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Sampling and degradation of biodegradable plastic and paper mulches in field after tillage incorporation
Journal Pre-proof Sampling and degradation of biodegradable plastic and paper mulches in field after tillage incorporation
Shuresh Ghimire, Markus Flury, Ed J. Scheenstra, Carol A. Miles PII:
S0048-9697(19)35572-X
DOI:
https://doi.org/10.1016/j.scitotenv.2019.135577
Reference:
STOTEN 135577
To appear in:
Science of the Total Environment
Received date:
31 August 2019
Revised date:
29 October 2019
Accepted date:
15 November 2019
Please cite this article as: S. Ghimire, M. Flury, E.J. Scheenstra, et al., Sampling and degradation of biodegradable plastic and paper mulches in field after tillage incorporation, Science of the Total Environment (2018), https://doi.org/10.1016/j.scitotenv.2019.135577
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© 2018 Published by Elsevier.
Journal Pre-proof Sampling and Degradation of Biodegradable Plastic and Paper Mulches in Field after Tillage Incorporation
Shuresh Ghimire1,4, Markus Flury2, Ed J. Scheenstra3, and Carol A. Miles3
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Department of Extension, University of Connecticut, Tolland County Extension Center, 24
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Department of Crop and Soil Sciences, Washington State University, Pullman, WA 99164 and
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Hyde Ave, Vernon, CT 06066, USA
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Puyallup, WA 98371, USA
Department of Horticulture, Washington State University, Northwestern Washington Research
Corresponding author: [email protected]
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and Extension Center, 16650 State Route 536, Mount Vernon, WA 98273, USA
Declarations of interest: none
Journal Pre-proof Sampling and Degradation of Biodegradable Plastic and Paper Mulches in Field after Tillage Incorporation
Abstract: Plastic biodegradable mulch (plastic BDM) is tilled after use, but there is concern about incomplete degradation and potential impact on subsequent crops, and we lack a reliable
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method to measure mulch degradation post soil-incorporation. We conducted two field experiments to (i) develop a sampling method to estimate the amount of mulch (fragments size >
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2.36 mm) in the field post soil-incorporation, and (ii) assess the amount of BDM in the soil after
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four consecutive years of mulch incorporation. In Expt. 1, we used the quartering method to
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reduce soil from a 1 m2 field sample area to a representative 19 L sample. In Expt. 2, we applied
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and tilled four plastic BDMs: BioAgri, Naturecycle, Organix AG, and an experimental mulch; and one paper mulch, WeedGuardPlus, in their respective plots for four consecutive years.
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Starting in year 2, we sampled soil with the quartering method each spring and fall to determine
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mulch recovery. With respect to the total amount of mulch applied, average mulch recovery in the fall for the three commercial plastic BDMs was 71%, 50%, and 35% after second, third and
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fourth applications, respectively. For the experimental mulch, the average recovery was 80%, 69%, and 54% in the fall after second, third, and fourth applications, respectively. Recovery was slightly lower in spring than preceding fall all years. For WeedGuardPlus, average recovery was 14%-20% in each fall, and no recovery in any spring (complete degradation). The results show that the quartering method reliably estimates the amount of mulch in a field and BDMs degrade over time in field even with repeated applications, but complete degradation takes more than 1 year. While a few standards (e.g., ASTM D5988) specify how to determine biodegradation of plastics in soil under controlled laboratory conditions, our sampling method measures plastic
Journal Pre-proof degradation under diverse field conditions.
Keywords: biodegradable plastic mulch; paper mulch; mulch degradation; microplastics;
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sampling method; mulch accumulation;
Journal Pre-proof 1. Introduction Plastic mulching has been integral to vegetable production systems for many decades as mulches control weeds, moderate soil temperature, conserve soil moisture, minimize nutrient loss, reduce disease incidence, and ultimately increase crop yield and quality. Most plastic mulches currently being used are made of polyethylene (PE), which needs to be removed from the field after use. As PE mulch fragments into smaller pieces during use and at removal, they form micro- to
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macroplastics leading to plastic pollution of terrestrial systems, which is becoming a major
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environmental issue. Plastic biodegradable mulches (plastic BDMs) provide crop production
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benefits comparable to PE mulch but in addition BDMs can be tilled into the soil after use
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thereby offering a sustainable alternative to PE mulch, and alleviating problems with agricultural plastic pollution (Cowan et al., 2014; DeVetter et al., 2017; Ghimire et al., 2018; Martin-Clossas
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et al., 2016; Miles et al., 2012; Miles et al., 2017; Moreno et al., 2009; Saglam et al., 2017).
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However, there is concern about incomplete biodegradation of BDMs and potential negative impacts on soil quality and subsequent crop production (Brodhagen et al., 2017).
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Incomplete biodegradation would lead to accumulation of plastics in soils; and is thus
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important to quantitatively assess the amount of plastics remaining in soils after use of BDMs. Such assessment is challenging because plastics tend to fragment into micro- and nanoparticles, which are difficult to extract and detect from soil samples (de Souza Machado et al., 2018; Duis and Coors, 2016). Further, the spatial distribution of plastics is highly non-uniform, and care needs to be taken to obtain representative soil samples. Different methods have been used to sample plastics from soils and sediments. For biodegradation studies, plastics are often enclosed into meshbags (Li et al., 2014), so that samples can readily be recovered from soil. Sampling of soils to assess amount of plastics is commonly done with taking soil samples with the quadrat
Journal Pre-proof method or with soil cores. Table 1 summarizes sampling methods that researchers have used to quantify plastics in terrestrial systems. Several studies have evaluated in-soil degradation of commercial BDMs under field conditions using meshbag or soil sampling methods, but the results have been inconsistent (Calmon et al., 1999; Cowan et al., 2013; Li et al., 2014; Moreno et al., 2017; Wortman et al., 2016). Calmon et al. (1999) buried 19 mulches including biodegradable plastics, PE and paper
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using PE meshbags (5 mm by 5 mm mesh size). Mulch recovery after 20 months was 1.6 times
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the weight that was buried in some samples because soil and mycelium adhered to the samples
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even after cleaning. Li et al. (2014) also used meshbags and measured mulch surface area to
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evaluate degradation of BDMs and found that 2 years after burial, average recovery of one of the mulches (BioAgri; BioBag Americas, Palm Harbor, FL) was 99% at Mount Vernon, WA and 2%
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at Lubbock, TX. In the meshbag method, unlike farmers’ practices, the mulch is not incorporated
system.
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into the soil using typical tillage practices nor is it exposed to field conditions within a cropping
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Cowan et al. (2013) used a golf hole cutter (10 cm diameter and 15 cm deep) to collect
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soil samples to measure the amount of mulch following post-soil incorporation. The authors found that the mulch recovery after 13 months in some samples was 1.25 times the amount that was tilled in. Wortman et al. (2016) used the same sized golf hole cutter to collect soil samples post-soil incorporation of mulch and measured the weight of recovered mulch fragments. In some samples, mulch recovery was 20% immediately after incorporation but increased to 55% nine months after incorporation. Moreno et al. (2017) rototilled six BDMs into the soil following pepper (Capsicum annuum L.) harvest. The authors collected soil samples at 0, 70, 160 and 200 days post-incorporation using a cylindrical soil core of 5.75 cm diameter and 8.95 cm depth. The
Journal Pre-proof authors concluded there was high variability in mulch recovery within each mulch treatment (e.g., 0.43–2664 mm2 recovery for a mulch product), thus median was reported instead of mean. Weng et al. (2013) buried three types of biodegradable plastics (5 cm by 10 cm) in the soil at 20 cm and 40 cm depths. The surface morphology of the plastic samples was documented by a digital camera, scanning electron microscope (SEM), and Fourier transform-infrared (FTIR) spectrophotometer every month for up to 5 months. For the most part, plastics degraded
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more quickly at 20 cm depth compared to 40 cm depth, and most samples were completely
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degraded by 5 months of burial. Such imaging methods that start with a known quantity of
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plastic mulch can be robust methods to measure in-soil degradation of plastic mulch in
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experimental conditions but are not applicable in a farmers’ field where mulches are tilled into the soil following crop harvest.
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Standards and methodologies (e.g., ASTM D5988, ISO 17556, and EN 17033) have been
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developed to measure the inherent biodegradation of the major ingredients of plastic BDM in soil under laboratory conditions where temperature, humidity, and oxygen levels are controlled. The
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European Standard EN 17033 (European Committee for Standardization, 2018) is the first
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standard that regulates the biodegradation requirements for plastic BDMs, i.e., > 90% of the organic carbon in the plastic polymers must be converted to CO2 in a controlled laboratory test within 2 years or less. However, there are no standards or established protocols for measuring infield biodegradation of plastic mulches. In most studies on in-field biodegradation of plastic mulches, mulch degradation was assessed by measuring surface area or fragmentation (Cowan et al., 2013; Moreno et al., 2017; Wortman et al., 2016). Ghimire et al. (2017) tested the reliability of the soil core sampling method to sample plastics and concluded this method was inaccurate and unreliable.
Journal Pre-proof There is a need for a sampling method that can accurately estimate the amount of BDM, and plastics in general, remaining in a field after tillage incorporation of the mulch. The presence of visible mulch fragments provides an estimation of the degree of mulch degradation in the field and provides an assessment of accumulation of visible mulch fragments, but it does not directly measure the rate or full extent of degradation. Since temperature, sunlight, moisture, mechanical stresses, and their interactions can affect mulch degradation (Hablot et al., 2014; Kijchavengkul
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et al., 2008; Lucas et al., 2008), mulches should be tested in the weather and soil conditions
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where they will be used in order to accurately evaluate the mulch functionality during the
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cropping season and the mulch degradation after tillage incorporation. Thus, the objective of this
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study was to (i) develop a reliable sampling method to assess the amount of mulch fragments remaining in the field after incorporation, and (ii) use this method to assess the amount of BDM
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into the respective plots every year.
