Since its introduction to agriculture in the 1950s, polyethylene plastic has become the standard mulch used by specialty crop growers to control weeds, conserve soil moisture, increase crop yield, modify soil temperature, and shorten the time to harvest (Hill et al., 1982; Schonbeck, 1998; Schonbeck and Evanylo, 1998; Shogren, 2000). Plastic mulch has contributed significantly to the economic viability of farmers worldwide (Takakura and Fang, 2001); by 2006, it was estimated that plastic mulch covered ≈162,000 ha in the United States alone (P. Bergholtz, personal communication). Disadvantages of plastic mulches include their long-term persistence in the environment as well as the economic costs of yearly removal and disposal. These costs hinge on available labor, equipment, and infrastructure (Olsen and Gounder, 2001; Schonbeck, 1995) and in 2004 were estimated to be ≈$250 per hectare (Shogren and Hochmuth, 2004).
The recycling of agricultural plastics is available in limited areas of the United States. Major obstacles to the recycling of agricultural plastics include: 1) contamination resulting from soil and mixing of different plastic (resin) types; 2) high cost of long-distance transport of plastic wastes from remote collection sites; and 3) high price of the recycled resin as compared with virgin resin on the open market (Garthe and Kowal, 1993). As a result of the difficulty and expense of agricultural plastic recycling, many growers choose to dispose of this waste through local landfills (Olsen and Gounder, 2001; Shogren, 2000). However, some growers burn their plastic waste (Shogren and Hochmuth, 2004), which has deleterious effects on environmental and human health.
The ideal agricultural mulch would be made from renewable, natural, and sustainable raw products; have low overall environmental impact (for raw materials, manufacturing, and disposal); be affordable (in purchase, application, removal, and disposal); suppress weeds; sustain crop yields; and retain functionality throughout the cropping season (Miles et al., 2009). Costs associated with mulch removal and disposal could be avoided if degradable or biodegradable mulches were tilled into the soil after harvest (Anderson et al., 1995; Olsen and Gounder, 2001). Environmental costs could also be reduced if the mulches were compostable.
Degradable plastic agricultural mulches were first introduced in the 1980s, but those products were photodegradable rather than biodegradable, and they disintegrated or fragmented into smaller pieces of plastic (Riggle, 1998). A major issue at the time these products entered the market was a lack of consensus regarding the definition of “degradable.” In the last decade, U.S. stakeholders (researchers, manufacturers, marketers, etc.) have created standards to define degradation and established widely accepted testing strategies to evaluate the behavior of degradable products [American Society for Testing and Materials/Institute for Standards Research (ASTM, 2004)]. According to ASTM D 883-11 (ASTM, 2011), “degradation” is “a deleterious change in the chemical structure, physical properties, or appearance of a plastic” irrespective of cause, whereas “biodegradation” “results from the action of naturally-occurring micro-organisms such as bacteria, fungi, and algae.” Although there are currently ASTM standards for measuring biodegradation in specific environments such as accelerated bioreactor landfill conditions [D 5526-94(2011)e1 (ASTM, 2011c), D 7475-11 (ASTM, 2011d)], anaerobic digesters [D 5511-11 (ASTM, 2011b)], controlled composting [D 5338-11 (ASTM, 2011a), D 5988-03 (ASTM, 2003), D 6340-98(2007) (ASTM, 2007b)], marine environment [D 6691-09 (ASTM, 2009], and municipal sewage sludge [D 5210-92(2007) (ASTM, 2007a), D 6340-98(2007) (ASTM, 2007b)], currently there is none for biodegradation in the soil environment, although one is under development [R. Narayan, personal communication; ASTM WK 29802, 2012 (ASTM, 2011f)]. In contrast to degradation and biodegradation, “deterioration” is the loss of physical or mechanical strength as observed through physical testing, microscopic imaging, or visual assessment and may be the result of abiotic and/or biotic factors.
The use of degradable or biodegradable mulches could reduce or eliminate costs associated with plastic mulch removal and disposal as well as decrease the total amount of disposed plastic waste. However, there are many questions and concerns regarding the efficacy, degradability, and potential residues of biodegradable petroleum-based mulches (Greer and Dole, 2003; Hochmuth, 2001; Shogren, 2000). As a result, alternative mulches such as paper and starch have been created from non-petroleum feedstocks. Numerous paper mulch products have been tested over the last century with variable results (Brault and Stewart, 2002; Flint, 1928; Knavel and Mohr, 1967; Shogren, 2000). The density and fiber orientation of paper mulches may impact light penetration, which can influence both weed seed germination and growth beneath the mulch. Weeds growing under paper mulch can push the mulch off the soil surface resulting in tearing (Miles et al., 2005). Use of paper mulch is further limited by its heavier weight, which is two to four times greater than that of plastic mulch and incurs increased costs and labor for shipping and handling. In addition, paper mulch can be very difficult to lay with mechanical mulch laying equipment (Sorkin, 2006). However, as a result of its natural fiber composition, paper mulch remains of high interest to growers concerned about the negative impacts of non-degradable plastic mulch.
Mulch film products composed primarily of starch are relatively new to the market, are similar in appearance (weight, density, and texture) and handling properties compared with standard polyethylene mulches, and may be a viable alternative to polyethylene-based mulch (Kijchavengkul et al., 2008; Rangarajan et al., 2003). New experimental materials such as spunbond (SB) polylactic acid (PLA) have recently been developed as potential alternatives to polyethylene mulches (Wadsworth et al., 2009). However, it is unclear how starch- and PLA-based mulch alternatives perform in high tunnel (HT) and open field (OF) production systems across diverse environments. The objective of this study was to evaluate and compare three mulch products marketed as biodegradable (two starch-based and one cellulose-based), and one experimental PLA-based product, to black plastic mulch under two production systems (HTs and OF) in three contrasting regions (Knoxville, TN; Lubbock, TX; Mount Vernon, WA) in the United States. Mulch deterioration, weed control attributes, and impact on tomato yield were the principal performance factors measured.
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