The effect of soil water content on the biodegradation dynamics of polyesters from a commercial biodegradable mulch film

Plastic mulch films are increasingly used in agriculture to enhance productivity by assisting in weed control, saving water, and extending the growing season. Many commercial, certified soil-biodegradable mulch films (including the film Bionov B tested herein) contain varying amounts of the two polyesters poly(butylene adipate-co-terephthalate) (PBAT) and polylactic acid (PLA). These polyesters are biodegraded by soil microorganisms as demonstrated under favorable soil incubation conditions. These biodegradable mulch films are seen as viable alternatives to the non-biodegradable conventional mulch films typically composed of low-density polyethylene (LDPE). While the tested biodegradable mulch is certified biodegradable according to the EN:17033:2018 norm, this biodegradability was tested under controlled laboratory incubation conditions with constant temperature and soil moisture. An in-depth understanding of the impact of soil moisture content and the interplay between moisture and temperature on the biodegradation dynamics of these biodegradable mulch films is still missing. In this work, we assessed the effect of soil moisture content on the biodegradation dynamics of PBAT and PLA from the tested mulch film and compared it to that of LDPE-based mulch film (negative control) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) powder as positive control. Laboratory incubations in three soils (a German standard clay soil (LUFA 6S), a Dutch silty loam soil (LOESS), and a Swiss silt clay loam soil (AGR2)) were performed over two years at 23°C at four soil moisture contents (20, 35, 50, and 65% of the soil water holding capacities (WHC)) and, in addition, at 15°C in LUFA6S at all four water contents. Biodegradation was assessed by quantifying residual PHBH, PBAT, and PLA in the soils after 6 and 12 months of incubation by Soxhlet extraction coupled to proton nuclear magnetic resonance spectroscopy analysis (¹H-NMR) and, for PE, gravimetrically. PE did not biodegrade under any conditions. After six months, PHBH had extensively biodegraded in all soils at all water contents, except for LUFA6S and AGR2 at the lowest water content with much smaller extents of biodegradation. Residual amounts of both PBAT and PLA after six months were generally higher than for PHBH, demonstrating slower biodegradation of mulch film polyesters than of the positive control. PBAT and PLA exhibited very similar trends. LOESS showed a continuous decrease in biodegradation of PBAT and PLA with increasing water content; ARG2 showed the highest biodegradation at the highest water content; LUFA6S showed optimal biodegradation at intermediate water content. These findings as well as results after 12 months, clearly show that the effect of soil water content on biodegradation of PBAT and PLA is large and highly soil dependent. By comparison, lowering the temperature from 23°C to 15°C had a smaller effect on biodegradation. Ongoing efforts are directed towards linking the effects of soil moisture to soil texture. Our results provide the basis on which to assess the effect of soil water content on the biodegradation dynamics of biodegradable mulch films in field soils under in situ conditions.