FormalPara Contents

1 Introduction

Plasticulture refers to the use of plastics in agriculture. Plastics began to be used after the Second World War, to avoid the high costs of glass, employed as traditional material in the greenhouses (Kasirajan and Ngouajio 2012). The introduction of plastic technology caused a true revolution in agriculture, with important advantages related to quantity and quality of crops, due to their particular properties: sound and electrical insulation, thermal stability, resistance to chemical corrosion and biodegradation, repellency activity against molds and bacteria, easy and inexpensive processing, and unique mechanical properties (Kyrikou and Briassoulis 2007). Due to these properties and advantages, polymeric materials have been employed for a wide range of agricultural applications, among them mulch films (Kasirajan and Ngouajio 2012).

According to Kader et al. (2017), mulches are defined as layers of a material or materials that consist of covering the soil surface. However, mulching is a water conservation technique in order to increase water retention by soil and prevent soil erosion. Historically, farmers have employed this agricultural practice to regulate soil temperatures, prevent the loss of moisture, reduce the growth of weeds, enhance crop productivity, obtain earlier harvests, and improve the protection of food products (Bandopadhyay et al. 2018). Hence, mulches provide a more sustainable agricultural efficiency from a resource management perspective (Yang et al. 2020). Among plastic mulch films, low-density polyethylene (PE) is the most used due to its easy processing, and because it is flexible and rigid enough to be removed from different growing environments (Kasirajan and Ngouajio 2012).

Traditional PE mulch films are of fossil oil origin and present several disadvantages (Hayes et al. 2012). The use of PE as crop mulching is associated with some economic issues. For example, at the end of the season crop, removal of plastic mulch films from the land is time-consuming, taking approximately 42 h per hectare (ha) (Velandia et al. 2020). Recycling and reuse of synthetic mulches are an expensive and a difficult method since these films contain a wide variety of agrochemicals, remains of soil, and organic matter (Kapanen et al. 2008). Marí et al. (2019) reported that in some regions of Spain, PE film final disposal costs represent 176.5 €/ha for removal, landfill 186 €/ha, and recycling 192 €/ha. As a consequence of these economic disadvantages, PE mulch film wastes are accumulated or tilled on the field; some growers even abandon them along rivers or rural areas. In other cases, these wastes are burned in open air and uncontrolled conditions (Scarascia-Mugnozza et al. 2008). Besides final disposal disadvantages, plastic mulch films represent a source of macro- and microplastic contamination in agricultural areas (Huang et al. 2020).

To provide a potential solution to the final disposal of PE crop mulches, and to increase their sustainability in the agriculture field, biodegradable and renewable films derived from biomass have been developed (Barragán et al. 2016). In contrast to PE-based films, biodegradable mulch films (BDMs) can be incorporated directly into the soil or placed in a composting plant, being degraded by microorganisms at the end of the crop season (Kapanen et al. 2008). After the fragmentation phase, microflora transforms the residual breakdown products of BDMs into carbon dioxide, methane, water, or biomass through the mineralization process, without harm. Figure 1 shows a starch-based BDM for a tomato crop at laboratory scale.

Fig. 1
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Illustration of starch-based biodegradable mulch film collocation on tomato crop.

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In relation to this environmental safety perspective, sustainable development, and eco-friendly design, natural biopolymers (agro-polymers) extracted/removed from agro-resources have offered a promising and viable alternative to replace PE-based mulch films and to overcome agricultural plastic problems (Averous and Boquillon 2004). Bio-based polymers are those polymers formed under natural conditions over growth cycles of all organisms. They include polysaccharides, proteins, and lipids (Briassoulis 2004). Ready availability, low cost, non-toxicity, biodegradability, and biocompatibility are some benefits of using natural polymers into agricultural applications. In nature polysaccharides such as cellulose and starch are abundant and can be acquired from plants, while alginate can be obtained from brown macroalgae. Other polysaccharides such as chitin/chitosan are extracted from the exoskeleton of arthropods, fungi, and crustaceous. All of them are complex carbohydrate polymers made up of monosaccharide units linked by glycosidic bonds. Several reactive functional groups including hydroxyl, amino, and carboxyl groups are present in their structures, conferring them a highly chemical versatility. Additionally, polysaccharides have different chemical structures, in terms of their charge: negative, positive, and neutral. Table 1 shows the chemical structures of the main polysaccharides employed to prepare BDMs, along with a brief description of each of them.

