Deterioration of Biodegradable Plastic Mulch in Pumpkin Production in Northwest Washington

Use of polyethylene (PE) mulch has become a standard practice in vegetable production around the world. However, removal and disposal of used PE mulch can be costly and PE fragments often become a source of pollution to the soil and environment. Mulch that can function similarly to PE mulch but biodegrades in the soil after tillage incorporation within a year or two, without negative environmental impacts, would be a sustainable alternative to PE mulch. In 2015, a field study at Mount Vernon, WA evaluated four potentially biodegradable mulches (Metabolix, Organix, Naturecycle and BioAgri) for above-soil deterioration during a pumpkin (Cucurbita pepo L.) cropping season and in-soil biodegradation after tillage incorporation. Cellulose mulch (100% biodegradable) and PE mulch (non-biodegradable) were included as positive and negative control treatments, respectively. Percent soil exposure (PSE) (where 0% PSE indicated intact mulch and 100% PSE indicated completely deteriorated mulch) at the end of the cropping season was high for Naturecycle (61.3%) and low for PE (0.3%), Metabolix (1.2%), WeedGuardPlus (2.5%), Organix (3.5%) and BioAgri (8%) (P < 0.0001). To measure in-soil biodegradation after soil incorporation, we collected five soil samples [15.24 cm deep and 10.16 cm diameter (total 6178 cm³)] from each plot and extracted mulch fragments to establish a baseline measure of mulch per unit volume of soil. Mulch fragments were weighed (‘dirty weight’), cleaned, dried and weighed again (‘clean weight’), and the mulch area per plot was calculated as clean weight per plot over the weight per unit area of the mulch. Weight per unit area of mulch was calculated by measuring the area and weight of ten relatively intact mulch pieces. Percent mulch extracted was calculated by dividing total area of recovered mulch fragments by the theoretical maximum mulch area. The theoretical maximum mulch area (405 cm²) was calculated as the surface area of a sub-soil sample multiplying by 5 (number of sub-samples per plot). Percent mulch extracted for the baseline measurement was highest for PE (61.3%), followed by Metabolix (56.3%), Organix (54.4%), and BioAgri (47.8%), and was lowest for Naturecycle (13.8%) and WeedGuardPlus (25.3%) (P = 0.03). Over the next 4 years, each mulch will be re-layed, cropped, and then tilled into each respective plot; PSE at the end of each cropping season and recovered mulch area at every 6 months will be measured as described above.