Sampling and degradation of biodegradable plastic and paper mulches in field after tillage incorporation

Plastic biodegradable mulch (plastic BDM) is tilled after use, but there is concern about incomplete degradation and potential impact on subsequent crops, and we lack a reliable method to measure mulch degradation post soil-incorporation. We conducted two field experiments to (i) develop a sampling method to estimate the amount of mulch (fragments size >2.36 mm) in the field post soil-incorporation, and (ii) assess the amount of BDM in the soil after four consecutive years of mulch incorporation. In Expt. 1, we used the quartering method to reduce soil from a 1 m² field sample area to a representative 19 L sample. In Expt. 2, we applied and tilled four plastic BDMs: BioAgri, Naturecycle, Organix AG, and an experimental mulch; and one paper mulch, WeedGuardPlus, in their respective plots for four consecutive years. Starting in year 2, we sampled soil with the quartering method each spring and fall to determine mulch recovery. With respect to the total amount of mulch applied, average mulch recovery in the fall for the three commercial plastic BDMs was 71%, 50%, and 35% after second, third and fourth applications, respectively. For the experimental mulch, the average recovery was 80%, 69%, and 54% in the fall after second, third, and fourth applications, respectively. Recovery was slightly lower in spring than in preceding fall all years. For WeedGuardPlus, average recovery was 14%-20% in each fall, and no recovery in any spring (complete degradation). The results show that the quartering method reliably estimates the amount of mulch in a field and BDMs degrade over time in the field even with repeated applications, but complete degradation takes >1 year. While a few standards (e.g., ASTM D5988) specify how to determine biodegradation of plastics in soil under controlled laboratory conditions, our sampling method assesses plastic degradation under diverse field conditions.