2019 ASHS Annual Conference

Biodegradable plastic mulch is incorporated into the soil after use, eliminating environmental and economic issues associated with mulch removal and disposal. Although biodegradable plastic mulches have been shown to provide comparable weed control, moisture conservation, crop yield and quality benefits to polyethylene mulch, there are concerns about incomplete biodegradation and potential negative impacts on soil quality and subsequent crop production. A few standards and methodologies (for example ASTM D 5988 or International Standards Organization; ISO 17556) measure in-soil biodegradation of the major ingredients of biodegradable plastic mulch under laboratory conditions where temperature, humidity, and oxygen levels are defined; but this is not representative of field conditions. We conducted a field study at Mount Vernon, WA to assess the amount of biodegradable mulch (fragment size > 2.36 mm) remaining in the soil each fall (within 1 month of mulch incorporation) and spring (6 months after mulch incorporation) for 3 years (2016-2018) in a field where five biodegradable mulches were applied and tilled into the respective plots each year. Treatments were four biodegradable plastic mulches, BioAgri [Mater-Bi^® (grade EF04P), poly(butylene adipate-co-terephthalate) (PBAT); 20-25% biobased], Naturecycle (Starch-polyester blend; ≥20% biobased), Organix [BASF ecovio^® (grade M2351), PBAT, polylactic acid (PLA); 10% biobased], and Experimental (Ingeo^® PLA, Mirel^TM amorphous Polyhydroxyalkanoates (PHA); 86% biobased), and one paper mulch, WeedGuardPlus (cellulose; 100% biobased). Percent mulch recovery was calculated with respect to the total amount of mulch applied. Average mulch recovery for the three commercial biodegradable plastic mulches in the fall was 71% (range 30-95%), 50% (41-69%), and 21% (18-25%) after first, second, and third applications, respectively. In the spring, average recovery for the same three mulches was 50% (31-67%) and 36% (21-53%) after the first and second applications, respectively (third sampling time is April 2019). For Experimental mulch, the average recovery was 60% (32-80%) in the fall and 66% (39-93%) in the spring. For WeedGuardPlus, average recovery was 15% in the fall, and no much was recovered in the spring (complete degradation). These results show that biodegradable plastic mulches degrade over time in the field, but complete degradation takes more than 1 year. Further, biobased content does not determine the biodegradation rate of the mulch. Tests additional to the current laboratory tests are needed to ascertain mulch biodegradation under field conditions, and long-term studies are needed to follow the degradation and fate of biodegradable plastic mulch in the field.