, and there were five raised beds per plot. Mulch samples were removed from the soil surface in September 2015 after pumpkin harvest, and physicochemical properties of these field-weathered mulches were determined. Separate pieces of mulch (10 cm × 12 cm) were placed into white nylon mesh bags (250-μm mesh opening, Industrial Netting, Inc., Minneapolis, MN). In this study chose a mesh with 250-μm opening so that we could optimize the capture of mulch fragments that have not been degraded while minimizing loss of fragments falling through the mesh bags.

Composting of mulches

An aerated static pile of compost was built at the Washington State University Research & Extension Center, Puyallup, WA. The compost was prepared using broiler litter (28% vol.), dairy manure solids (28%), fish carcasses (2%), bedding (14%), and yard wastes (28%). The feedstock was mixed with water to obtain a gravimetric water content of 55–65% (wt.) and C: N (g/g) ratio of 25–30:1. The compost pile was under cover to protect it from rain, and it was ≈2-m wide × 4-m long × 2-m high. Meshbags containing the mulch pieces were attached along a 4-mm thick nylon string, with each meshbag placed 2 cm apart. Each nylon string consisted of 24 meshbags and represented one sampling date. Temperature was monitored continuously at different locations at 60-cm depth in the compost pile with TMC50-HD temperature sensors and U12-008 data loggers (Onset Computer Corp., Bourne, MA). The meshbags were retrieved every two weeks for the first seven sampling times and then after 18 weeks of composting, making a total of eight sampling times.

To test degradation in soil, the meshbags containing the field weathered mulches from the 2015 growing season were buried at Knoxville, TN, and Mount Vernon, WA, in October 2015. The meshbags were attached to a 4-mm thick nylon string and aligned, with each meshbag placed 2 cm apart, with six meshbags per plot so that we could sample them destructively six times. The meshbags were placed at about 10-cm depth in the respective plots from where they were sampled. Thus, the meshbags were in soil directly underneath laid mulches during the subsequent growing season. Paper mulch could not be retrieved from the Knoxville field in October 2015 because it had disintegrated during the growing season. To test the degradation of paper mulch in Knoxville, buried weathered paper mulch from Mount Vernon in Knoxville in May 2018. The meshbags were sampled for analysis at 6-month intervals for 3 years. During major field operations each year, such as tillage, the meshbags not yet sampled were temporarily removed from the soil, placed in plastic bags, stored in a refrigerator at 4 °C, and re-buried within two weeks.

The compost and soil temperatures are chosen as the ones measured at the location of the meshbags in the compost and in the soil at 10-cm depth. Thermal time was set to 0 °C-day if the average temperature for a given day was below the base temperature.

Measurement and quantification of mulch degradation

After removal from soil, the surface area of the mulch was measured by image analysis (SI Appendix “Estimation of Mulch Degradation in Soil”). The percentage of visual mulch degradation was then plotted as a function of time. Mulch degradation was further assessed by attenuated total reflectance FTIR spectroscopy, NMR, TGA, and molecular weight analysis. FTIR spectra (4000–600 cm−1 ) were obtained with a IRAffinity-1 spectrometer (Shimadzu Co., Tokyo, Japan) equipped with a single reflection ATR system (MIRacle ATR, PIKE Technologies, Madison, WI).

Results

Mulch degradation in compost

In compost, all biodegradable plastic mulches degraded at least 95% in 2015 and 85% in 2016 based on surface area measurements. PLA/PHA degraded rapidly within the first two weeks. The paper mulch did not degrade within the first 12 weeks in 2015 or within the first six weeks in 2016; but it degraded rapidly thereafter, reaching N85% degradation after 18 weeks both years. The temperature in the compost increased rapidly within a few days to about 80 °C, which likely impeded the activity of fungi, the major decomposers of cellulose.