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present in the soil every 6 months for 4 years in a field where five BDMs were applied and tilled
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2. Materials and Methods
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This study included two separate field experiments carried out at the Washington State University Northwestern Washington Research and Extension Center in Mount Vernon, WA (48°43'24" N, 122°39'09" W, elevation 6 m). The field site was located in the maritime Pacific Northwest with Mediterranean-type climate, where year round weather is mild and humid with average annual temperature 10.5 °C and annual rainfall 829 mm. Average soil temperature at 5 cm depth from May to November is 16 °C (range 8-21 °C) and from December to April is 8 °C (range 3-13 °C) (2016-2019 monthly data: AgWeatherNet, 2019). The site has poorly drained
Journal Pre-proof Skagit silt loam soil characterized as a fine-silty, mesic Fluvaquentic Endoaquepts with a pH of 6.2 and organic matter content of 2.8% by weight.
2.1 Experiment 1 – Developing a sampling protocol for macro- and microplastics. Raised beds were formed (15-20 cm high and 0.8 m wide) and a BDM (Organix AG, 17.8 μm; Organix Solutions, Maple Grove, MN) was laid by machine (Model 2600 Bed Shaper; Rain-Flo
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Irrigation, East Pearl, PA) on 11 Oct. 2017. Three replicates each of five beds measuring 10 m
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long, spaced 2 m center-to-center, were formed. Replicates were separated from each other by 8
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m so the tractor-tiller could turn without entering an adjacent replicate. No crop was planted to
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minimize mulch degradation, and 61 days later (11 Dec. 2017) beds were disced, first in the direction parallel to the beds and then a 45° diagonal to the beds, as is the common grower
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practice in this region. The mulch was exposed to field conditions only for 2 months so that
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elasticity of the mulch resembled a field-weathered mulch, but significant degradation had not occurred. Thus, the amount of mulch incorporated was similar to the amount of mulch applied to
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the bed, and mulch fragmentation and distribution in the plot was similar to a typical
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incorporation of a field-weathered mulch. In each replicate, the outer 1 m at each end of plot was excluded from sampling and 24 soil samples were collected 1–3 days after discing: 12 samples were from the center of the three center mulched beds. In each bed, four sampling areas were randomly selected from a total of eight 1 m × 1 m areas. Likewise, 12 samples were collected from the center of the three center alleys (four samples per alley) (Fig. 1a). For each sample, soil was collected from a 1 m × 1 m area to a depth of 15 cm using a shovel. The amount of soil in each sample was then reduced using the quartering method (ASTM International, 2018): the soil was placed on a piece of
Journal Pre-proof plywood, mixed thoroughly, divided into quarters by two lines intersecting at right angles at the center of the pile, and two diagonally opposite quarters were discarded. This procedure was carried out for a total of three times so that the final sample size (19 L) was 1/8 of the original sample size. Mulch fragments were then recovered by wet sieving each soil sample using a 2.36 mm sieve, the mulch fragments were air dried in the laboratory until they reached a constant weight, which was recorded, and the mulch area (cm2) was calculated using the weighing method
|Mass of recovered mulch per sample| Apparent density
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Total mulch area recovered =
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described by Ghimire et al. (2017), i.e.:
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where the apparent density is the mass of the mulch per surface area (g/cm2). Percent mulch
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recovery was then calculated based on the total area of mulch tilled into the soil and area of
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mulch recovered per sample, i.e., as the mulch of width 1.2 m was evenly distributed throughout 2 m width of the bed and alley, 100% mulch recovery would be 0.6 m2 mulch recovered from 1
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m2 soil surface area.
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2.2 Experiment 2 – Assessing mulch accumulation in the field after tillage.
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In Expt. 2, five BDMs were laid in the beginning of the crop growing season, mulch was rototilled into the soil after the crop harvest in the fall, and this cycle was repeated for a total of 4 years (2015 to 2018). Each mulch treatment included five beds spaced 2.4 m apart and 9.1 m long, and was replicated four times with 1.5 m buffer between each block. The same mulch laying machine was used as for Expt. 1. Detailed information about BDM treatments is presented in Table 2, while detailed physical and mechanical properties of these mulches are reported elsewhere (Hayes et al., 2017). Briefly, the apparent density and peak load are linearly proportional to the thickness for Organix AG, BioAgri, and experimental PLA/PHA. However,
Journal Pre-proof Naturecycle possesses a higher density and lower peak load per unit thickness than three other plastic BDMs. All four plastic BDMs possess similar values of % elongation at maximum tensile strength. Paper mulch, as expected, possesses very different physical values compared to the plastic mulches: higher density, thickness, and peak load, and a very low % elongation, the latter reflecting its brittleness. Mulch laying, crops planted, rototilling, and soil sampling dates are listed in Table 3. The
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plots were tilled with a rototiller (tiller 2 m wide and 20 cm deep, power take-off operating speed
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of 540 rpm, forward operating speed of 2.4 km/h; Terranova rototiller, Maschio Gaspardo North
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America Inc., East DeWitt, IA). This type of rototiller is commonly used by farmers to
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incorporate mulch and crop debris into the soil, and to prepare the soil for planting a winter cover crop. Rototilling occurred twice a year, once in fall before collecting samples and second in
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spring after collecting samples. Each time, the field was rototilled twice to incorporate mulch (in
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fall) or cover crop (in spring), once in each direction parallel to the plots, with the rototiller centered on the mulched row to minimize dragging mulch fragments beyond the plot. At the end
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of each plot, the rototiller was lifted, and mulch fragments and soil adhering to the blades were
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removed and redistributed randomly over the respective plot area and buried up to 15 cm deep with a shovel so that all mulch was incorporated into the plots. One soil sample (1 m × 1 m) from the center of row 2 or 4 (based on one-time randomization) in each plot was collected within a month of mulch incorporation in fall and 6 months thereafter in spring, from fall 2015 to spring 2019, such that the final sample was collected 42 months after the first mulch incorporation. In fall 2015 and spring 2016, we used a soil core to collect soil samples for mulch recovery, but this method was found to be inaccurate and unreliable (Ghimire et al., 2017); therefore, mulch recovery data for the first year are not
Journal Pre-proof reported in this paper. Each sampled area was marked and excluded for subsequent samples. Mulch fragments were recovered from each sample as described in Expt. 1 except for the paper mulch, for which dry sieving was done. Mulch area and percent mulch recovery were calculated as described in Expt. 1. Mulch recovery (%) at every measurement time was based on the total amount of mulch applied by that time, so the recovery accounts for the cumulative amount of
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mulch applied to the soil for four years.
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2.3 Statistical analysis
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2.3.1 Experiment 1.
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Percent mulch recovery from Expt. 1 was plotted using response surface analysis (Statistical Analysis System Version 9.4 for Windows; SAS Institute, Cary, NC) to determine the
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distribution of mulch fragments across rows or columns after soil incorporation. The response
distribution).
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2.3.2 Experiment 2.
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surface method uses row and column values to see curvature effects (any peaks or valleys in the
Percent mulch recovery from Expt. 2, which included 3 years of mulch recovery data (fall 2016 to spring 2019), were subjected to analysis of variance using generalized linear mixed model (GLIMMIX) procedure in SAS (Statistical Analysis System Version 9.4 for Windows). Data were analyzed as a randomized complete bock design with repeated measures. The slice statement was used to subdivide means by measurement dates to simplify means’ comparisons. The assumptions of normality and homogeneity of variances were assessed using the Shapiro– Wilk test (W > 0.80) and the Levene’s test ( = 0.05), respectively. The MMAOV macro
Journal Pre-proof (Saxton, 2010) in SAS was used to build all PROC GLIMMIX procedures. Fisher’s least significant difference test at = 0.05 was used to compare treatment means for significant differences.
3. Results and Discussion 3.1 Experiment 1 - Developing a sampling protocol for macro- and microplastics.
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In Expt. 1, the 95% confidence interval for the mulch recovery rate was 92-112%. The mean ±
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SE for mulch recovery in replicates 1, 2, and 3 were 107 ± 9 %, 97 ± 8 %, and 101 ± 8 %,
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respectively. There was no strong evidence to support the differences in mulch recovery among
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any columns or rows (response surface method; R2 = 0.05, P = 0.64 for rows and P = 0.53 for columns) (Fig. 1b). These results indicate that the quartering method was reliable and repeatable
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and the distribution of the mulch fragments in the plot after soil incorporation was uniform
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regardless of the original bed location. Therefore, random sampling using the quartering method can accurately estimate the amount of mulch remaining in the field post-incorporation.
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Previous studies that used a cylindrical soil core to collect soil samples to estimate the
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amount of mulch post-soil incorporation had much higher variability in the mulch recovery from one sample to another. For example, in our previous study, BioAgri recovery ranged from 14% to 95% among four samples and the range of percent mulch recovery within each treatment was between 16 and 81 for four plastic BDM treatments (Ghimire et al. 2017). Moreno et al. (2017) also used a cylindrical soil core to collect three subsamples per treatment. In all five mulch treatments, the minimum number of mulch fragments recovered was zero and the maximum ranged from 9 to 18 fragments. The average size of the smallest fragment among all treatments was 1.6 mm2 and the average size of the largest fragment was 1086 mm2. However, in this
Journal Pre-proof current study, mulch recovery was consistent and close to 100% across different plots indicating the quartering method is repeatable. The main reason for the better performance of the quartering method used here is that the sample support (the volume of soil taken from the ground and which was used for measurements; Webster and Oliver, 1990) is much larger (1 m × 1 m) compared to core samples (typically 5 to 10 cm in diameter), and so a more representative sample can be taken and analyzed. This is particularly important for plastic mulch pieces, which are distributed
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in the soil as sparse, discrete samples (i.e., nuggets; Webster and Oliver, 1990).
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3.2 Experiment 2. – Assessing mulch accumulation in the field after tillage.
