Is Biodegradable Plastic Mulch the Solution to Agriculture's Plastic Problem?

T global use of agricultural plastic films, which includes films used for greenhouses, mulching, and silage, is expected to grow 69% from 4.4 million tons in 2012 to 7.4 million tons in 2019. The global market for agricultural plastic films is expected to grow at a compound annual growth rate of 7.6% from 2013 to 2019, reaching a market of 9.66 billion USD, with China and the Middle East driving future demand. In 2012, plastic mulch films accounted for over 40% of the total plastic films used in agriculture. Plastic mulch films provide multiple benefits for crop production. They help to control weeds and insects, increase soil and air temperature, reduce evaporation, minimize soil erosion, and prevent soil splashing on fruits or vegetables (Figure 1). These benefits translate into reduced pesticide use, early planting in spring, water conservation, and increased crop yield and quality. Given the world’s projected population growth and need for more food, the use of plastic mulch film in agriculture has great potential to increase food production and security. Plastic mulch films consist mostly of low-density and linear low-density polyethylene, which do not readily biodegrade. As a result, these polyethylene-based mulches must be retrieved and disposed after usage. Agricultural plastic mulch films are often 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 are high, causing many producers to amass mulches or even burn them on-farm. 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. 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. 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 enhancement of crop yield and quality. However, the potential environmental consequences of using biodegradable plastic mulches have not been thoroughly studied, and the international standards (ISO 17088, ASTM D6400, ISO 17556, ASTM D5988) for validating their safeness are, in our opinion, not rigorous enough. For instance, the standards for degradation in compost (ISO 17088, ASTM D6400) specify that at least 90% of the organic carbon need to be converted to CO2, leaving room for up to 10% of the organic carbon to remain. The breakdown products of biodegradable plastic mulch must not be toxic or persist in the environment, but should be completely assimilated by soil organisms. Extensive reports on the breakdown of biodegradable plastic mulch in the laboratory are available, but little is known about effects of biodegradable mulches on soil quality and their breakdown into microand nanometer-sized fragments. Degraded mulch products can be invisible to the naked eye, but out-of-sight does not mean they are safe. Standards used to assess the degradability of plastics under aerobically composted conditions (ISO 17088, ASTM D6400) and in the soil environment (ISO 17556, ASTM D5988) are based on macroscopic (i.e., visual by eye) disintegration and organic carbon conversion to CO2, but do