Viewpoint pubs.acs.org/est
Is Biodegradable Plastic Mulch the Solution to Agriculture’s Plastic Problem? Henry Y. Sintim and Markus Flury* Department of Crop & Soil Sciences, Washington State University, Pullman, Washington 99164 and Puyallup, Washington 98371 United States burn them on-farm.3 The release of plastic fragments from amassed mulches or the incomplete removal of plastic mulches from fields is of major global concern as plastic fragments from all sources ultimately accumulate in the sea.4 In an effort to mitigate the adverse impacts of polyethylene mulch, biodegradable plastic mulch was introduced in the 1980s, but low biodegradation in soils has hindered widespread use so far.2 However, new promising formulations of biodegradable polymers have been developed in recent years. The advertised advantage of biodegradable plastic mulch is that it can be tilled into the soil after use and that it would be degraded by the action of soil organisms, thus saving labor and disposal costs. To be a competitive alternative to polyethylene mulch, biodegradable plastic mulch must (a) maintain a conducive microclimate for plant growth, (b) be flexible to allow mechanical installation, (c) remain intact during the majority of the cropping season, (d) undergo complete degradation after soil incorporation or composting, (e) have no adverse impact on the environment, and (f) be economical. Studies have shown that some of the commercially available biodegradable plastic mulches perform similarly to polyethylene mulches in terms of ease of laying, weed prevention, and he global use of agricultural plastic films, which includes enhancement of crop yield and quality.5 However, the potential films used for greenhouses, mulching, and silage, is environmental consequences of using biodegradable plastic expected to grow 69% from 4.4 million tons in 2012 to 7.4 mulches have not been thoroughly studied, and the internamillion tons in 2019.1 The global market for agricultural plastic tional standards (ISO 17088, ASTM D6400, ISO 17556, films is expected to grow at a compound annual growth rate of ASTM D5988) for validating their safeness are, in our opinion, 7.6% from 2013 to 2019, reaching a market of 9.66 billion USD, 1 not rigorous enough. For instance, the standards for with China and the Middle East driving future demand. In degradation in compost (ISO 17088, ASTM D6400) specify 2012, plastic mulch films accounted for over 40% of the total 1 that at least 90% of the organic carbon need to be converted to plastic films used in agriculture. CO2, leaving room for up to 10% of the organic carbon to Plastic mulch films provide multiple benefits for crop remain. production. They help to control weeds and insects, increase The breakdown products of biodegradable plastic mulch soil and air temperature, reduce evaporation, minimize soil must not be toxic or persist in the environment, but should be erosion, and prevent soil splashing on fruits or vegetables completely assimilated by soil organisms. Extensive reports on (Figure 1). These benefits translate into reduced pesticide use, the breakdown of biodegradable plastic mulch in the laboratory early planting in spring, water conservation, and increased crop 2,3 are available,2 but little is known about effects of biodegradable yield and quality. Given the world’s projected population mulches on soil quality6 and their breakdown into micro- and growth and need for more food, the use of plastic mulch film in nanometer-sized fragments. Degraded mulch products can be agriculture has great potential to increase food production and invisible to the naked eye, but out-of-sight does not mean they security. are safe. Standards used to assess the degradability of plastics Plastic mulch films consist mostly of low-density and linear under aerobically composted conditions (ISO 17088, ASTM low-density polyethylene, which do not readily biodegrade. As a D6400) and in the soil environment (ISO 17556, ASTM result, these polyethylene-based mulches must be retrieved and D5988) are based on macroscopic (i.e., visual by eye) disposed after usage. Agricultural plastic mulch films are often disintegration and organic carbon conversion to CO2, but do contaminated with soil, and therefore are not accepted by many recycling facilities. This limits disposal options for polyethylene mulches, which often have to be landfilled. Costs of landfilling Received: November 29, 2016 are high,2 causing many producers to amass mulches or even
T
© XXXX American Chemical Society
A
DOI: 10.1021/acs.est.6b06042 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Viewpoint
Environmental Science & Technology