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Percent mulch recovery differed due to mulch treatment (P < 0.0001 overall and every sampling time except for fall 2018, where P = 0.0002) and over time (P < 0.0001). There also was a
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significant interaction between time and treatment (P = 0.03). The amount of mulch recovered
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declined over time as the mulch was continuously degrading. Average mulch recovery for the three commercial plastic BDMs (BioAgri, Naturecycle and Organix AG) was 71% in the second
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fall (range 30-95%), 50% in the third fall (range 41-69%), and 35% in the fourth fall (range 30-
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42%) (Fig. 2). In the spring, average recovery for the same three mulches was 50% (range 3167%), 36% (range 21-53%) and 44% (range 23-64%) after the second, third and fourth applications, respectively. For experimental mulch, the average recovery was 80%, 69%, and 54% in the fall and 93%, 39%, and 55% in the spring after the second, third, and fourth applications, respectively. The main ingredient of the experimental mulch was polylactic acid (PLA), which does not degrade rapidly in soil because of its higher glass transition temperature (Tg ~55-60 °C) compared to BDMs that have poly (butylene adipate-co-terephthalate (PBAT; Tg = -30 °C) as the major ingredient (Table 2). The experimental mulch had the lowest amount of
Journal Pre-proof degradation, yet it also had the highest biobased content, i.e. 86%, compared to other BDMs which had 10-25% biobased content, validating that biobased content of a mulch does not determine its biodegradability. For WeedGuardPlus, average recovery was, 14%, 18%, and 20% in the fall after the second, third, and fourth applications, respectively, and no much was recovered in the spring any year (complete degradation). The variability in the amount of mulch recovered within a treatment
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(as shown by standard error bars; Fig. 2) decreased as the amount of recovered mulch declined.
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Mulch recovery was generally only slightly lower in the spring sampling compared to the
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preceding fall because degradation was slow during the winter months.
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This recovery of mulch fragments over time indicates that plastic BDMs tested in this experiment take more than 1 year to completely degrade. However, the amount of mulch
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fragments remaining in the soil 4 years after annual mulch application is significantly less than
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the total amount of mulch that was incorporated, indicating there appears to be no significant macroscopic mulch accumulation following repeated applications. However, plastic mulch
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fragments smaller than 2.36 mm would have passed through the sieve and so would not have
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been captured by the sieving method. Thus, the sieving method may underestimate the total mulch remaining, though any missed pieces would be small and be continuing to degrade quickly as they would have more specific surface area (i.e., edge area) exposed to microbes as compared to the same quantity of one intact mulch fragment. At the same field site as this study, Sintim (2018) buried the same set of field-weathered mulch treatments mulches at 10 cm depth using meshbags in fall 2015. For each mulch treatment, a meshbag was removed from the soil every 6 months for 3 years, and mulch area was measured using Image J software. Paper mulch was 100% degraded within 1 year whereas
Journal Pre-proof average mulch area remaining for commercial plastic BDMs were 84%, 80%, and 59% at the end of years 2, 3 and 4, respectively. The average mulch area remaining for experimental mulch were 97%, 90% and 74% at the end of years 2, 3 and 4. Reduced mulch degradation in the meshbags could have been because of the lack of soil tillage, which would have mechanically decreased the size of mulch fragments, providing more edge area for biodegradation to occur. Although the amount of mulch remaining in the soil was greater in the meshbag study compared
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to this current study, except for the paper mulch, the relative amount of degradation was similar
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for individual mulch treatments. That is, experimental mulch had the least degradation,
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commercial plastic BDMs had intermediate degradation, and paper mulch fully degraded. Li et
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al. (2014) also used the meshbag method to estimate the amount of mulch remaining in the soil in an adjacent field at this site. The authors also found 100% degradation of the paper mulch
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(WeedGuardPlus) within 6 months of burial. However, after 24 months of burial in the soil,
respectively.
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mulch area remaining for BioAgri and Bio360 (formerly BioTelo) were 99% and 89%,
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Sintim (2018) and Li et al. (2014) also used meshbags to evaluate mulch degradation in
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soil in Knoxville, TN, where temperatures are on average 8 °C higher May - Sept. compared to Mount Vernon, WA. Mulch degradation in both studies was significantly higher at Knoxville compared to Mount Vernon. Li et al. (2014) found that mulch area remaining after 2 years of burial in soil in Knoxville was 52% for BioAgri and 43% for BioTelo. Sintim (2018) found that mulch area remaining after 3 years of burial in soil at Knoxville ranged from 60% to 85% for all plastic BDM treatments. These results indicate that mulch degradation rate is greater in warmer climates and a BDM should thus be evaluated in the environmental and soil conditions where it will be used as its performance will vary with different weather and soil conditions.
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4. Conclusions From this study, we found that the quartering method is reliable and repeatable for estimating the amount of macro- to microscopic mulch fragments (≥ 2.36 mm) remaining in field soil after tillage incorporation, where mulch fragments are dispersed sparsely but uniformly in the field. This discrete spatial distribution of the mulch fragments requires a large sample support, i.e., a
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large volume of soil needs to be sampled to provide a representative measurement. The
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quartering method can reliably reduce a large sample size (1 m × 1 m) to a reasonable amount
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that then be used for plastic extraction without compromising the representativeness of the initial
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sample. Our sampling protocol, used here for quantifying the remaining macro- to microscopic fragments from plastic BDM in agricultural soil, can also be used for recovery of macro-, meso-
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and microplastics in other terrestrial systems.
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We also found a significant reduction in mulch fragment recovery over time after tillage incorporation, which indicates that the BDMs tested in this study will not accumulate to a
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significant level in the soil even after repeated applications. As a plastic BDM breaks down into
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smaller and smaller pieces over time, it becomes increasingly difficult to recover the pieces by sieving. Additional sampling methods are needed to measure the smaller micro- and nanoparticles of BDM in soil, and to determine their residence time as the mulch products fully biodegrade. Further studies are also needed to determine the amount of time needed for complete biodegradation of BDMs under field soil conditions.
Journal Pre-proof 5. Acknowledgement We appreciate technical assistance by Henry Sintim, and field assistance by Carolyn Klismith, and Paul Morgan at Washington State University. We thank Arnold Saxton at University of Tennessee (UT) for statistical advice and Douglas Hayes (UT) for thorough review of the manuscript. Funding: This article is based upon work that is supported by the National Institute of Food and
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Agriculture (NIFA), U.S. Department of Agriculture (USDA), under award number 2014-51181-
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22382, and NIFA Hatch projects 1017286 and 1014527. Any opinions, findings, conclusions, or
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recommendations expressed in this article are those of the authors and do not necessarily reflect
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the view of the USDA.
Journal Pre-proof 6. References AgWeatherNet. 2019. Washington State Univ. AgWeatherNet, Mount Vernon, WA. Last accessed 8 July, 2019.
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estuarine shorelines. Environ. Sci. Technol. 44:3404–3409. Calmon, A., S. Guillaume, V. Bellon-Maurel, P. Feuilloley, and F. Silvestre. 1999. Evaluation of
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material biodegradability in real conditions – Development of a burial test and an analysis
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methodology based on numerical vision. J. Environ. Polymer Degradation 7:157-166. Cowan, J.S., C.A. Miles, P.K. Andrews, and D.A. Inglis. 2014. Biodegradable mulch performed
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comparably to polyethylene in high tunnel tomato (Solanum lycopersicum L.) production. J.
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Sci. Food Agr. 94:1854-1864.
Cowan, J.S., D.A. Inglis, and C.A. Miles. 2013. Deterioration of three potentially biodegradable plastic mulches before and after soil incorporation in a broccoli field production system in northwestern Washington. HortTechnology 23:849-858. de Souza Machado, A.A., Kloas, W., Zarfl, C., Hempel, S., M.C. Rillig. Microplastics as an emerging threat to terrestrial ecosystems. Glob Change Biol.2018;24:1405–1416. https://doi.org/10.1111/gcb.14020
Journal Pre-proof DeVetter, L.W., H. Zhang, S. Ghimire, S. Watkinson, and C.A. Miles. 2017. Plastic biodegradable mulches reduce weeds and promote crop growth in day-neutral strawberry in western Washington. HortScience 52(12):1700–1706. doi: 10.21273/HORTSCI12422-17 Duis, K. and Coors, A. 2016. Microplastics in the aquatic and terrestrial environment: sources (with a specific focus on personal care products), fate and effects. Environ Sci Eur 28: 2. https://doi.org/10.1186/s12302-015-0069-y
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European Committee for Standardization. 2018. EN 17033: Plastics-biodegradable mulch films
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for use in agriculture and horticulture-Requirements and test methods. European Standard,
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European Committee for Standardization, Brussels, Belgium.
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Ghimire, S., A. Saxton, A.L. Wszelaki, J.C. Moore, and C.A. Miles. 2017. Reliability of soil sampling method to assess visible biodegradable mulch fragments remaining in the field
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post soil-incorporation. HortTechnology 27:650-658.
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Ghimire, S., A.L. Wszelaki, J.C. Moore, D.A. Inglis, and C.A. Miles. 2018. Use of biodegradable mulches in pie pumpkin production. HortScience 53:288-294.
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Hablot, E., S. Dharmalingam, D.G. Hayes, L.C. Wadsworth, C. Blazy, and R, Narayan. 2014.
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Effect of simulated weathering on physicochemical properties and inherent biodegradation of PLA/PHA nonwoven mulches. J. Polymers and Environ. 22:417-429. Hayes, D. G., L. C. Wadsworth, H. Y., Sintim, M. Flury, M. English, S. M. Schaeffer, and A. M. Saxton. 2017. Effect of diverse weathering conditions on the physicochemical properties of biodegradable plastic mulches. Polymer Testing 62:454-467. Hurley, R., J. Woodward, and J.J. Rothwell. 2018. Microplastic contamination of river beds significantly reduced by catchment-wide flooding. Nature Geoscience 11:251-257. https://doi.org/10.1038/s41561-018-0080-1
Journal Pre-proof Kijchavengkul, T., R. Auras, M. Rubino, M. Ngouajio, and R.T. Fernandez. 2008. Assessment of aliphatic-aromatic copolyester biodegradable mulch films. Part I: Field study. Chemosphere 71:942-953. Li, C., J. Moore-Kucera, C. Miles, K. Leonas, J. Lee, A. Corbin, and D. Inglis. 2014. Degradation of potentially biodegradable plastic mulch films at three diverse U.S. locations. Agroecol. Sustain. Food Syst. 38:861-889.