Table 1 Chemical structure and characteristics of the main polysaccharides employed to prepare biodegradable mulch films
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In spite of previous advantages, BDMs also exhibit two clear disadvantages related to their mechanical and water barrier performances: sensitive to moisture and brittleness (Abdul Khalil et al. 2018).

The development of polysaccharide-based BDMs must retain similar mechanical properties as PE mulch films as well as barrier properties. In this sense, several authors have already reported the specifications regarding thermoplastic BDMs for agriculture/horticulture practices: mechanical (Briassoulis 2004) and biodegradability (Rudnik and Briassoulis 2011). More recently, the European Union (EU) standardized specific norms for BDMs, EN 17033 (European Committee for Standardization 2018) which regulate their mechanical properties and biodegradability in soil (Table 2). The standard EN 17033 does not include a particular value to water vapor permeability (WVP); however, this property is significantly important for mulch films, since WVP low values enhance soil moisture and provide a sustainable water management. In Table 2, we included PE WVP as another specific parameter that BDMs should achieve.

Table 2 EN 17033 EU standard: mechanical and biodegradability criteria for biodegradable mulch films (BDMs).
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A common way to obtain eco-friendly mulch films, overcome their own issues, and comply with the international requirements is through the formulation of multiphase materials such as surface modification, employment of polymer blends, and introduction of reinforcements (Mohanty et al. 2002). Specifically, bio-based composites can be defined as composite materials constituted by two or more distinct biodegradable phases, which generally consist of a continuous weak matrix, and embedded reinforcements providing strength and stiffness (Fowler et al. 2006). Fillers have been introduced into biodegradable matrices as a discontinuous phase, creating a complex structure (Abdul Khalil et al. 2018). When reinforcements and matrix are combined, the result is a biomaterial with improved properties compared to those of each single component (Fowler et al. 2006). Through bio-composite materials, BDMs should achieve PE mulch film parameters and for that reason, in the last years, the scientific community has increased its attention regarding these biodegradable thermoplastics (Sander 2019).

In parallel, some polysaccharide-based multiphase mulch films are already commercialized and they occupy an important place among bio-based formulations (Sarkar et al. 2018), although their scope is limited by economical (Velandia et al. 2020) and sociological issues (Goldberger et al. 2013). For example, in the USA, BDM consumption represents only 5% of the total plastic mulch films (Guerrini et al. 2019). In this context of continued plastic production growth (Briassoulis and Giannoulis 2018) and low BDMs adoption, efforts should be made to foster technological progress in order to reduce BDMs costs and to promote their usage. In this direction, one way to transfer technology is through intellectual property (IP) rights, such as patents (Van Norman and Eisenkot 2017).

Until now, there already exist some works that explain the importance of employing BDMs from biopolymers like paper mulches (Haapala et al. 2014), polyhidroxyalkanoates (PHAs) (Tian and Wang 2020), and renewable resources (Yang et al. 2020). Also, these reviews include comparisons between PE-based mulches and BDMs, but none of them covers the use of polysaccharide-based multiphase materials, as matrices and/or fillers, and their impact on mechanical, barrier, and biodegradability properties. This review includes a comparative analysis between different mulch film treatments for their performance for different crops, and their impact on soil quality and cultivated plants. In addition, in this review, we summarize commercial challenges and limitations related to composite-based BDMs, we list polysaccharide-based multiphase materials available commercially, and we analyze the evolution of patents as instruments of intellectual protection for innovation over the last 15 years.

