The Role of Biodegradable Plastic Mulches in Soil Organic Carbon Cycling.

Biodegradable plastic mulches (BDMs) have become a viable alternative to polyethylene (PE) agricultural mulches, a frequently used material in specialty crop production for lengthening the growing season and preventing weeds. BDMs provide the same functions as PE mulches with the added benefits of reducing manual labor costs for mulch retrieval and generation of plastic waste because they are tilled into the soil at the end of the season, where they biodegrade into carbon dioxide, water, and microbial biomass. However, little is known about their long-term impacts on soil health including their impact on soil carbon dynamics. In addition to being a direct addition of carbon (C), BDMs change soil moisture and temperature, which affects soil organic matter formation and potentially alters C cycling. We utilized two years of soil quality data (2015-2017) to asses changes in C cycling for each of 7 treatments (four BDMs and three controls: PE, cellulosic (paper) mulch, and bare ground). Pools of active and passive carbon were measured including 24-hr incubations, total organic carbon, dissolved organic carbon, permanganate-oxidizable (POXC) and HOOH-oxidizable carbon. Across multiple seasons, PE plots had lower respiration compared to other treatments. After the fall 2016 season, respiration rates of soils from PE mulch treatments were significantly lower than Organix^® and BioAgri^® treatments (2.3 ± 0.4, 7.1 ± 0.3, 7.3 ± 1.5 mg C g^-1 dry soil) respectively. Because of the unique isotopic (δ¹³C) signatures of the BDM feedstocks of PLA/PHA (-13.0), Naturecycle^® (-31.5), and Organix^® (-29.8) compared to soil (-23.1), it may be possible to track the fate of mulch carbon using isotopic analyses of microbial biomass carbon and total organic carbon. The combination of flux measurements, repeated soil organic carbon measurements, and isotopic analyses help answer questions about the overall impacts of these materials on the soil carbon cycle.