Carol Miles

Biodegradable plastic agricultural mulches and key features of microbial degradation

The development of biodegradable plastic mulch films for use in agriculture has been ongoing for decades. These films consist of mixtures of polymers with various additives. As a result, their physical and chemical properties differ from those of the pure polymers often used for in vitro enzymatic and microbial degradation studies, raising questions about the biodegradation capability of mulch films. Currently, standards exist for the biodegradation of plastics in composting conditions but not in soil. Biodegradation in soil or compost depends on a complex synergy of biological and abiotic degradative processes. This review discusses the physicochemical and structural properties of biodegradable plastic mulches, examines their potential for on-site decomposition in light of site-to-site variance due to environmental and biological conditions, and considers the potential for long-term effects on agroecosystem sustainability and functionality.

Degradation of Potentially Biodegradable Plastic Mulch Films at Three Diverse U.S. Locations

For widespread adoption of biodegradable plastics as agricultural mulches, dependable biodegradation across contrasting conditions is necessary. The in situ degradation of four potentially biodegradable mulches (two commercially available starch-based films, one commercially available cellulose paper mulch, and one experimental spunbond polylactic acid mulch) were evaluated by measuring percentage of area remaining (PMAR) after burial for 6, 12, 18, and 24 months in high tunnel and open field tomato production systems at three geographically distinct U.S. locations (Knoxville, TN; Lubbock, TX; Mount Vernon, WA). The PMAR of the mulches did not differ between high tunnel and open field systems at any location, and PMAR of cellulose mulch was 0% within 12 months but >90% for experimental spunbond at 24 months. The PMAR of the two starch-based products did vary by location and was lowest at Lubbock (˜2%) compared to Knoxville (49%) or Mount Vernon (89%). Relative to the other two locations, Lubbock had the greatest soil diurnal temperature range, maximum daily soil temperature, an alkaline soil pH, and a microbial community structure characterized by a relatively high abundance of fungi. Mulch type and geographic location exerted a greater influence on PMAR than did production system, and abiotic and biotic variables influenced degradation.

Barriers and bridges to the adoption of biodegradable plastic mulches for US specialty crop production

Commercial farmers have been using polyethylene plastic mulch since the 1950s. Despite the affordability and effectiveness of polyethylene mulch, the disposal process is financially and environmentally costly. Biodegradable plastic mulches, an ecologically sustainable alternative to polyethylene mulch films, were introduced in the 1980s. Biodegradable plastic mulches can be tilled into the soil or composted at the end of the season, reducing the labor and environmental costs associated with plastic removal and disposal. However, research results are mixed as to the effectiveness, degradability and ease-of-use of biodegradable plastic mulches. In 2008–2012, researchers, funded by a USDA Specialty Crop Research Initiative grant, conducted surveys and focus groups in three different agricultural regions of the USA to better understand the barriers and bridges to the adoption of biodegradable plastic mulches for specialty crop production systems. Data on the experiences and views of specialty crop growers, agricultural extension agents, agricultural input suppliers, mulch manufacturers and other stakeholders showed that the major adoption barriers were insufficient knowledge, high cost and unpredictable breakdown. The major bridges to adoption were reduced waste, environmental benefits and interest in further learning. These findings are discussed with reference to the classic innovation diffusion model, specifically work on the innovation–decision process and the attributes of innovations. The study results can be used to guide the activities of those involved in the design, development and promotion of biodegradable plastic mulches for US specialty crop production systems.

High Tunnels Are My Crop Insurance: An Assessment of Risk Management Tools for Small-Scale Specialty Crop Producers

High tunnels are being used by specialty crop producers to enhance production yields and quality, extend growing seasons, and protect crops from extreme weather. The tunnels are unheated, plastic-covered structures under which crops are planted directly in the soil, and they provide greater environmental protection and control than open-field production. This study uses field-level experiments to evaluate high-tunnel production. The results suggest that investments in high tunnels can provide increased profits and superior protection against adverse risks relative to crop insurance.

Isolation of native soil microorganisms with potential for breaking down biodegradable plastic mulch films used in agriculture.

Fungi native to agricultural soils that colonized commercially available biodegradable mulch (BDM) films were isolated and assessed for potential to degrade plastics. Typically, when formulations of plastics are known and a source of the feedstock is available, powdered plastic can be suspended in agar-based media and degradation determined by visualization of clearing zones. However, this approach poorly mimics in situ degradation of BDMs. First, BDMs are not dispersed as small particles throughout the soil matrix. Secondly, BDMs are not sold commercially as pure polymers, but rather as films containing additives (e.g. fillers, plasticizers and dyes) that may affect microbial growth. The procedures described herein were used for isolates acquired from soil-buried mulch films. Fungal isolates acquired from excavated BDMs were tested individually for growth on pieces of new, disinfested BDMs laid atop defined medium containing no carbon source except agar. Isolates that grew on BDMs were further tested in liquid medium where BDMs were the sole added carbon source. After approximately ten weeks, fungal colonization and BDM degradation were assessed by scanning electron microscopy. Isolates were identified via analysis of ribosomal RNA gene sequences. This report describes methods for fungal isolation, but bacteria also were isolated using these methods by substituting media appropriate for bacteria. Our methodology should prove useful for studies investigating breakdown of intact plastic films or products for which plastic feedstocks are either unknown or not available. However our approach does not provide a quantitative method for comparing rates of BDM degradation.

Reliability of Soil Sampling Method to Assess Visible Biodegradable Mulch Fragments Remaining in the Field after Soil Incorporation

Biodegradable mulches (BDMs) provide a unique advantage to growers in that they can be tilled into the soil after use, eliminating disposal costs that include time, labor, and equipment needs. Biodegradation of BDMs in the soil can be assessed by the presence of visible mulch fragments; although this is not a direct measure of biodegradation, it provides an initial estimation of mulch biodegradation. We carried out three field experiments to develop a protocol for quantifying BDM fragments in the soil after soil incorporation of mulch. Expt. 1 was done at Mount Vernon, WA, and Knoxville, TN, using five BDMs in four replications, including a polyethylene (PE) mulch reference treatment (three replications and at Mount Vernon only), and a ʽCinnamon Girl’ pumpkin (Cucurbita pepo) test crop. At the end of the growing season, mulches were tilled into the soil to a depth of 6 inches and within 16 days, five soil samples were collected with a golf hole cutter (4 inches diameter and 6 inches deep). Fifty-nine percent of the PEmulch fragments were recovered from the reference treatment. Among the remaining treatments, there was a high plot-to-plot variation as to the percent of the BDM recovered (3% to 95% at Mount Vernon, 2% to 88% at Knoxville). To exclude the possibility of mulch degradation impacting mulch recovery, in Expts. 2 and 3 (at Mount Vernon only), one BDM was laid, then tilled into the soil and sampled using the same sampling core as in Expt. 1, but all in 1 day. In Expt. 2, 15 soil samples were collected per plot, which recovered 70% of the mulch, and in Expt. 3, the entire plot was sampled by collecting 128 soil samples per plot, which recovered 62% of the mulch. In summary, sampling with a relatively large core recovered less than 70% of tilled-in mulch, there was high variability between plots within each treatment because of uneven distribution of the mulch fragments in the plot, and even 50 samples per plot did not provide an accurate estimate of the amount of mulch remaining in the field. Thus, soil sampling with a large core was ineffective, and new sampling methods are needed to assess the amount of BDM remaining in the field after soil incorporation.