2 Polysaccharide-based bio-composite mulch films and their properties

In this section, we will carefully review studies focused on microscale bio-composites which offer advantages to be employed as mulch films. Particularly, polysaccharide-based bio-composites, as matrices and/or reinforcements, will be prioritized to interpret the impacts on mechanical, biodegradability, and barrier properties of these promising materials in agriculture. According to Briassoulis (2006), BDMs should, at least: (1) show appropriate mechanical properties at installation stage: tensile strength (TS) and elongation at break (ε), (2) conserve mechanical properties during BDMs useful lifetime, and (3) be 100% biodegradable in the soil before the start of the next crop season. Besides mechanical property and biodegradability capacity, WVP is another important characteristic for mulching applications, especially in arid regions. For this reason, it is necessary to achieve similar PE water vapor barrier value (Table 2). In Fig. 2, we summarized mechanical, barrier, and degradability properties related to crop season.

Fig. 2
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Mechanical, barrier, and biodegradability properties related to crop season.

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2.1 Mechanical properties

Once the material has been loaded with reinforcements, the stability and resistance of the bio-composite are determined by evaluating their mechanical properties. Specifically, for mulching applications, it is fundamental that bio-composites ensure their integrity under certain load: ability to stretch and cover the soil (Yang et al. 2020). Three parameters that are usually reported and/or measured to inform bio-based material mechanical properties are: TS, Young’s modulus (YM), and ε. Each one provides the following information: TS is evaluated in order to determine how resistant the material is, YM is measured to report the stiffness, and ε parameter informs about flexibility, elasticity, or ductility (Abdul Khalil et al. 2018). Tables 3 and 4 summarize the mechanical properties of BDMs with different organic filler contents.