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Liu, M., S. Lu, Y. Song, L. Lei, J. Hu, W. Lv, W. Zhou, C. Cao, H. Shi, X. Yang, and D. He.
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2018. Microplastic and mesoplastic pollution in farmland soils in suburbs of Shanghai,
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China. Environmental Pollution 242:855–862. doi:10.1016/J.ENVPOL.2018.07.051
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Lucas, N., C. Bienaime, C. Belloy, M. Queneudec, F. Silvestre, and J.E. Nava-Saucedo. 2008. Polymer biodegradation: Mechanisms and estimation techniques - A review. Chemosphere
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Martín-Closas, L., J. Costa, A. Cirujeda, J. Aibar. Zaragoza, A. Pardo, M. L. Suso, M. M. Moreno, C. Moreno, I. Lahoz, J. I. Mácua, and A. M. Pelacho. 2016. Above-soil and in-soil
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54:225-236.
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degradation of oxo- and bio-degradable mulches: a qualitative approach. Soil Research
Miles, C. A., L. DeVetter, S., Ghimire, and D.G. Hayes. 2017. Suitability of biodegradable plastic mulches for organic and sustainable agricultural production systems. HortScience 52:10-15. Miles, C., R. Wallace, A. Wszelaki, J. Martin, J. Cowan, T. Walters, and D. Inglis. 2012. Deterioration of potentially biodegradable alternatives to black plastic mulch in three tomato production regions. HortScience 47:1270-1277.
Journal Pre-proof Moreno, M.M., A. Moreno, and I. Mancebo. 2009. Comparison of different mulch materials in a tomato (Solanum lycopersicum L.) crop. Spanish J. Agr. Res. 7:454-464. Moreno, M.M., S. Gonzalez-Mora, J. Villena, J.A. Campos, and C. Moreno. 2017. Deterioration pattern of six biodegradable, potentially low-environmental impact mulches in field conditions. Journal of Environmental Management 200:490-501. Piehl, S., A. Leibner, M.G.J. Löder, R. Dris, C. Bogner, and C. Laforsch.2018. Identification and
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quantification of macro- and microplastics on an agricultural farmland. Sci Rep.
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8(1):17950. https://doi.org/10.1038/s41598-018-36172-y.
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Saglam, M., H.Y. Sintim, A. Bary, C. Miles, S. Ghimire, D. Inglis, and M. Flury. 2017.
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Modeling the effect of biodegradable paper and plastic mulch on soil moisture dynamics. Agricultural Water Management 193:240-250.
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Tennessee, Knoxville, TN.
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Saxton A.M. 2010. DandA.sas Design and analysis macro collection version 1.29. Univ. of
Scheurer, M. and M. Bigalke. 2018. Microplastics in Swiss floodplain soils. Environmental
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Science & Technology 52 (6):3591-3598. DOI: 10.1021/acs.est.7b06003
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Sintim, H.Y., Biodegradable Plastic Mulch: Degradation and Impacts on Soil Health, PhD Dissertation, 2018, Washington State University, Pullman, WA. Webster, R. and M.A. Oliver. 1990. Statistical Methods in Soil and Land Resource Survey. Oxford University Press, New York. Weng, Y., L. Wang, M. Zhang, X. Wang, and Y. Wang. 2013. Biodegradation behavior of P(3HB,4HB)/PLA blends in real soil environments. Polymer Testing 32:60-70. Wortman, S. E., I. Kadoma, and M.D. Crandall. 2016. Biodegradable plastic and fabric mulch performance in field and high tunnel cucumber production. HortTechnology 26:148-155.
Journal Pre-proof Zhang, D., H.B. Liu, W.L. Hu, X.H. Qin, X.W. Ma, C.R. Yan, and H.Y. Wang. 2016. The status and distribution characteristics of residual mulching film in Xinjiang, China. Journal of Integrative Agriculture 15(11):2639-2646. Zhang, G.S. and Y.F. Liu. 2018. The distribution of microplastics in soil aggregate fractions in southwestern China. Science of the Total Environment 642:12–20.
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doi:10.1016/J.SCITOTENV.2018.06.004
Journal Pre-proof Conflict of Interest Statement
All authors hereby affirm that we have no personal or financial conflict of interest, direct or indirect, regarding the content or support of the study submitted for publication.
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-Shuresh Ghimire, Markus Flury, Ed Scheenstra, and Carol Miles
Journal Pre-proof Figure 1. (a) Plot map showing the location of alleys, beds, and sampling areas for one of the three replicates. Beds were covered with plastic mulch film (Organix mulch), and borders were used to separate the replicates. Plot was disced parallel and then diagonal to the beds to incorporate mulch into the soil. (b) Distribution of mulch recovery of Organix plastic film 1 - 3 days after discing. The figure shows response contour lines of mulch recovery; number along the lines indicate percent mulch recovery at respective areas in the plots, and shading represents the
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standard error.
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Figure 2. Percent recovery of mulch fragments every 6 months for 3 years in the mulch
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degradation study at Mount Vernon, WA. Percent mulch recovery was based on the mulch
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application rate of 0.6 m2 mulch per 1 m2 soil surface area. Data are means of four replicates, and errors bars represent standard errors. The plots were rototilled twice a year, once in fall before
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collecting samples and second in spring after collecting samples.
Journal Pre-proof Table 1. Summary of sampling methods that have been used to quantify plastics in terrestrial systems. Terrestrial
Samplin
Area/volu
No. of
Target
Averag
Recover
Referen
system
g
me
replicates/subsampl
object
e
y (%)y
ce
method
sampled
esz
2-99%
Li et al.
recover y per
al soil
Meshba
10.15 cm ×
4 replicates per
gs
10.15 cm
treatment
Biodegradab
ro
Agricultur
of
sample
le mulch
2-99 cm2
(2014)
5 cm × 20
3 replicates per
gs
cm
treatment
re
al soil
Meshba
Soil
10 cm
al soil
core
diameter ×
3 subsamples per
na
Agricultur
lP
Agricultur
-p
film
sample
ur
15 cm
Biodegradab
0-100
le mulch
cm2
0-100%
Calmon et al.
film
(1999)
Biodegradab
0-563
le mulch
cm2
0-125%
Cowan et al.
film
(2013)
Jo
height
Agricultur
Soil
10 cm
5 or 8 subsamples
Biodegradab
0-660
al soil
core
diameter ×
per sample
le mulch
cm2
15 cm
0-55%
Wortma n et al.
film
(2016)
height Agricultur
Soil
5.75 cm
3 subsamples per
Biodegradab
1.5 -4.6
al soil
core
diameter ×
sample
le mulch
mg
et al.
film
(media
(2017)
8.95 cm height
n)
27-43%
Moreno
Journal Pre-proof Agricultur
Soil
10 cm
5 subsamples per
Biodegradab
60-540
al soil
core
diameter ×
sample
le mulch
cm2
15 cm
8-72%
Ghimire et al.
film
(2017)
height Agricultur al soil
Direct
10 cm × 5
5 replicates per
Biodegradab
soil
cm
treatment
le mulch
al.
film
(2013)
Quadrat
al soil
200 cm ×
5 replicates per
Plastic
0-502
100 cm ×
treatment
mulch film
kg ha–1
ro
Agricultur
cm × 5 cm
treatment
s
particle
al.
s kg−1
(2018)
Macro- and
78-
cm × 3 cm
treatment
microplastic
63 item
al.
s
s kg−1
(2018)
6 subsamples per
Microplastic
7100 to
sample
s
42,960
and Liu
particle
(2018)
na
na
Piehl et
3 replicates per
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al soil
Spade
na
50 cm × 50
height
Agricultur
(2016)
0.34
ur
al soil
al.
Microplastic
na
Quadrat
Zhang et
14 replicates per
height Agricultur
na
Weng et
32 cm × 32
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al soil
re
height Quadrat
na
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30 cm
Agricultur
of
burial
na
na
Liu et
Zhang
s kg−1 Floodplai n soils
Quadrat
8 cm × 8
5 subsamples per
Microplastic
56-593
cm × 5 cm
sample
s
mg
and
kg−1
Bigalke
height
na
Scheurer
(2018)
Journal Pre-proof Riverbed
Cylinder
sediment
42 cm
4 subsamples per
Microplastic
0.56 ±
diameter ×
sample
s
0.18 kg
et al.
km-2
(2018)
69 cm
na
Hurley
height Shoreline
Quadrat
50 mL of
5 replicates per
Plastic
3.2
sediment
treatment
debris
items
et al.
of
(2010)
Browne
of
collected
na
plastics
m2
ro
from 0.25
Subsamples were used to make a composite sample.
y
Recovery (%) is given for studies were initial plastic amount was known (i.e., plastic mulch
re
-p
z
between application and sampling.
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na
na: not available or not applicable.
