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The role of biodegradable plastic mulches in soil organic carbon cycling
Summary
This review examines the role of biodegradable plastic mulches in soil organic carbon cycling as an alternative to conventional polyethylene mulches, which shed microplastics into agricultural soils. Biodegradable mulches offer potential environmental benefits but their long-term soil impacts are still being studied.
Agricultural plastic mulching improves weed management and controls the soil moisture and temperature leading to large increases in yield. Unfortunately, the rise in popularity of mulching results in more plastic pollution due to a lack of recycling options for polyethylene (PE) mulch. Biodegradable plastic mulches (BDMs) are an alternative to the traditionally used PE mulches and are recommended for tillage into the soil at the end of the season. There is evidence that by altering soil moisture and temperature, the use of mulching is tightly tied to soil carbon (C) cycling. The purpose of this study was to gain insight on the effect of BDMs on soil C storage, an important part of soil health, using field and lab studies. The field study was replicated in two diverse climates, Knoxville, TN and Mt.Vernon, WA. Multiple C pools were measured over two years (Spring 2015- Spring 2017) to monitor the effect of 7 different mulching treatments (four BDMs and three controls: PE plastic mulch, cellulosic (paper) mulch, and no-mulch). After two years, results were variable across locations, but PE treatments accumulated less soil C than the no-mulch control in each location. These differences are likely due to increased mineralization from the higher soil moistures and temperatures in the PE treatment compared to the no-mulch treatment. The use and incorporation of BDMs showed no significant declines in soil C pools compared to the no-mulch control. Season and location significantly affected C pools as well. To improve estimates of BDM fate, BDM microplastics were measured from field samples using a novel method. The second part of this study was a lab experiment to investigate how air temperature affects the biodegradation of BDMs by combining time course biodegradation measures with soil C pools, soil microbial properties, and chemical properties of the BDMs. Collectively, these results showed significantly higher BDM biodegradation at higher temperatures with amorphous polymeric regions being degraded first. The positive effects of BDMs on soil C stocks compared to PE supports their use as a substitute to PE; however more research is needed on microplastic residence times.
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