Table 3 Mechanical properties of bio-composite polysaccharide-based matrix mulch films with different organic filler contents.
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Table 4 Mechanical properties of bio-composite biodegradable mulch films with different polysaccharide filler contents.
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Starch is, among agro-polymers, the most used for the preparation of biodegradable and economical thermoplastic films (Merino et al. 2019a). In the case of starch-based BDMs, this polysaccharide could be used in two different ways: (1) it may be employed as biodegradable reinforcement or (2) it may be used as biodegradable matrix. Regarding the use of starch as reinforcement, Chiellini et al. (2001b) and Flores et al. (2009) introduced starch as organic filler into films based on poly(vinyl alcohol) (PVA) matrix and poly(butylene succinate) (PBS) matrix, respectively. Chiellini et al. (2001b) used starch and different fibers (sugarcane, apple, and orange waste) to prepare bio-composite films based on PVA. These same authors obtained biodegradable films through casting method and studied the effect of increasing starch contents, from 0 to 25 wt%, on mechanical properties (Table 4). For PVA/orange pomace fibers and PVA/apple pomace fibers bio-composites, ε values considerably decreased by adding 25 wt% of starch content: from 105.4 and 149.7% without filler to 29.6 and 65.0%, respectively. TS did not suffer important changes in its values, showing slight improvements for PVA/orange pomace fibers. In the case of PVA/sugarcane fibers/starch, the introduction of reinforcement produced opposite changes. TS increased more than twice with 25 wt% of starch, and presented small changes for ε values. In contrast, Flores et al. (2009) developed PBS bio-composite films reinforced with starch filler by a hot-pressing method. Mechanical properties of these composite films were studied varying starch content: 0, 20, 40 and 60 wt% (Table 4). Regarding TS values, these same authors reported a reduction when starch content increased from 37.2 MPa with 0 wt% to 5.7 ± 0.9 MPa with 60 wt%. The introduction of low content of starch showed a small improvement in ε; however, this parameter decreased by adding contents over 20 wt%. Both Chiellini et al. (2001b) and Flores et al. (2009) confirm that loaded organic filler improves TS property until certain weight content and they attribute it to uniform dispersion of starch fillers into PVA and PBS matrix. They also agree that the incorporation of plasticizers enhanced ε values against non-plasticized films. However, they report TS values with significant reductions. In order to use starch as biodegradable matrix, this biopolymer should be plasticized and compatibilized with other polymers (Briassoulis 2004). For starch matrix-based BDMs, matrix filled to different organic reinforcements showed better mechanical properties compared with neat polysaccharide matrix (Abdul Khalil et al. 2018). Patil and Netravali (2016) used microfibrillated cellulose extracted from kraft pulp, to introduce them into a mango seed starch matrix. The biodegradable films were developed through casting method using an eco-friendly cross-linker, and the chemical similarity between matrix and fillers was exploited in order to improve interfacial bonding in the bio-composites developed. Additionally, to analyze the effect of different cellulose fibers loads (0, 20, 30, and 40 wt%), these same authors characterized the prepared bio-composites. Important enhancements in the TS and YM were observed with the incorporation of microfibrillated cellulose into a mango seed starch matrix (Table 3): YM increased from 1.347 ± 0.324 GPa to 2.407 ± 0.063 GPa when fillers were loaded from 0 to 40%, respectively; TS improved almost 90% by introducing 20% of micro fibrillated cellulose to neat starch film. The parameter ε showed significant improvements up to 30% filler content; when up to 40% was added, this parameter slightly decreased. According to Patil and Netravali (2016) the developed bio-composite films showed excellent mechanical properties due to the uniform dispersion of fillers; cellulose as reinforcement provided high aspect ratio and high surface area. In contrast, Spiridon et al. (2011) elaborated bio-composite films using corn starch as a matrix and chemically modified starch microparticles as biodegradable reinforcements. They incorporated lignin, from two different sources: boost and beech, into corn starch chemically modified and starch microparticle matrix. Chemically modified starch microparticles and lignin filler were prepared by adding 4 wt% by casting method and the effects on mechanical properties were investigated. In general, the presence of lignin decreased YM and ε but increased TS (Table 3). Starch-boost lignin and starch-beech lignin bio-composite materials are more rigid than the original films: TS increased almost 47 and 21%; YM decreased 8 and 31%; and ε decreased nearly 39 and 30%, respectively. This mechanical behavior was attributed to generating compact structures due to high intermolecular hydrogen bonds between the starch and lignin. Bodirlau et al. (2013) carried out similar researches. They prepared chemically modified starch microparticles and introduced them within corn starch matrix by casting process. Chemically modified starch microparticles and different fillers including keratin, lignin, and cellulose were incorporated into a corn starch matrix, by adding 4 wt%. In relation to mechanical properties, the incorporation of these natural fibers produced the following effects: TS values were improved, reaching almost 98% with keratin fibers; in an opposite way, ε values were reduced until 37% for the case of lignin fibers, and YM did not present a clear trend (Table 3). All these mechanical properties are associated with the development of brittle bio-composites, due to high intermolecular hydrogen bond between starch and natural fibers. In order to improve mechanical performance of the thermoplastic maize starch matrix, Stasi et al. (2020) used carbon ashes obtained from lignocellulosic agricultural waste. They evaluated mechanical properties to bio-composite films with two different filler contents: 0 and 7 wt% (Table 3). The effects of carbon ashes on thermoplastic maize starch mechanical response were as follows: an increase of 15% in YM, a decrease in ε from 0.66 ± 0.05 to 0.33 ± 0.03, and a reduction of 14% in the TS value. Ayu et al. (2020) studied bio-composite films based on PBS and modified tapioca starch as biodegradable matrix reinforced with empty fruit brunch fibers. The biodegradable films were prepared with fixed composition of modified tapioca starch and PBS (50:50 ratio) with different volume contents of fibers: 0, 30, 40, and 50%. TS and YM values were significantly reduced as more empty fruit brunch fiber contents: 37 and 28% for 50 wt% of fiber content, respectively (Table 3). The authors explained these matrix and fiber interactions on the basis of poor interfacial adhesion, difference between functional groups, and non-homogeneous dispersion of fillers.

