Molecular Plant Breeding 2024, Vol.15, No.1, 34-41 http://genbreedpublisher.com/index.php/mpb 35 In this case, biodegradable mulch has emerged, which is mainly divided into non-biodegradable mulch (e.g. photodegradation, oxidative degradation, erosion, etc.) and biodegradable mulch (Kasirajan and Ngouajio, 2012). Photodegradable mulches are mainly plastics degraded by light-induced chemical reactions, and the photodegradation pathway has been shown to be effective in past studies, but it is more expensive to use (Greer and Dole, 2003). Meanwhile, photodegradable materials are usually composed of polyethylene and associated additives that accelerate the rate of degradation of mulch in sunlight, but also at a slower rate in areas with less solar radiation (Greer and Dole, 2003). The same principle is true for oxidative degradation materials. Therefore, more and more researchers are focusing on biodegradable mulch. Biodegradable mulch is a more suitable alternative to plastic films. At the end of its useful life, biodegradable mulch can be integrated directly into the soil, where microbial communities convert it into carbon dioxide, methane, water, and other substances (Kyrikou and Briassoulis, 2007; Brodhagen et al., 2015). It has been concluded that this mulch has the same thermal insulation and yield-enhancing effect as plastic mulch (DeVetter et al., 2017). It can also effectively solve the problem of subsequent waste that is difficult to dispose of the mulch film (Kunioka et al., 2009). Therefore, biodegradable mulch is expected to replace the use of plastic mulch and be widely used. In this paper, we focus on the new biodegradable mulch films in recent years and describe the effects on soil microorganisms during the degradation process, as well as the future of biodegradable mulch films. 2 Types of Biodegradable Films and Their Introduction The principle of biodegradable mulch is to mix biodegradable biomass into plastic components, so that large pieces of plastic can be easily degraded into small pieces of plastic. However, this kind of degradation is not a real and complete degradation, the plastic that has been degraded into small particles still remains in the soil (Wong et al., 2020), and this kind of material is not able to do anything about the subsequent degradation of small pieces of plastic. With the deepening of human society's understanding of environmental issues, solving the "white pollution" caused by waste mulch has become a priority. The organic macromolecule of degradable mulch has a carbon base as its backbone. Therefore, there are two main research directions for biodegradable films (1) using natural materials, such as starch, cellulose, etc.; (2) synthetic materials, such as polylactic acid (PLA), polybutylene terephthalate-ethylene glycol-ethylene terephthalate co-polyester (PBAT), etc., and this section summarizes the above four types of biodegradable films. 2.1Starch Starch is one of the first biopolymers used for the development of sustainable materials to replace petroleum-based synthetic plastic production. Due to their low cost, renewability, and inherent biodegradability, starch-based polymers are high-potential feedstocks for the large-scale production of bio-plastic films (Bartolucci et al., 2023). It is a mixture of straight chain starch (Figure 1a) and branched chain starch (Figure 1b) (Vamadevan and Bertoft, 2015). Starch (the structure is shown in Figure 1) is a plant reserve carbohydrate synthesized from glucose, which in turn is produced by photosynthesis of carbon dioxide and water. On the one hand, starch has poor physical properties because it is a hydrophilic substance that is soluble in water and becomes brittle upon drying (Kaur et al., 2012); On the other hand, starch polymers have strong hydrogen bonds and are difficult to melt (Zhu et al., 2020), therefore, starch polymers need to be reprocessed in order to satisfy the requirements for mulching membranes (Bastioli, 1998). Taking thermoplastic starch (TPS) as an example, after adding some plasticizers to it, these additives include alcohols, polyolefins, and surfactants (Wong et al., 2020), thermoplastic starch can be prepared under the action of heat and shear, thus improving the processing and use properties of the starch (Zhang et al., 2017), and thus producing membranes without using properties. In short, starch cannot be used as a biodegradable mulch due to its poor water resistance and high melting point, but different types of biodegradable mulch can be produced by combining it with other plasticizers (Zhao et al., 2021), but this type of mulch is not fully biodegradable, and the residual non-biodegradable components, such as PE, can pollute the soil (Rudnik and Briassoulis, 2011).
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