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films used in agriculture). For biodegradable plastics, degradation of plastic may have occurred
Journal Pre-proof Table 2. Mulch treatments, manufacturers, mulch density, thickness, peak load, key ingredients, and percent biobased content provided by manufacturers for Experiment 2 at Mount Vernon, WA in 2015–2019. BioTreatment
Manufacturer
Density
Thickness
(g m−2)z
(μm)
Peak
Key based
Load (N)z ingredient(s)y
BioAgri
BioBag
22.8 ± 0.4
re
27.7 ± 0.4
25.0
ur
Custom
26.6 ± 0.5
EF04P)
86
MirelTM amorphous PHA 25.4
8.5 ± 0.3
Starch-
Bioplastics,
polyester
Burlington,
blend
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Naturecycle
(grade
17.4 ± 0.5 Ingeo® PLA /
na
filmw
20-25
(PBAT)
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Experimental
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Dunedin, FL
Exp. PLA/PHA
12.1 ± 0.6 Mater-Bi®
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Americas,
18.0
of
%x
≥20
WA Organix AG
Organix
19.8 ± 0.3
17.8
9.1 ± 0.4
BASF
Solutions,
ecovio® grade
Bloomington,
M2351
MN
(PBAT + PLA)
10
Journal Pre-proof WeedGuardPlus Sunshine
110.9 ± 0.5
240.0
88.2 ± 7.1 Cellulose
100
Paper, Aurora, CO z
Hayes et al. (2017).
y
PBAT= poly(butylene adipate-co-terephthalate), PLA=polylactic acid, PHA= polyhydroxy
alkanoate). Composition (%) of mulch that is from biological products or renewable materials, reported by
of
x
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na
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Not available commercially, prepared for this study by Metabolix, Inc., Cambridge, MA.
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w
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each company for their product.
Journal Pre-proof Table 3. Date of mulch laying, crop planted, date of tillage and soil sampling for Experiment 2 carried out at Mount Vernon, WA from 2015 to 2019. Study
Mulch
year
laying
2015
26 May
Crop plantedz
Mulch tillage
‘Cinnamon Girl’
Spring
Fall
Sampling
sampling
28 Sept.
-y
-
3 Oct.
12 Apr.y
5 Oct.
11 Apr.
9 Oct.
5 Oct.
12 Mar.
23 Oct.
++
12 Apr.
++
2016
25 May
‘Cinnamon Girl’
19 May
‘Xtra-Tender
6 Oct.
-p
2017
ro
pumpkin
‘Xtra-Tender
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17 May
re
2171’ sweet corn 2018
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pumpkin
2171’ sweet corn ++x
++
na
2019
Additionally, winter wheat was planted as cover crop each winter from 2015 to 2019.
y
No samples were collected as the experiment was not yet underway or data was unreliable.
x
Mulch was not installed, and a vegetable crop was not planted in 2019 as the field entered a
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z
winter wheat rotation for the next experiment.
Journal Pre-proof Highlights We developed a sampling method to quantify macro- and microplastics in soil. The quartering method reliably quantifies plastics in field soil. Biodegradable mulch does not accumulate in field even after repeated applications. Full degradation of biodegradable plastic mulch takes >1 year in field conditions.
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Biobased content does not determine the degradation rate of the mulch.
Figure 1
Figure 2
Shuresh Ghimire, Markus Flury, Ed J. Scheenstra, Carol A. Miles PII:
S0048-9697(19)35572-X
DOI:
https://doi.org/10.1016/j.scitotenv.2019.135577
Reference:
STOTEN 135577
To appear in:
Science of the Total Environment
Received date:
31 August 2019
Revised date:
29 October 2019
Accepted date:
15 November 2019
Please cite this article as: S. Ghimire, M. Flury, E.J. Scheenstra, et al., Sampling and degradation of biodegradable plastic and paper mulches in field after tillage incorporation, Science of the Total Environment (2018), https://doi.org/10.1016/j.scitotenv.2019.135577
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© 2018 Published by Elsevier.
Journal Pre-proof Sampling and Degradation of Biodegradable Plastic and Paper Mulches in Field after Tillage Incorporation
Shuresh Ghimire1,4, Markus Flury2, Ed J. Scheenstra3, and Carol A. Miles3
1
Department of Extension, University of Connecticut, Tolland County Extension Center, 24
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Department of Crop and Soil Sciences, Washington State University, Pullman, WA 99164 and
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Hyde Ave, Vernon, CT 06066, USA
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Puyallup, WA 98371, USA
Department of Horticulture, Washington State University, Northwestern Washington Research
Corresponding author: [email protected]
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and Extension Center, 16650 State Route 536, Mount Vernon, WA 98273, USA
Declarations of interest: none
Journal Pre-proof Sampling and Degradation of Biodegradable Plastic and Paper Mulches in Field after Tillage Incorporation
Abstract: Plastic biodegradable mulch (plastic BDM) is tilled after use, but there is concern about incomplete degradation and potential impact on subsequent crops, and we lack a reliable
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method to measure mulch degradation post soil-incorporation. We conducted two field experiments to (i) develop a sampling method to estimate the amount of mulch (fragments size >
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2.36 mm) in the field post soil-incorporation, and (ii) assess the amount of BDM in the soil after
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four consecutive years of mulch incorporation. In Expt. 1, we used the quartering method to
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reduce soil from a 1 m2 field sample area to a representative 19 L sample. In Expt. 2, we applied
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and tilled four plastic BDMs: BioAgri, Naturecycle, Organix AG, and an experimental mulch; and one paper mulch, WeedGuardPlus, in their respective plots for four consecutive years.
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Starting in year 2, we sampled soil with the quartering method each spring and fall to determine
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mulch recovery. With respect to the total amount of mulch applied, average mulch recovery in the fall for the three commercial plastic BDMs was 71%, 50%, and 35% after second, third and
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fourth applications, respectively. For the experimental mulch, the average recovery was 80%, 69%, and 54% in the fall after second, third, and fourth applications, respectively. Recovery was slightly lower in spring than preceding fall all years. For WeedGuardPlus, average recovery was 14%-20% in each fall, and no recovery in any spring (complete degradation). The results show that the quartering method reliably estimates the amount of mulch in a field and BDMs degrade over time in field even with repeated applications, but complete degradation takes more than 1 year. While a few standards (e.g., ASTM D5988) specify how to determine biodegradation of plastics in soil under controlled laboratory conditions, our sampling method measures plastic
Journal Pre-proof degradation under diverse field conditions.
Keywords: biodegradable plastic mulch; paper mulch; mulch degradation; microplastics;
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sampling method; mulch accumulation;
Journal Pre-proof 1. Introduction Plastic mulching has been integral to vegetable production systems for many decades as mulches control weeds, moderate soil temperature, conserve soil moisture, minimize nutrient loss, reduce disease incidence, and ultimately increase crop yield and quality. Most plastic mulches currently being used are made of polyethylene (PE), which needs to be removed from the field after use. As PE mulch fragments into smaller pieces during use and at removal, they form micro- to
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macroplastics leading to plastic pollution of terrestrial systems, which is becoming a major
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environmental issue. Plastic biodegradable mulches (plastic BDMs) provide crop production
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benefits comparable to PE mulch but in addition BDMs can be tilled into the soil after use
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thereby offering a sustainable alternative to PE mulch, and alleviating problems with agricultural plastic pollution (Cowan et al., 2014; DeVetter et al., 2017; Ghimire et al., 2018; Martin-Clossas
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et al., 2016; Miles et al., 2012; Miles et al., 2017; Moreno et al., 2009; Saglam et al., 2017).
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However, there is concern about incomplete biodegradation of BDMs and potential negative impacts on soil quality and subsequent crop production (Brodhagen et al., 2017).
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Incomplete biodegradation would lead to accumulation of plastics in soils; and is thus
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important to quantitatively assess the amount of plastics remaining in soils after use of BDMs. Such assessment is challenging because plastics tend to fragment into micro- and nanoparticles, which are difficult to extract and detect from soil samples (de Souza Machado et al., 2018; Duis and Coors, 2016). Further, the spatial distribution of plastics is highly non-uniform, and care needs to be taken to obtain representative soil samples. Different methods have been used to sample plastics from soils and sediments. For biodegradation studies, plastics are often enclosed into meshbags (Li et al., 2014), so that samples can readily be recovered from soil. Sampling of soils to assess amount of plastics is commonly done with taking soil samples with the quadrat
Journal Pre-proof method or with soil cores. Table 1 summarizes sampling methods that researchers have used to quantify plastics in terrestrial systems. Several studies have evaluated in-soil degradation of commercial BDMs under field conditions using meshbag or soil sampling methods, but the results have been inconsistent (Calmon et al., 1999; Cowan et al., 2013; Li et al., 2014; Moreno et al., 2017; Wortman et al., 2016). Calmon et al. (1999) buried 19 mulches including biodegradable plastics, PE and paper
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using PE meshbags (5 mm by 5 mm mesh size). Mulch recovery after 20 months was 1.6 times
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the weight that was buried in some samples because soil and mycelium adhered to the samples
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even after cleaning. Li et al. (2014) also used meshbags and measured mulch surface area to
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evaluate degradation of BDMs and found that 2 years after burial, average recovery of one of the mulches (BioAgri; BioBag Americas, Palm Harbor, FL) was 99% at Mount Vernon, WA and 2%
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at Lubbock, TX. In the meshbag method, unlike farmers’ practices, the mulch is not incorporated
system.
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into the soil using typical tillage practices nor is it exposed to field conditions within a cropping
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Cowan et al. (2013) used a golf hole cutter (10 cm diameter and 15 cm deep) to collect
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soil samples to measure the amount of mulch following post-soil incorporation. The authors found that the mulch recovery after 13 months in some samples was 1.25 times the amount that was tilled in. Wortman et al. (2016) used the same sized golf hole cutter to collect soil samples post-soil incorporation of mulch and measured the weight of recovered mulch fragments. In some samples, mulch recovery was 20% immediately after incorporation but increased to 55% nine months after incorporation. Moreno et al. (2017) rototilled six BDMs into the soil following pepper (Capsicum annuum L.) harvest. The authors collected soil samples at 0, 70, 160 and 200 days post-incorporation using a cylindrical soil core of 5.75 cm diameter and 8.95 cm depth. The
Journal Pre-proof authors concluded there was high variability in mulch recovery within each mulch treatment (e.g., 0.43–2664 mm2 recovery for a mulch product), thus median was reported instead of mean. Weng et al. (2013) buried three types of biodegradable plastics (5 cm by 10 cm) in the soil at 20 cm and 40 cm depths. The surface morphology of the plastic samples was documented by a digital camera, scanning electron microscope (SEM), and Fourier transform-infrared (FTIR) spectrophotometer every month for up to 5 months. For the most part, plastics degraded
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more quickly at 20 cm depth compared to 40 cm depth, and most samples were completely
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degraded by 5 months of burial. Such imaging methods that start with a known quantity of
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plastic mulch can be robust methods to measure in-soil degradation of plastic mulch in
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experimental conditions but are not applicable in a farmers’ field where mulches are tilled into the soil following crop harvest.
