PE mulch has been used in agriculture for 60 years, and is considered effective and affordable; however, its use leads to waste and pollution. In 2011, PE mulch was used on nearly 20 million hectares in China with its use projected to grow 7% or more annually, whereas in 2012, an estimated 98,300 tons of PE mulch was used in North America (Liu et al., 2014; Markets and Markets, 2012). Whereas PE mulch controls weeds, conserves soil moisture, and increases overall crop yield and quality (Cowan et al., 2014), this technology contributes to environmental pollution (Liu et al., 2014). Most PE mulch is removed from the field and disposed of in landfills, buried or burned on-site, or dumped in streams, rivers, or the ocean because used PE mulch is contaminated with soil and crop debris (up to 50% by weight) and not readily recyclable (Kasirajan and Ngouajio, 2012). Residual PE mulch is left in the field [estimated 5% to 10% (L. Martin-Closas, personal communication)], where it negatively impacts soil structure, water quality, and crop growth, and can enter water systems, thereby disrupting the agricultural ecosystem and overall environment (Steinmetz et al., 2016). It is worth noting that over 80% of the plastic waste found in oceans originates from disposal on land (Li et al., 2016).
Biodegradable plastics are degradable plastic in which the degradation results from the action of naturally occurring microorganisms such as bacteria, fungi, and algae (ASTM International, 2011). Biodegradable plastic offers a potential solution to some of the issues associated with PE mulch; however, BDM users are concerned about the extent and rate of mulch biodegradation in the field after soil incorporation and impacts on soil health and the productivity of subsequent crops (Goldberger et al., 2015; Miles et al., 2009; Yamamoto-Tamura et al., 2015). Many studies have evaluated crop yield with BDM as well as mulch functionality and deterioration (loss of physical or mechanical strength, as observed through physical strength testing, microscopic imaging, or sizable macroscopic alteration of morphology) during the growing season (Anzalone et al., 2010; Cowan et al., 2014; Jenni et al., 2007; Kasirajan and Ngouajio, 2012; Li et al., 2014; Miles et al., 2012; Moreno et al., 2009; Ngouajio et al., 2008; Waterer, 2010; Wortman et al., 2016). Deterioration of BDM aboveground is driven by temperature, sunlight, moisture, mechanical stresses, and their interactions, and can affect biodegradation, which primarily occurs belowground (Hablot et al., 2014; Kijchavengkul et al., 2008; Lucas et al., 2008; Singh and Sharma, 2008). Biodegradation is the disintegration of materials by microorganisms or other biological means, producing carbon dioxide (CO2) or methane (CH4), water, and microbial biomass (Kyrikou and Briassoulis, 2007). In the field, the extent of biodegradation of soil-incorporated BDM can be assessed by the presence of visible mulch fragments in soil samples. This does not directly measure the degree of biodegradation, but does provide an estimation of the initial stage of mulch biodegradation.
Currently, there is no established field method to measure the amount of BDM remaining in the soil after incorporation, and the few studies that have attempted to make assessments have used somewhat similar methods. Calmon et al. (1999) buried a known surface area (5 × 20 cm) and mass of 19 films made from polyhydrobutyrate hydroxyvalerate, polycaprolactone (PCL), PCL-starch, poly (lactic acid) (PLA), starch-PLA, cellophane, protein, PE, PE-starch, and paper, where each material represented a different level of biodegradability. Each sample was placed in a PE mesh (0.5 × 0.5 cm) bag, and samples were placed in the soil at five different depths (0–20 cm) at a 45° angle to enhance water drainage. Biodegradation was determined by measuring the mass of recovered samples (after cleaning) and the area (image analysis, method not specified) at 4, 8, 12, 16, 20, and 24 months after burial. Results indicated that the mass of PLA films in some samples increased up to 160% after 20 months because of high adherence of soil particles and mycelium on fragments even after cleaning, whereas image analysis of these same samples showed that the surface area decreased. The authors concluded that image analysis was more robust compared with mass measurements for assessing biodegradation in the field. Although this study included a positive control treatment (PE), the mulches were not exposed to field conditions within a cropping system before burial, nor were they incorporated into the soil using typical tillage practices as they would when used on a farm.
