Impact of Biodegradable Plastic Mulches on Soil Health.

Plastic mulch films reduce weeds, conserve moisture, and increase soil temperatures. Conventional polyethylene (PE) plastic mulch films are not readily biodegradable, and thus impose considerable end-of-life costs in terms of removal and disposal expenses, and environmental impacts. Biodegradable plastic mulches (BDMs) made from biodegradable polymers (e.g. starch, PBAT, PLA, and other polyesters) can be tilled into the soil at the end of the growing season, and may be a viable alternative to PE. However, BDM decomposition in soils, and impact on soil health, is not well understood; this is a primary concern with growers who are considering adoption of biodegradable plastics and with regulators considering allowing BDMs for organic production. The goal of our study was to assess impacts of BDMs on soils in terms of the physical, chemical and biological soil properties, with special emphasis on microbial communities and carbon cycling and storage. Randomized block trials of four BDMs, conventional PE film, cellulosic mulch and bare ground were conducted at two locations (Tennessee and Washington). Like PE, BDMs increased soil temperature and moderated soil moisture compared to bare ground. There were significant differences in soil quality between the two locations, but no significant differences in soil quality parameters among the BDM and PE treatments. An in situ decomposition test revealed that relative rates of BDM decomposition in soil were not consistent between the two locations. Laboratory soil microcosms demonstrated that degradation of BDMs was enhanced under higher temperature with the percent biodegraded more than doubling with each 10 degree C rise in temperature. Taken together, our work shows that BDMs performed comparably to PE films and had negligible effects on soil quality over the short term, but may alter carbon storage potential of soils over the long term.