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Standards and methodologies (e.g., ASTM D5988, ISO 17556, and EN 17033) have been
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developed to measure the inherent biodegradation of the major ingredients of plastic BDM in soil under laboratory conditions where temperature, humidity, and oxygen levels are controlled. The
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European Standard EN 17033 (European Committee for Standardization, 2018) is the first
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standard that regulates the biodegradation requirements for plastic BDMs, i.e., > 90% of the organic carbon in the plastic polymers must be converted to CO2 in a controlled laboratory test within 2 years or less. However, there are no standards or established protocols for measuring infield biodegradation of plastic mulches. In most studies on in-field biodegradation of plastic mulches, mulch degradation was assessed by measuring surface area or fragmentation (Cowan et al., 2013; Moreno et al., 2017; Wortman et al., 2016). Ghimire et al. (2017) tested the reliability of the soil core sampling method to sample plastics and concluded this method was inaccurate and unreliable.
Journal Pre-proof There is a need for a sampling method that can accurately estimate the amount of BDM, and plastics in general, remaining in a field after tillage incorporation of the mulch. The presence of visible mulch fragments provides an estimation of the degree of mulch degradation in the field and provides an assessment of accumulation of visible mulch fragments, but it does not directly measure the rate or full extent of degradation. Since temperature, sunlight, moisture, mechanical stresses, and their interactions can affect mulch degradation (Hablot et al., 2014; Kijchavengkul
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et al., 2008; Lucas et al., 2008), mulches should be tested in the weather and soil conditions
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where they will be used in order to accurately evaluate the mulch functionality during the
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cropping season and the mulch degradation after tillage incorporation. Thus, the objective of this
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study was to (i) develop a reliable sampling method to assess the amount of mulch fragments remaining in the field after incorporation, and (ii) use this method to assess the amount of BDM
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into the respective plots every year.
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present in the soil every 6 months for 4 years in a field where five BDMs were applied and tilled
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2. Materials and Methods
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This study included two separate field experiments carried out at the Washington State University Northwestern Washington Research and Extension Center in Mount Vernon, WA (48°43'24" N, 122°39'09" W, elevation 6 m). The field site was located in the maritime Pacific Northwest with Mediterranean-type climate, where year round weather is mild and humid with average annual temperature 10.5 °C and annual rainfall 829 mm. Average soil temperature at 5 cm depth from May to November is 16 °C (range 8-21 °C) and from December to April is 8 °C (range 3-13 °C) (2016-2019 monthly data: AgWeatherNet, 2019). The site has poorly drained
Journal Pre-proof Skagit silt loam soil characterized as a fine-silty, mesic Fluvaquentic Endoaquepts with a pH of 6.2 and organic matter content of 2.8% by weight.
2.1 Experiment 1 – Developing a sampling protocol for macro- and microplastics. Raised beds were formed (15-20 cm high and 0.8 m wide) and a BDM (Organix AG, 17.8 μm; Organix Solutions, Maple Grove, MN) was laid by machine (Model 2600 Bed Shaper; Rain-Flo
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Irrigation, East Pearl, PA) on 11 Oct. 2017. Three replicates each of five beds measuring 10 m
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long, spaced 2 m center-to-center, were formed. Replicates were separated from each other by 8
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m so the tractor-tiller could turn without entering an adjacent replicate. No crop was planted to
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minimize mulch degradation, and 61 days later (11 Dec. 2017) beds were disced, first in the direction parallel to the beds and then a 45° diagonal to the beds, as is the common grower
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practice in this region. The mulch was exposed to field conditions only for 2 months so that
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elasticity of the mulch resembled a field-weathered mulch, but significant degradation had not occurred. Thus, the amount of mulch incorporated was similar to the amount of mulch applied to
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the bed, and mulch fragmentation and distribution in the plot was similar to a typical
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incorporation of a field-weathered mulch. In each replicate, the outer 1 m at each end of plot was excluded from sampling and 24 soil samples were collected 1–3 days after discing: 12 samples were from the center of the three center mulched beds. In each bed, four sampling areas were randomly selected from a total of eight 1 m × 1 m areas. Likewise, 12 samples were collected from the center of the three center alleys (four samples per alley) (Fig. 1a). For each sample, soil was collected from a 1 m × 1 m area to a depth of 15 cm using a shovel. The amount of soil in each sample was then reduced using the quartering method (ASTM International, 2018): the soil was placed on a piece of
Journal Pre-proof plywood, mixed thoroughly, divided into quarters by two lines intersecting at right angles at the center of the pile, and two diagonally opposite quarters were discarded. This procedure was carried out for a total of three times so that the final sample size (19 L) was 1/8 of the original sample size. Mulch fragments were then recovered by wet sieving each soil sample using a 2.36 mm sieve, the mulch fragments were air dried in the laboratory until they reached a constant weight, which was recorded, and the mulch area (cm2) was calculated using the weighing method
|Mass of recovered mulch per sample| Apparent density
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Total mulch area recovered =
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described by Ghimire et al. (2017), i.e.:
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where the apparent density is the mass of the mulch per surface area (g/cm2). Percent mulch
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recovery was then calculated based on the total area of mulch tilled into the soil and area of
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mulch recovered per sample, i.e., as the mulch of width 1.2 m was evenly distributed throughout 2 m width of the bed and alley, 100% mulch recovery would be 0.6 m2 mulch recovered from 1
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m2 soil surface area.
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2.2 Experiment 2 – Assessing mulch accumulation in the field after tillage.
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In Expt. 2, five BDMs were laid in the beginning of the crop growing season, mulch was rototilled into the soil after the crop harvest in the fall, and this cycle was repeated for a total of 4 years (2015 to 2018). Each mulch treatment included five beds spaced 2.4 m apart and 9.1 m long, and was replicated four times with 1.5 m buffer between each block. The same mulch laying machine was used as for Expt. 1. Detailed information about BDM treatments is presented in Table 2, while detailed physical and mechanical properties of these mulches are reported elsewhere (Hayes et al., 2017). Briefly, the apparent density and peak load are linearly proportional to the thickness for Organix AG, BioAgri, and experimental PLA/PHA. However,
Journal Pre-proof Naturecycle possesses a higher density and lower peak load per unit thickness than three other plastic BDMs. All four plastic BDMs possess similar values of % elongation at maximum tensile strength. Paper mulch, as expected, possesses very different physical values compared to the plastic mulches: higher density, thickness, and peak load, and a very low % elongation, the latter reflecting its brittleness. Mulch laying, crops planted, rototilling, and soil sampling dates are listed in Table 3. The
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plots were tilled with a rototiller (tiller 2 m wide and 20 cm deep, power take-off operating speed
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of 540 rpm, forward operating speed of 2.4 km/h; Terranova rototiller, Maschio Gaspardo North
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America Inc., East DeWitt, IA). This type of rototiller is commonly used by farmers to
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incorporate mulch and crop debris into the soil, and to prepare the soil for planting a winter cover crop. Rototilling occurred twice a year, once in fall before collecting samples and second in
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spring after collecting samples. Each time, the field was rototilled twice to incorporate mulch (in
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fall) or cover crop (in spring), once in each direction parallel to the plots, with the rototiller centered on the mulched row to minimize dragging mulch fragments beyond the plot. At the end
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of each plot, the rototiller was lifted, and mulch fragments and soil adhering to the blades were
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removed and redistributed randomly over the respective plot area and buried up to 15 cm deep with a shovel so that all mulch was incorporated into the plots. One soil sample (1 m × 1 m) from the center of row 2 or 4 (based on one-time randomization) in each plot was collected within a month of mulch incorporation in fall and 6 months thereafter in spring, from fall 2015 to spring 2019, such that the final sample was collected 42 months after the first mulch incorporation. In fall 2015 and spring 2016, we used a soil core to collect soil samples for mulch recovery, but this method was found to be inaccurate and unreliable (Ghimire et al., 2017); therefore, mulch recovery data for the first year are not
Journal Pre-proof reported in this paper. Each sampled area was marked and excluded for subsequent samples. Mulch fragments were recovered from each sample as described in Expt. 1 except for the paper mulch, for which dry sieving was done. Mulch area and percent mulch recovery were calculated as described in Expt. 1. Mulch recovery (%) at every measurement time was based on the total amount of mulch applied by that time, so the recovery accounts for the cumulative amount of
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mulch applied to the soil for four years.
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2.3 Statistical analysis
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2.3.1 Experiment 1.
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Percent mulch recovery from Expt. 1 was plotted using response surface analysis (Statistical Analysis System Version 9.4 for Windows; SAS Institute, Cary, NC) to determine the
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distribution of mulch fragments across rows or columns after soil incorporation. The response
distribution).
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2.3.2 Experiment 2.
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surface method uses row and column values to see curvature effects (any peaks or valleys in the
Percent mulch recovery from Expt. 2, which included 3 years of mulch recovery data (fall 2016 to spring 2019), were subjected to analysis of variance using generalized linear mixed model (GLIMMIX) procedure in SAS (Statistical Analysis System Version 9.4 for Windows). Data were analyzed as a randomized complete bock design with repeated measures. The slice statement was used to subdivide means by measurement dates to simplify means’ comparisons. The assumptions of normality and homogeneity of variances were assessed using the Shapiro– Wilk test (W > 0.80) and the Levene’s test ( = 0.05), respectively. The MMAOV macro
Journal Pre-proof (Saxton, 2010) in SAS was used to build all PROC GLIMMIX procedures. Fisher’s least significant difference test at = 0.05 was used to compare treatment means for significant differences.