To improve the properties of bio-composites, lignocellulosic fillers have been studied (Briassoulis 2004). Chiellini et al. (2001a) studied and mechanically evaluated bio-composite films based on gelatin and fillers from sugarcane bagasse obtained by casting method. Sugarcane bagasse was introduced within the gelatin matrix with 20 wt%. In the same study, the authors compared tensile properties with neat gelatin film, using glutaraldehyde as a cross-linker agent. The addition of sugarcane fibers to the gelatin matrix produced considerable modifications in YM, which increased from 0.033 ± 1.0 × 10−3 to 0.283 ± 2.1 × 10−3 GPa and ε value, which was reduced almost 90%. TS resulted without variation (Table 4). According to Chiellini et al. (2001a), the incorporation of lignocellulosic fibers and glutaraldehyde as cross-linker provided a hard and brittle material compared to gelatin without sugarcane fibers. Thus, they are not recommended for agricultural films. Alternatively, Flores et al. (2009) used different cellulose filler contents: 0, 20, 40, and 60 wt% to develop PBS-based bio-composite films. They evaluated their mechanical properties and obtained two clear trends: TS decreased with cellulose content from 37.2 MPa with 0 wt% to 7.8 ± 0.2 MPa with 60 % wt, and ε showed significant improvements regardless of cellulose contents (Table 4). Finkenstadt and Tisserat (2010) evaluated poly(lactic acid) (PLA) bio-composite films with 0, 10, and 25 wt% Osage orange wood fibers with different particle sizes (Table 4). In general, BDMs based on PLA/Osage orange wood fibers showed slight improvements related to YM values with 25 wt%, compared with neat PLA. In contrast, ε and TS values decreased with Osage wood fibers with 400 nm of size and 25 wt%: ε decreased from 18.65 to 8 ± 3.2% and TS decreased from 57.3 to 36.6 ± 4.2 MPa. In the same sense, França et al. (2018) prepared bio-composite films with babassu lignocellulosic fibers in a blend of polybutyrate adipate terephthalate (PBAT) and PLA (50/50 ratio) and studied mechanical properties of different BDMs containing 0, 10, 20, 30, and 50 wt% of fibers. TS and ε values were reduced, while YM values improved regardless of fiber contents (Table 4). Similarly, Hoffmann et al. (2019) obtained close results for films based on different weight ratio of PBAT and poly(3-hydroxybutyrate) (PHB) as biodegradable matrix, filled by babassu fibers (see Table 4). Babassu filler yielded an enhancement in stiffness and a decrease in ductility regardless of lignocellulosic content added. In both studies, the authors explained the high stiffness due to reinforcing effect of the cellulose-based particles, and an increase in TS and in ε was attributed to the possibility of the agglomeration of particles which might act as breaking points in each material. Kumar et al. (2018) developed bio-composite films based on cellulose fibrils acquired from African Napier grass as filler and cellulose as a matrix, by a regeneration process. Different cellulose fibers were loaded: 5 to 25 wt% and all composite films were characterized by tensile testing techniques. These authors confirmed that both control and bio-composite films were rigid and brittle (Table 3). TS decreased with the increase of cellulose fibers content from 107.9 ± 6.5 MPa with 0 wt% to 49.7 ± 3.5 MPa with 25 wt%. However, ε improved its values almost 63% with 25 wt%. The decrease of TS in bio-composite films was associated with size differences of cellulose fibers and their random orientation within the matrix (Kumar et al. 2018). In relation to chitosan-based BDMs, Merino and Alvarez (2019) used natural seaweed microparticles as filler in chitosan and thermoplastic starch blend matrix. Bio-composite films were obtained by adding fillers in 0.5, 1, 2, and 10 wt% to thermoplastic starch and chitosan blend (Table 3). The effects that introducing natural seaweed microparticles had on mechanical properties led to the following findings: the addition of reinforcements did not provide major changes on TS values; low contents of natural seaweed microparticles caused improvements on ε, from 70.5 ± 5.2% with 0 wt% to 162.0 ± 28.9% with 0.5 wt%; YM increased notably for bio-composite with low fillers content; however, YM value decreased when 10 wt% of filler was added. These same authors explained that YM trend is related to a good adhesion and chemical compatibility of seaweed microparticles and the matrix; however, at high quantities of fillers, discontinuities could appear. A similar explanation was associated with ε behavior. For the bio-composite film with 10 wt% of reinforcements, the discontinuities caused by particle agglomeration decreased the ε value.