Figure 1. Schematic of agricultural cropping system with use of plastic mulch film (left) and without mulch film (right). (Parts of the schematic are courtesy of the Integration and Application Network, University of Maryland Center for Environmental Science, ian.umces.edu/symbols). (2) Kasirajan, S.; Ngouajio, M. Polyethylene and biodegradable mulches for agricultural applications: a review. Agron. Sustainable Dev. 2012, 32, 501−529. (3) Steinmetz, Z.; Wollmann, C.; Schaefer, M.; Buchmann, C.; David, J.; Tröger, J.; Munoz, K.; Frör, O.; Schaumann, G. E. Plastic mulching in agriculture. Trading short-term agronomic benefits for long-term soil degradation? Sci. Total Environ. 2016, 550, 690−705. (4) Barnes, D. K. A.; Galgani, F.; Thompson, R. C.; Barlaz, M. Accumulation and fragmentation of plastic debris in global environments. Philos. Trans. R. Soc., B 2009, 364, 1985−1998. (5) Miles, C.; Wallace, R.; Wszelaki, A.; Martin, J.; Cowan, J.; Walters, T.; Inglis, D. Deterioration of potentially biodegradable alternatives to black plastic mulch in three tomato production regions. HortScience 2012, 47, 1270−1277. (6) Li, C.; Moore-Kucera, J.; Lee, J.; Corbin, A.; Brodhagen, M.; Miles, C.; Inglis, D. Effects of biodegradable mulch on soil quality. Appl. Soil. Ecol. 2014, 79, 59−69. (7) Lö nnstedt, O. M.; Eklö v , P. Environmentally relevant concentrations of microplastic particles influence larval fish ecology. Science 2016, 352, 1213−1216. (8) Teuten, E. L.; Saquing, J. M.; Knappe, D. R. U.; Barlaz, M. A.; Jonsson, S.; Bjorn, A.; Rowland, S. J.; Thompson, R. C.; Galloway, T. S.; Yamashita, R.; Ochi, D.; Watanuki, Y.; Moore, C.; Viet, P. H.; Tana, T. S.; Prudente, M.; Boonyatumanond, R.; Zakaria, M. P.; Akkhavong, K.; Ogata, Y.; Hirai, H.; Iwasa, S.; Mizukawa, K.; Hagino, Y.; Imamura, A.; Saha, M.; Takada, H. Transport and release of chemicals from plastics to the environment and to wildlife. Philos. Trans. R. Soc., B 2009, 364, 2027−2045. (9) Rochman, C. M.; Kross, S. M.; Armstrong, J. B.; Bogan, M. T.; Darling, E. S.; Green, S. J.; Smyth, A. R.; Verissimo, D. Scientific evidence supports a ban on microbeads. Environ. Sci. Technol. 2015, 49, 10759−10761.
not prescribe determination of micro- and nanoparticle residues, which could accumulate in soils or compost. We feel that biodegradable plastic mulches must be thoroughly investigated to ensure they are safe and sustainable for use in agricultural systems. If biodegradable plastic mulches are to be tilled into the soil after use, their complete breakdown needs to be ensured and verified under the wide variety of soils and environments where they may be applied. Global use of plastic mulch is high and is increasing, thus there is a growing market for biodegradabale plastic mulches. Incomplete breakdown of biodegradable plastic, however, could lead to an accumulation of plastic fragments and particulates in soils. While very little is known about the effects of biodegradable plastics in soil, it has been shown that plastic microparticles can be toxic to aquatic organisms.7 Also, agricultural systems are heavy users of pesticides and other hydrophobic chemicals that can adsorb to residual plastics from mulches, and plastic fragments or particles can act as transport conduit for toxic chemicals through soil or the atmosphere.8 The issue of biodegradable plastic mulches in agriculture is analogous to the use of plastic microbeads in cosmetics and health care products where the environmental consequences were not thoroughly investigated until the beads found their way to the ocean and caused harm to the environment,9 and only recently led the U.S. government to ban their usage through the Microbead-Free Waters Act of 2015. Biodegradable plastic mulches are a promising alternative to the currently used polyethylene-based mulches, but rigorous testing is needed to ensure their use is environmentally safe. In-field testing of biodegradation under different soil and climatic conditions is needed, with particular attention to release of micro- and nanoparticles from plastics and their long-term accumulation in soils and their effects on soil quality.
■
AUTHOR INFORMATION
Corresponding Author
*Phone: 1-253-445-4522; e-mail: fl
[email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Transparency Market Research. Agricultural Films Market for Greenhouse, Mulching and Silage ApplicationsGlobal Industry Analysis, Size, Share, Growth, Trends and Forecast, 2013−2019; Transparency Market Research: Albany, NY, 2013; http://www. transparencymarketresearch.com/agricultural-film.html. B
DOI: 10.1021/acs.est.6b06042 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