3. Results and Discussion 3.1 Experiment 1 - Developing a sampling protocol for macro- and microplastics.
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In Expt. 1, the 95% confidence interval for the mulch recovery rate was 92-112%. The mean ±
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SE for mulch recovery in replicates 1, 2, and 3 were 107 ± 9 %, 97 ± 8 %, and 101 ± 8 %,
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respectively. There was no strong evidence to support the differences in mulch recovery among
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any columns or rows (response surface method; R2 = 0.05, P = 0.64 for rows and P = 0.53 for columns) (Fig. 1b). These results indicate that the quartering method was reliable and repeatable
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and the distribution of the mulch fragments in the plot after soil incorporation was uniform
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regardless of the original bed location. Therefore, random sampling using the quartering method can accurately estimate the amount of mulch remaining in the field post-incorporation.
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Previous studies that used a cylindrical soil core to collect soil samples to estimate the
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amount of mulch post-soil incorporation had much higher variability in the mulch recovery from one sample to another. For example, in our previous study, BioAgri recovery ranged from 14% to 95% among four samples and the range of percent mulch recovery within each treatment was between 16 and 81 for four plastic BDM treatments (Ghimire et al. 2017). Moreno et al. (2017) also used a cylindrical soil core to collect three subsamples per treatment. In all five mulch treatments, the minimum number of mulch fragments recovered was zero and the maximum ranged from 9 to 18 fragments. The average size of the smallest fragment among all treatments was 1.6 mm2 and the average size of the largest fragment was 1086 mm2. However, in this
Journal Pre-proof current study, mulch recovery was consistent and close to 100% across different plots indicating the quartering method is repeatable. The main reason for the better performance of the quartering method used here is that the sample support (the volume of soil taken from the ground and which was used for measurements; Webster and Oliver, 1990) is much larger (1 m × 1 m) compared to core samples (typically 5 to 10 cm in diameter), and so a more representative sample can be taken and analyzed. This is particularly important for plastic mulch pieces, which are distributed
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in the soil as sparse, discrete samples (i.e., nuggets; Webster and Oliver, 1990).
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3.2 Experiment 2. – Assessing mulch accumulation in the field after tillage.
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Percent mulch recovery differed due to mulch treatment (P < 0.0001 overall and every sampling time except for fall 2018, where P = 0.0002) and over time (P < 0.0001). There also was a
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significant interaction between time and treatment (P = 0.03). The amount of mulch recovered
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declined over time as the mulch was continuously degrading. Average mulch recovery for the three commercial plastic BDMs (BioAgri, Naturecycle and Organix AG) was 71% in the second
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fall (range 30-95%), 50% in the third fall (range 41-69%), and 35% in the fourth fall (range 30-
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42%) (Fig. 2). In the spring, average recovery for the same three mulches was 50% (range 3167%), 36% (range 21-53%) and 44% (range 23-64%) after the second, third and fourth applications, respectively. For experimental mulch, the average recovery was 80%, 69%, and 54% in the fall and 93%, 39%, and 55% in the spring after the second, third, and fourth applications, respectively. The main ingredient of the experimental mulch was polylactic acid (PLA), which does not degrade rapidly in soil because of its higher glass transition temperature (Tg ~55-60 °C) compared to BDMs that have poly (butylene adipate-co-terephthalate (PBAT; Tg = -30 °C) as the major ingredient (Table 2). The experimental mulch had the lowest amount of
Journal Pre-proof degradation, yet it also had the highest biobased content, i.e. 86%, compared to other BDMs which had 10-25% biobased content, validating that biobased content of a mulch does not determine its biodegradability. For WeedGuardPlus, average recovery was, 14%, 18%, and 20% in the fall after the second, third, and fourth applications, respectively, and no much was recovered in the spring any year (complete degradation). The variability in the amount of mulch recovered within a treatment
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(as shown by standard error bars; Fig. 2) decreased as the amount of recovered mulch declined.
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Mulch recovery was generally only slightly lower in the spring sampling compared to the
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preceding fall because degradation was slow during the winter months.
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This recovery of mulch fragments over time indicates that plastic BDMs tested in this experiment take more than 1 year to completely degrade. However, the amount of mulch
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fragments remaining in the soil 4 years after annual mulch application is significantly less than
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the total amount of mulch that was incorporated, indicating there appears to be no significant macroscopic mulch accumulation following repeated applications. However, plastic mulch
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fragments smaller than 2.36 mm would have passed through the sieve and so would not have
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been captured by the sieving method. Thus, the sieving method may underestimate the total mulch remaining, though any missed pieces would be small and be continuing to degrade quickly as they would have more specific surface area (i.e., edge area) exposed to microbes as compared to the same quantity of one intact mulch fragment. At the same field site as this study, Sintim (2018) buried the same set of field-weathered mulch treatments mulches at 10 cm depth using meshbags in fall 2015. For each mulch treatment, a meshbag was removed from the soil every 6 months for 3 years, and mulch area was measured using Image J software. Paper mulch was 100% degraded within 1 year whereas
Journal Pre-proof average mulch area remaining for commercial plastic BDMs were 84%, 80%, and 59% at the end of years 2, 3 and 4, respectively. The average mulch area remaining for experimental mulch were 97%, 90% and 74% at the end of years 2, 3 and 4. Reduced mulch degradation in the meshbags could have been because of the lack of soil tillage, which would have mechanically decreased the size of mulch fragments, providing more edge area for biodegradation to occur. Although the amount of mulch remaining in the soil was greater in the meshbag study compared
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to this current study, except for the paper mulch, the relative amount of degradation was similar
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for individual mulch treatments. That is, experimental mulch had the least degradation,
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commercial plastic BDMs had intermediate degradation, and paper mulch fully degraded. Li et
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al. (2014) also used the meshbag method to estimate the amount of mulch remaining in the soil in an adjacent field at this site. The authors also found 100% degradation of the paper mulch
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(WeedGuardPlus) within 6 months of burial. However, after 24 months of burial in the soil,
respectively.
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mulch area remaining for BioAgri and Bio360 (formerly BioTelo) were 99% and 89%,
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Sintim (2018) and Li et al. (2014) also used meshbags to evaluate mulch degradation in
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soil in Knoxville, TN, where temperatures are on average 8 °C higher May - Sept. compared to Mount Vernon, WA. Mulch degradation in both studies was significantly higher at Knoxville compared to Mount Vernon. Li et al. (2014) found that mulch area remaining after 2 years of burial in soil in Knoxville was 52% for BioAgri and 43% for BioTelo. Sintim (2018) found that mulch area remaining after 3 years of burial in soil at Knoxville ranged from 60% to 85% for all plastic BDM treatments. These results indicate that mulch degradation rate is greater in warmer climates and a BDM should thus be evaluated in the environmental and soil conditions where it will be used as its performance will vary with different weather and soil conditions.
Journal Pre-proof
4. Conclusions From this study, we found that the quartering method is reliable and repeatable for estimating the amount of macro- to microscopic mulch fragments (≥ 2.36 mm) remaining in field soil after tillage incorporation, where mulch fragments are dispersed sparsely but uniformly in the field. This discrete spatial distribution of the mulch fragments requires a large sample support, i.e., a
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large volume of soil needs to be sampled to provide a representative measurement. The
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quartering method can reliably reduce a large sample size (1 m × 1 m) to a reasonable amount
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that then be used for plastic extraction without compromising the representativeness of the initial
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sample. Our sampling protocol, used here for quantifying the remaining macro- to microscopic fragments from plastic BDM in agricultural soil, can also be used for recovery of macro-, meso-
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and microplastics in other terrestrial systems.
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We also found a significant reduction in mulch fragment recovery over time after tillage incorporation, which indicates that the BDMs tested in this study will not accumulate to a
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significant level in the soil even after repeated applications. As a plastic BDM breaks down into
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smaller and smaller pieces over time, it becomes increasingly difficult to recover the pieces by sieving. Additional sampling methods are needed to measure the smaller micro- and nanoparticles of BDM in soil, and to determine their residence time as the mulch products fully biodegrade. Further studies are also needed to determine the amount of time needed for complete biodegradation of BDMs under field soil conditions.
Journal Pre-proof 5. Acknowledgement We appreciate technical assistance by Henry Sintim, and field assistance by Carolyn Klismith, and Paul Morgan at Washington State University. We thank Arnold Saxton at University of Tennessee (UT) for statistical advice and Douglas Hayes (UT) for thorough review of the manuscript. Funding: This article is based upon work that is supported by the National Institute of Food and
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Agriculture (NIFA), U.S. Department of Agriculture (USDA), under award number 2014-51181-
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22382, and NIFA Hatch projects 1017286 and 1014527. Any opinions, findings, conclusions, or
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recommendations expressed in this article are those of the authors and do not necessarily reflect
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the view of the USDA.
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Ghimire, S., A. Saxton, A.L. Wszelaki, J.C. Moore, and C.A. Miles. 2017. Reliability of soil sampling method to assess visible biodegradable mulch fragments remaining in the field
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Ghimire, S., A.L. Wszelaki, J.C. Moore, D.A. Inglis, and C.A. Miles. 2018. Use of biodegradable mulches in pie pumpkin production. HortScience 53:288-294.