In contrast to the paper mulch, which degraded about 100% within 1 year, the biodegradable plastics showed degradation of only 20% in Knoxville and 15% in Mount Vernon within 1 year. Over the 3 years of the study, the biodegradable plastic mulches degraded continuously, with a pronounced seasonal pattern of increased degradation during summer. However, it degraded more rapidly thereafter, becoming equivalent to the other biodegradable plastics.  The comparative rate of biodegradation of pure polymers in soils has been reported as high (starch), to moderately high (cellulose), to moderate (PHA), to low moderate (PBAT), to low (PLA), and extremely low (polyethylene) which is consistent with observed mulch degradation. For PLA/PHA, the –OH bending and stretching regions (1082–1052 cm−1 , 1076–1000 cm−1 ) increased after soil burial, particularly at Mount Vernon, reflecting hydrolysis of ester bonds. In addition, the COO stretching region (1800–1700 cm−1) became narrow, with a maximum occurring at 1760 cm−1 and the portion of the band between 1700 and 1750 cm−1 decreasing. These results likely represent the preferred utilization of PHA as a carbon source, leading to a relative increase of PLA’s carbonyl band (~1760 cm−1 ) and decrease of PHA’s carbonyl band (~1720 cm−1). This is consistent with observed increase of PLA’s mass fraction and decrease of PHAs fraction observed via NMR analysis. The three commercially-available PBAT-based mulches underwent more substantial spectral changes due to incubation in the soil: decrease of peak intensities at 1750–1650 cm−1 (COO stretching) and 1450–1300 cm−1 (C-H2 bend) and increase of intensities between 1127 and 800 cm−1 (C\O stretching), suggesting hydrolysis of these mulches. For the polyethylene mulch, the spectral band occurring at 1127–800 cm−1 is attributable to photochemical reactions that occurred during the 2015 field season. After soil burial, these peaks disappeared, but we do not know the reason for this. Thermo gravimetric analyses indicate that soil burial led to a shift to higher temperatures of main heating stages of PLA/PHA and polyethylene mulch. Peaks for the two main heating stages occur at 270 °C and 320 °C, which represent PHA and PLA, respectively. An increase in temperature for various heating stages of a material can also reflect an increase of a component’s thermo stability due to the formation of cross-links. Peak of PBAT heating stage (390 °C), one of the main polymeric components of BioAgri, decreased and shifted to lower temperature after soil burial. This trend likely reflects a loss of molecular weight. However, for starch’s heating stage (310 °C), the peak shifted to higher temperature and eventually started to even out after longer duration of soil burial. This reflects the utilization of lower molecular weight and lower crystalline morphological regions of the plastic mulch as a carbon source by microorganisms throughout the soil burial of 36 months. Soil burial of mulches increased the weight (%) remaining at 600 °C as determined by TGA. Any residual material remaining at 600 °C is likely due to inorganic components of the mulch or soil particulates adhered to the mulch, and are expected to increase in relative percentage if biodegradation of polymers occurs. After soil incubation, the weight % remaining at 600 °C was highest after 36 months of burial, for both BioAgri (63% and 41%) and PLA/PHA (35% and 28%) in Knoxville and Mount Vernon, respectively. Polyethylene did not show a weight change at 600 °C after soil burial. The molecular weights of the polymers in BioAgri and PLA/PHA decreased substantially after soil burial in both Knoxville and Mount Vernon, suggesting degradation of the plastic polymers in the mulches.

Limitations and implications

Meshbags were temporarily removed from the soil during tillage operations, and this prevented physical disturbances to the mulches that occur during tillage; however, the mulch samples buried were small and well embedded into the soil after tillage, so that the lack of tillage in the presence of mulch would not have affected mulch degradation. The meshbags themselves protected the mulches to some extent from degradation by reducing the surface area available for degradation. The standard for measuring biodegradation would be to quantify the amount of organic carbon from the mulches converted to CO₂; however, that is not possible under in situ composting and soil conditions.

While it is known that the biodegradable plastics used in this study indeed undergo biodegradation in laboratory tests (e.g., ASTM D5338, D5988), their incomplete macroscopic disintegration in field soil after 36 months is indicative of incomplete biodegradation. Laboratory tests for biodegradation in compost and soil often use plastic polymers in powder form which increases the degradation rate as compared to when entire plastic film pieces are tested. Slower degradation is thus expected in agricultural fields where plastic is present in the form of films. Macroscopic degradation of the biodegradable plastic mulches after 36 months in soil ranged from 61 to 83% in Tennessee and from 26 to 63% in Washington. This low level of degradation, particularly in cooler climates like in Washington, raises questions about whether the application of biodegradable plastic mulches will be sustainable in the long-term when the mulches are repeatedly tilled into the soil. Plastic mulch may persist in the environment for longer periods when mulch fragments are washed into water bodies by soil erosion, or when fragments leach from the actively degrading topsoil into less degrading subsoil. Paper mulch degraded well in soil: ~100% macroscopic degradation occurred in both the warmer and cooler site in less than 12 months. While the biodegradable plastics did not degrade as well as paper in soil, they did degrade well in compost, making composting a viable disposal method, particularly in locations where soil degradation is slow.

Reference:

Sintim, H.Y., Bary, A.I., Hayes, D.G., Wadsworth, L.C., Anunciado, M.B., English, M.E., Bandopadhyay, S., Schaeffer, S.M., DeBruyn, J.M., Miles, C.A. and Reganold, J.P., 2020. In situ degradation of biodegradable plastic mulch films in compost and agricultural soils. Science of The Total Environment, 727, p.138668.