Information concerning BDMs and mechanical properties showed that these bio-based plastics partially satisfy the specifications of the EN 17033 EU standard. In order to overcome BDM mechanical problems, several authors have implemented different modifications to get better mechanical properties, among them: surface modification (Merino et al. 2019b; Zhang et al. 2008), optimization of compatibility between multiphase materials (Sun et al. 2019; Wei et al. 2015), or the introduction of nanoparticles (Baheti et al. 2013). Particularly, Sun et al. (2019), in an effort to provide a solution to the problem of rupture, developed starch/chitosan BDMs modifying the following ratios: chitosan to starch and plasticizers. They obtained biodegradable starch/chitosan films by casting method, with significant increases of mechanical values: ε value reached up to 104.1% and TS value was 56.18 MPa.

Polysaccharide-based bio-composite films showed that the addition of reinforcements from organic origin is able to enhance the overall mechanical performance regarding neat polysaccharides films. The mechanical properties could be adjusted through different chemical/physical methodologies and/or the introduction of additives, in order to comply with international standards. Significant scientific works could be addressed to study BDM mechanical properties over different crop seasons.

2.2 Biodegradability

Unlike some biopolymers, natural polymers biodegrade when exposed to compost and/or in situ soil degradation (Fig. 2). Particularly, polysaccharides’ biodegradability is related to the high number of hydrophilic polar function groups in their polymeric chain (Table 1). For composite materials, the intrinsic biodegradability of polysaccharides is modified when they are incorporated as fillers and/or used as the matrix and, consequently, bio-composite film biodegradability is influenced by the chemical structure (Garrison et al. 2016)/the nature of components (Thompson et al. 2019), cross-linking density (Garrison et al. 2016), and soil conditions (Sintim et al. 2020). According to European standard EN 17033:2018 (European Committee for Standardization 2018) and American ISO 17556-2019 (International Organization for Standardization, 2019), a given material is considered to be completely biodegradable if it reaches a complete biodegradation in less than 24 months (Briassoulis et al. 2020). Unfortunately, EN 17033: 2018 (European Committee for Standardization 2018) and ISO 17556-2019 (International Organization for Standardization 2019) cannot simulate the real biodegradability of BDMs in soil environment, since biodegradation process depends on climates and locations (Sintim et al. 2020).

Historically, the first biodegradability studies related to bio-composite-based BDMs were focused on the evaluation of mineralization tests by soil microorganisms. That is the case of Corti et al. (2002), Imam et al. (2005), and Chiellini et al. (2008). In their works, they introduced polysaccharide fillers or blended polysaccharides in order to increase the biodegradability rate of PVA. Among vinyl polymers, PVA possesses oxidizable functional groups in its polymeric chain, and it is easily biodegradable by secondary alcohol peroxidases isolated from soil (Briassoulis 2004). Corti et al. (2002) studied the effect of incorporating sugar cane bagasse as natural fillers into PVA matrix on bio-composite biodegradability. The authors reported that after 160 days of incubation, the mineralization of PVA with 50 wt% of sugar cane approached a plateau near 23.7%. This value represented an improvement of more than twice in PVA film mineralization, and it was related to the incorporation of hydrophilic organic filler within PVA matrix (Table 5). Imam et al. (2005) developed bio-composites based on PVA and PVA/lignocellulosic fibers reinforced with corn starch, and they compared their biodegradability through 120 days of exposure in soil under prevailing environmental conditions. After 90 days, the bio-composite films showed a significant reduction in size as well as in their total weights. Regarding corn starch filled PVA composite, this film lost approximately 51% of their initial weight, unlike corn starch filled PVA/lignocellulosic fiber film decreasing to 41% (Table 5). This difference between these bio-composite films could be attributed to better cross-linked for the PVA/lignocellulosic film and, consequently, a lower rate of film deterioration. Chiellini et al. (2008) used materials from renewable resources of marine origin to produce bio-composite films and evaluated their biodegradability. This composite was based on green algae and corn starch as fillers and PVA as a matrix including algae and corn starch fibers as reference. After 150 days of incubation, bio-composite film mineralization was affected by organic fillers, and increased approximately 50% compared with PVA as reference (≈ 5%) (Table 5).