Li et al. (2014) also used mesh bags to evaluate biodegradation of four BDMs in three climatically distinct locations of the United States (Knoxville, TN; Lubbock, TX; Mount Vernon, WA) by measuring loss of mulch surface area. At all three locations, the authors found that after 24 months of burial, there was no decrease in the surface area of spunbond PLA [experimental nonwoven PLA (feedstock from NatureWorks, Blair, NE)], and the average surface area loss of two commercial BDMs [BioAgri (BioBag Americas, Palm Harbor, FL) and BioTelo Agri (Dubois Agrinovation, Waterford, ON, Canada)] was 52% at Knoxville, 98% at Lubbock, and 6% at Mount Vernon. The biodegradability of the BDMs at all sites may have been reduced by the lack of subsequent soil tillage and the protection provided by the mesh bags. To address these two issues, Cowan et al. (2013) tilled three BDMs into the soil following a broccoli (Brassica oleracea var. italica) crop at Mount Vernon, WA, and 13 months after incorporation, randomly sampled the soil to a 6-inch depth with a large soil core (golf hole cutter, 4 inches diameter). The authors bulked three soil samples per plot, and extracted the mulch through a wet sieve (1.18 mm) process. Mulch fragments were placed on a glass plate, photographed (EOS Digital Rebel XT; Canon USA, Lake Success, NY), and the surface area was calculated using image-processing software (ImageJ; National Institutes of Health, Bethesda, MD) (Rasband, 1997). The authors calculated percent loss of surface area relative to that of the total soil sample, and found that surface area of spunbond PLA did not decrease whereas surface area of BioAgri mulch had decreased 60%, and no fragments of Crown 1 mulch [currently marketed as Naturecycle (Custom Bioplastics, Burlington, WA)] were found. In a similar study, Wortman et al. (2016) grew cucumber (Cucumis sativus) on four experimental biofabrics [all spunbond nonwoven PLAs, but with different thickness and color (3M Co., Saint Paul, MN)], incorporated the mulch following the final cucumber harvest, collected soil samples with the same-sized soil core as Cowan et al. (2013) 9 months after incorporation, and measured the mass of recovered mulch fragments. Mulch recovery ranged from 5% to 55%, which was in contrast with the results of Cowan et al. (2013) and Li et al. (2014) where 125% and 100% of the initial amount of spunbond PLA were recovered 13 and 24 months after soil incorporation, respectively. This difference in mulch recovery could have been related to different spunbond PLA formulations used in the different studies, but also could be due to inaccuracy of the soil sampling method. Moreno et al. (2014) reported a method to determine the amount of residual mulch remaining at the end of the season using image analysis, but that study did not incorporate the mulch into the soil; mulch was measured on the soil surface only.
Organic and other growers concerned about environmental sustainability frequently are interested in knowing if BDM is indeed biodegrading fully after soil incorporation. Although laboratory tests can assess the potential of a mulch product to biodegrade under certain conditions (ASTM International, 2012), results may vary widely under field conditions as demonstrated by the studies cited previously. Because sampling errors often have been much greater than analytical errors for general soil sampling studies in the past, developing reliable soil sampling methods is essential (Webster and Oliver, 1990). Random soil sampling is suitable only in a highly homogenous field. A reliable and efficient soil sampling protocol is needed that enables growers and agricultural professionals to estimate mulch biodegradation after soil incorporation.
The specific objectives of this study were to 1) compare three methods (graph paper, image, and weight) to measure the amount of mulch recovered in soil samples; 2) determine the distribution of mulch fragments in the plot after soil incorporation; and 3) calculate the number of soil samples needed per unit area to assess the amount of mulch remaining in the soil.
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