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Effect of simulated weathering on physicochemical properties and inherent biodegradation of PLA/PHA nonwoven mulches. J. Polymers and Environ. 22:417-429. Hayes, D. G., L. C. Wadsworth, H. Y., Sintim, M. Flury, M. English, S. M. Schaeffer, and A. M. Saxton. 2017. Effect of diverse weathering conditions on the physicochemical properties of biodegradable plastic mulches. Polymer Testing 62:454-467. Hurley, R., J. Woodward, and J.J. Rothwell. 2018. Microplastic contamination of river beds significantly reduced by catchment-wide flooding. Nature Geoscience 11:251-257. https://doi.org/10.1038/s41561-018-0080-1
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Miles, C. A., L. DeVetter, S., Ghimire, and D.G. Hayes. 2017. Suitability of biodegradable plastic mulches for organic and sustainable agricultural production systems. HortScience 52:10-15. Miles, C., R. Wallace, A. Wszelaki, J. Martin, J. Cowan, T. Walters, and D. Inglis. 2012. Deterioration of potentially biodegradable alternatives to black plastic mulch in three tomato production regions. HortScience 47:1270-1277.
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Scheurer, M. and M. Bigalke. 2018. Microplastics in Swiss floodplain soils. Environmental
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Science & Technology 52 (6):3591-3598. DOI: 10.1021/acs.est.7b06003
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Sintim, H.Y., Biodegradable Plastic Mulch: Degradation and Impacts on Soil Health, PhD Dissertation, 2018, Washington State University, Pullman, WA. Webster, R. and M.A. Oliver. 1990. Statistical Methods in Soil and Land Resource Survey. Oxford University Press, New York. Weng, Y., L. Wang, M. Zhang, X. Wang, and Y. Wang. 2013. Biodegradation behavior of P(3HB,4HB)/PLA blends in real soil environments. Polymer Testing 32:60-70. Wortman, S. E., I. Kadoma, and M.D. Crandall. 2016. Biodegradable plastic and fabric mulch performance in field and high tunnel cucumber production. HortTechnology 26:148-155.
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doi:10.1016/J.SCITOTENV.2018.06.004
Journal Pre-proof Conflict of Interest Statement
All authors hereby affirm that we have no personal or financial conflict of interest, direct or indirect, regarding the content or support of the study submitted for publication.
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-Shuresh Ghimire, Markus Flury, Ed Scheenstra, and Carol Miles
Journal Pre-proof Figure 1. (a) Plot map showing the location of alleys, beds, and sampling areas for one of the three replicates. Beds were covered with plastic mulch film (Organix mulch), and borders were used to separate the replicates. Plot was disced parallel and then diagonal to the beds to incorporate mulch into the soil. (b) Distribution of mulch recovery of Organix plastic film 1 - 3 days after discing. The figure shows response contour lines of mulch recovery; number along the lines indicate percent mulch recovery at respective areas in the plots, and shading represents the
of
standard error.
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Figure 2. Percent recovery of mulch fragments every 6 months for 3 years in the mulch
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degradation study at Mount Vernon, WA. Percent mulch recovery was based on the mulch
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application rate of 0.6 m2 mulch per 1 m2 soil surface area. Data are means of four replicates, and errors bars represent standard errors. The plots were rototilled twice a year, once in fall before
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collecting samples and second in spring after collecting samples.
Journal Pre-proof Table 1. Summary of sampling methods that have been used to quantify plastics in terrestrial systems. Terrestrial
Samplin
Area/volu
No. of
Target
Averag
Recover
Referen
system
g
me
replicates/subsampl
object
e
y (%)y
ce
method
sampled
esz
2-99%
Li et al.
recover y per
al soil
Meshba
10.15 cm ×
4 replicates per
gs
10.15 cm
treatment
Biodegradab
ro
Agricultur
of
sample
le mulch
2-99 cm2
(2014)
5 cm × 20
3 replicates per
gs
cm
treatment
re
al soil
Meshba
Soil
10 cm
al soil
core
diameter ×
3 subsamples per
na
Agricultur
lP
Agricultur
-p
film
sample
ur
15 cm
Biodegradab
0-100
le mulch
cm2
0-100%
Calmon et al.
film
(1999)
Biodegradab
0-563
le mulch
cm2
0-125%
Cowan et al.
film
(2013)
Jo
height
Agricultur
Soil
10 cm
5 or 8 subsamples
Biodegradab
0-660
al soil
core
diameter ×
per sample
le mulch
cm2
15 cm
0-55%
Wortma n et al.
film
(2016)
height Agricultur
Soil
5.75 cm
3 subsamples per
Biodegradab
1.5 -4.6
al soil
core
diameter ×
sample
le mulch
mg
et al.
film
(media
(2017)
8.95 cm height
n)
27-43%
Moreno
Journal Pre-proof Agricultur
Soil
10 cm
5 subsamples per
Biodegradab
60-540
al soil
core
diameter ×
sample
le mulch
cm2
15 cm
8-72%
Ghimire et al.
film
(2017)
height Agricultur al soil
Direct
10 cm × 5
5 replicates per
Biodegradab
soil
cm
treatment
le mulch
al.
film
(2013)
Quadrat
al soil
200 cm ×
5 replicates per
Plastic
0-502
100 cm ×
treatment
mulch film
kg ha–1
ro
Agricultur
cm × 5 cm
treatment
s
particle
al.
s kg−1
(2018)
Macro- and
78-
cm × 3 cm
treatment
microplastic
63 item
al.
s
s kg−1
(2018)
6 subsamples per
Microplastic
7100 to
sample
s
42,960
and Liu
particle
(2018)
na
na
Piehl et
3 replicates per
Jo
al soil
Spade
na
50 cm × 50
height
Agricultur
(2016)
0.34
ur
al soil
al.
Microplastic
na
Quadrat
Zhang et
14 replicates per
height Agricultur
na
Weng et
32 cm × 32
lP
al soil
re
height Quadrat
na
-p
30 cm
Agricultur
of
burial
na
na
Liu et
Zhang
s kg−1 Floodplai n soils
Quadrat
8 cm × 8
5 subsamples per
Microplastic
56-593
cm × 5 cm
sample
s
mg
and
kg−1
Bigalke
height
na
Scheurer
(2018)
Journal Pre-proof Riverbed
Cylinder
sediment
42 cm
4 subsamples per
Microplastic
0.56 ±
diameter ×
sample
s
0.18 kg
et al.
km-2
(2018)
69 cm
na
Hurley
height Shoreline
Quadrat
50 mL of
5 replicates per
Plastic
3.2
sediment
treatment
debris
items
et al.
of
(2010)
Browne
of
collected
na
plastics
m2
ro
from 0.25
Subsamples were used to make a composite sample.
y
Recovery (%) is given for studies were initial plastic amount was known (i.e., plastic mulch
re
-p
z
between application and sampling.
Jo
ur
na
na: not available or not applicable.
lP
films used in agriculture). For biodegradable plastics, degradation of plastic may have occurred
Journal Pre-proof Table 2. Mulch treatments, manufacturers, mulch density, thickness, peak load, key ingredients, and percent biobased content provided by manufacturers for Experiment 2 at Mount Vernon, WA in 2015–2019. BioTreatment
Manufacturer
Density
Thickness
(g m−2)z
(μm)
Peak
Key based
Load (N)z ingredient(s)y
BioAgri
BioBag
22.8 ± 0.4
re
27.7 ± 0.4
25.0
ur
Custom
26.6 ± 0.5
EF04P)
86
MirelTM amorphous PHA 25.4
8.5 ± 0.3
Starch-
Bioplastics,
polyester
Burlington,
blend
Jo
Naturecycle
(grade
17.4 ± 0.5 Ingeo® PLA /
na
filmw
20-25
(PBAT)
lP
Experimental
-p
Dunedin, FL
Exp. PLA/PHA
12.1 ± 0.6 Mater-Bi®
ro
Americas,
18.0
of
%x
≥20
WA Organix AG
Organix
19.8 ± 0.3
17.8
9.1 ± 0.4
BASF
Solutions,
ecovio® grade
Bloomington,
M2351
MN
(PBAT + PLA)
10
Journal Pre-proof WeedGuardPlus Sunshine
110.9 ± 0.5
240.0
88.2 ± 7.1 Cellulose
100
Paper, Aurora, CO z
Hayes et al. (2017).
y
PBAT= poly(butylene adipate-co-terephthalate), PLA=polylactic acid, PHA= polyhydroxy
alkanoate). Composition (%) of mulch that is from biological products or renewable materials, reported by
of
x
ur
na
lP
re
-p
Not available commercially, prepared for this study by Metabolix, Inc., Cambridge, MA.
Jo
w
ro
each company for their product.
Journal Pre-proof Table 3. Date of mulch laying, crop planted, date of tillage and soil sampling for Experiment 2 carried out at Mount Vernon, WA from 2015 to 2019. Study
Mulch
year
laying
2015
26 May
Crop plantedz
Mulch tillage
‘Cinnamon Girl’
Spring
Fall
Sampling
sampling
28 Sept.
-y
-
3 Oct.
12 Apr.y
5 Oct.
11 Apr.
9 Oct.
5 Oct.
12 Mar.
23 Oct.
++
12 Apr.
++
2016
25 May
‘Cinnamon Girl’
19 May
‘Xtra-Tender
6 Oct.
-p
2017
ro
pumpkin
‘Xtra-Tender
lP
17 May
re
2171’ sweet corn 2018
of
pumpkin
2171’ sweet corn ++x
++
na
2019
Additionally, winter wheat was planted as cover crop each winter from 2015 to 2019.
y
No samples were collected as the experiment was not yet underway or data was unreliable.
x
Mulch was not installed, and a vegetable crop was not planted in 2019 as the field entered a
Jo
ur
z
winter wheat rotation for the next experiment.
Journal Pre-proof Highlights We developed a sampling method to quantify macro- and microplastics in soil. The quartering method reliably quantifies plastics in field soil. Biodegradable mulch does not accumulate in field even after repeated applications. Full degradation of biodegradable plastic mulch takes >1 year in field conditions.
Jo
ur
na
lP
re
-p
ro
of
Biobased content does not determine the degradation rate of the mulch.
Figure 1
Figure 2