PE plastic mulch is used widely in specialty crop production to control weeds, conserve soil moisture, increase crop yields, modify soil temperature, and shorten the time to harvest (Hill et al., 1982; Schonbeck, 1998; Schonbeck and Evanylo, 1998; Shogren, 2000). These benefits provide farmers worldwide with significant horticultural and economic advantages (Takakura and Fang, 2001). However, the widespread use of PE mulch creates removal and disposal costs to growers as well as costs to the environment due to waste plastics being buried in landfills and on farm, or burned. As an alternative, plastic mulches that are biodegradable have been developed. These products first appeared on the market in the 1980s, and may be made from renewable, natural, or sustainable feedstocks (Hayes et al., 2012; Miles et al., 2009). They provide the benefits of PE mulch, such as weed suppression and improved crop yields, and are purported to completely degrade between crop seasons (Cowan et al., 2014; Miles et al., 2012; Minuto et al., 2008; Moreno and Moreno, 2008). Because farmers must bear the annual cost of mulch removal and disposal, estimated at ≈$250 per hectare (Shogren and Hochmuth, 2004), the ability to till mulch into the soil following harvest with confidence that biodegradation will occur is an economic incentive (Olsen and Gounder, 2001). Additionally, the environmental benefit of a mulch that degraded completely in soil without producing toxic byproducts would be highly desirable.
Biodegradable plastics typically undergo a two-phase degradation process: disintegration or weathering during use in the field, and biodegradation, which occurs after being incorporated into the soil (Krzan et al., 2006; Kyrikou and Briassoulis, 2007). Moisture, temperature, and light are key factors that impact disintegration, and interactions among these factors may further enhance degradation (Hakkarainen, 2002; Ho et al., 1999; Krzan et al., 2006). Disintegration can lead to decreased molecular weight and increased water solubility of the plastic, which subsequently facilitates biodegradation (Lucas et al., 2008). During biodegradation, biotic processes mineralize the polymer fragments to carbon dioxide, water, and microbial biomass (Lucas et al., 2008). In this study, the word “deterioration” is used to reflect a reduction in the function or visual intactness of the mulch without respect to any particular mode of action or biological process.
Parameters typically measured in the field and laboratory to evaluate deterioration of biodegradable mulch films include reduction in soil coverage, weight loss, and changes in mechanical properties. Measuring the decrease in soil coverage provided by mulch is relatively inexpensive and can be performed in the field without the use of special equipment. Measuring changes in mulch weight requires either large samples, or sensitive laboratory-quality balances, and cannot be reliably performed in the field. Although measuring mulch weight is less subjective than a visual assessment, weight measurements are subject to error because of soiled samples. Measuring changes in mulch mechanical properties, including breaking force and elongation to break, among others, are objective and accurate, but requires specialized laboratory equipment and climate-controlled facilities. If visual assessment of soil coverage reliably predicts degradation of biodegradable plastic mulches, the need for more expensive and less accessible methods may be obviated, despite the potential for greater accuracy.
To quantify biodegradable mulch deterioration and compare mulch performance in the field, Miles et al. (2012) measured the number of rips, tears, and holes (RTH), as well as the PVD in three distinct climates of the United States. However, the authors found RTH to be an unreliable measure of deterioration due to the coalescence of RTH over time, leading to underestimation of deterioration. Other studies have used similar observations to visually rate or evaluate loss of integrity of biodegradable mulch on the soil surface (Minuto et al., 2008; Moreno and Moreno, 2008; Moreno et al., 2009; Ngouajio and Ernest, 2005; Ngouajio et al., 2008), but none have been adopted as a standard of measurement. In addition, other investigators have evaluated changes in mulch mechanical properties over time to evaluate deterioration (Briassoulis, 2006, 2007; Candido et al., 2006; Cascone et al., 2008; Kijchavengkul et al., 2008; Scarascia-Mugnozza et al., 2006; Tocchetto et al., 2001). Martín-Closas et al. (2007 and 2008) implied that visual observations of biodegradable mulch deterioration were related to changes in mechanical properties, though no statistical comparisons were made between the measures.
Studies that statistically analyze the relationships between visual assessments of mulch deterioration in the field and mechanical properties in the laboratory have not yet been reported. However, an increased awareness and understanding of such relationships could contribute to more accurate interpretations about visual measures of deterioration. The research reported here builds on the work of Miles et al. (2012), and seeks to determine whether visual assessments of biodegradable mulch intactness during the growing season (independent variable) predict statistically significant changes in mechanical properties (dependent variables). Additionally, the study was designed to contrast mulch deterioration in open-field and high tunnel production environments to expand on potential agricultural settings. Tomato (Solanum lycopersicum cv. Celebrity) was selected as the model crop for this study because it is an important vegetable crop throughout the United States and plastic mulch is a common and important component of tomato production systems (Lamont, 1993).
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