Evaluation of the mulch films biodegradation in soil: a methodological review

Plastic mulch films are widely used in agriculture, but most are not biodegradable in soil. Biodegradable mulch films are blends of different polymers whose composition-ratios vary notably from one product to another. Their degradation rates vary greatly according to the physio-chemical characteristics of the product and according to the properties of the soil and its microbial activity. The objective of this review is to provide an overview of the methods used to estimate the biodegradation performances of biodegradable plastics in the soil. In line with this objective, 80 papers were selected and systematically analyzed to extract information on the characteristics of the soil used in the experiments, the type of polymer analyzed, and the methods used to estimate biodegradation in soil environment. Our systematic analysis showed that studies were carried out under both laboratory-controlled and open-field conditions, with different approaches involving visual analysis, mass loss measurements, spectroscopy, and CO2 measurements. A linear estimation of biodegradation performance for four of the most common biodegradable polymers (i.e., polybutylene succinate, polybutylene succinate-co-adipate, polylactic acid, and polybutylene adipate-co-terephthalate), either pure or blended, showed a very wide range of results that appear only partially comparable. Many of the analyzed papers did not report soil characteristics at all, despite soil being one of the most important factors in the biodegradation process. Although methodologies for estimating biodegradation are well developed, at least under laboratory-controlled conditions, there is a need for a shared methodology to make results comparable among different experiments. Within such a shared methodology, visual analysis or mass loss measurements, despite not being able to scientifically prove the biodegradation of polymers, should not be discarded a priori as they might be useful indicators especially for open field experiments. When using indirect biodegradation indicators such as visual analysis or mass loss, it is necessary to couple them with CO2 measurements or to use materials whose biodegradability in the soil environment has already been tested.

Deterioration of Soil-biodegradable Mulch Films during Storage and Its Impact on Specialty Crop Production

Plastic mulch films contribute to improved crop yield and quality for vegetable and small fruit cropping systems. Although the single-season agronomic performance of conventional polyethylene mulches and soil-biodegradable mulches (BDMs) are similar, over time BDMs can begin to break down during storage and subsequently not provide season-long soil coverage. In this study, the changes in physicochemical properties of BDMs were investigated over 3 years of indoor storage (2015–18) under ideal environmental conditions in two laboratories. Mulches evaluated were black, 20–40 µm thick, suitable for annual vegetable production, and included three BDMs: two polybutylene adipate terephthalate (PBAT)-enriched mulches that are commercially available in North America, an experimental polylactic acid (PLA) and polyhydroxyalkanote-based film, and a conventional polyethylene mulch as a control. Tensile properties, specifically peak load and elongation at maximum tensile stress, decreased during storage, particularly for the PBAT-based BDMs, indicating a loss of strength. During year 3 of storage, the tensile properties declined extensively, suggesting embrittlement. The average molecular weight of PLA and PBAT slightly increased during year 1, perhaps due to release of monomers or oligomers, and then decreased extensively during years 2 and 3 due to hydrolysis of ester bonds (confirmed by Fourier transform infrared spectroscopic analysis). The structural integrity of BDMs was assessed during years 2 and 3 of the study (2017–18) in field trials at the locations where they were stored, Knoxville, TN, and Mount Vernon, WA, for vegetable production. The degradation of the BDMs during the cropping season was higher in 2018 compared with 2017, suggesting that degradation of mechanical and chemical properties while in storage may have contributed to rapid degradation of mulches in the field. In summary, BDMs undergo degradation even under ideal storage conditions and may perform best if deployed within 2 years of their receipt date. The farmer should verify that proper storage conditions have been used before receipt and that manufacturing date precedes the receipt date by no more than 6 months.

Pseudomonas putida Isolation and Quantification by Real-Time PCR in Agricultural Soil Biodegradable Mulching

To reduce the plastic waste problem in agriculture, biodegradable plastic (BP) mulch films have become of key importance thanks to their biodegradability and beneficial effects on crops. However, at present, BPs cannot always replace conventional plastics, because biodegradation is governed by many biotic and abiotic factors under field conditions. This research aimed at isolating and identifying, from soil particles directly attached to the surface of BP samples, the microorganisms responsible of degradation through a combined approach based on biodegradation and molecular tests. For this purpose, a field trial within a Mediterranean apricot orchard was carried out to study the biodegradation of a commercial BP mulch with respect to a no-BP, a conventional apricot management, following the standard agricultural practices, and a subterranean clover cover cropping, either incorporating or leaving its dead mulches on the soil surface. After BP film appeared visibly degraded in field, we isolated from soil particles attached to the polymer surface, a mesophilic bacterium with certain degradative potential assessed by plate and liquid assays, identified by sequencing as Pseudomonas putida. Quantitative real time PCR analysis showed the P. putida was significantly more abundant in PB plots than the other plot treatments. These preliminary results are potentially applicable to accelerate the degradation of BP mulch films and decrease the plastic pollution in agriculture.