Table 5 Biodegradation process of bio-composite biodegradable mulch films with different organic filler contents.
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More recently, other biopolymers are getting attention as matrices and/or reinforcements in order to evaluate their biodegradability, such as kenaf fibers (Pua et al. 2013), PBAT (Wang et al. 2015), cotton fibers (Tan et al. 2016), chitosan (Arrieta et al. 2016), and carnauba wax (Oliveira et al. 2019). Pua et al. (2013) developed kenaf/PVA composite films having 5, 10, and 15 wt% of fiber loadings, modified by NaOH and citric acid. The biodegradability tests of these bio-composite films were evaluated by soil burial test and the weight of the samples was recorded 15 days before and 15 days after the test. After the soil burial exposition, the films showed weakness, brittleness, and weight loss. For both NaOH and citric acid-modified kenaf/PVA films, this deterioration increased with fiber loading. After 15 days, PVA film without fibers had lower weight loss (near 1.63%) than kenaf/PVA bio-composites. However, 15 wt% citric acid-modified kenaf/PVA resulting in the highest weight loss, followed by 5 wt% NaOH-modified kenaf/PVA: 8.91 and 7.73%, respectively (Table 5). All samples presented higher weight loss than neat PVA film and also in other works, the authors associated this performance with the high degradability of kenaf fibers. Wang et al. (2015) studied the biodegradability of novel antimicrobial bio-composite films based on PBAT/starch thermoplastic by soil burial tests for 3 months. These authors loaded three filler contents of thermoplastic starch into PBAT matrix: 0 (neat PBAT), 20, and 40 wt%. After 3-month soil burial tests, neat PBAT films kept the original weight of 97.7 ± 0.5%, while PBAT/starch composite films with 20% and 40 wt% filler content exhibited 88 ± 1.0 and 65.9 ± 0.3% of the original weight, respectively (Table 5). The authors concluded that the incorporation of the polysaccharide facilitated the biodegradation of the synthetic biodegradable polyester. Tan et al. (2016) prepared bio-composite mulch films from natural fibers and biodegradable polymers. More specifically, the biodegradable composites were made up of cotton fiber/starch and cotton fiber/PVA loaded with 16 wt% of the biodegradable polymer. Biodegradation was studied by soil burial tests in the laboratory and in the field. The biodegradation degree was almost 54% weight loss for cotton fiber/starch film after 30 days and near 80% weight loss for cotton fiber/PVA after 45 days (Table 5). Field and laboratory tests indicated that cotton fiber/starch could be used for crops with short growth periods due to the high rate of degradation in soil. Arrieta et al. (2016) studied the effect of addition of low amounts of chitosan on disintegration property into PLA/PHB matrix. Three different bio-composites were studied in order to obtain a flexible and degradable PLA-PHB-based electrospun mats: 0, 1, and 5 wt% of chitosan. They examined films’ biodegradability under compositing conditions at laboratory scale for 37 days and reported that all bio-composite films were disintegrated (Table 5). These authors concluded that chitosan, as organic filler, increased the disintegration rate. To develop alternative bio-composite mulch films, Oliveira et al. (2019) used PBAT as a biodegradable polymeric matrix, and sugar cane and carnauba wax as bio-based additives. The authors monitored the biodegradation of these films through burial soil tests for 75 days by mass loss. In the same work, they evaluated five different films: neat PBAT, PBAT loaded with 2.5% and 5 wt% of sugar cane, PBAT loaded with 2.5% and 5 wt% of sugar cane, and with 2 wt% of carnauba wax in each one. During 60 days, biodegradation rates increased with sugar cane content in comparison with neat PBAT. Moreover, they confirmed that the addition of carnauba wax did not change the biodegradation pattern of PBAT/sugar cane films; however, this additive accelerated the process. After 75 days, regardless of the bio-composite films prepared, the highest value of weight loss was for neat PBAT (Table 5). The same authors divided this behavior into two parts: (1) during 60 days of burial test, bio-composite films presented higher biodegradation rates than neat PBAT, which was related to the incorporation of organic/hydrophilic filler (sugar cane) and, consequently, greater ease to water penetration; (2) the remaining 15 days, when sugar cane content was consumed, biodegradation rates decreased and, therefore, lower weight losses were achieved as compared to neat PBAT film.

Regarding PVA-based mulches, biodegradation rates improve their values when polysaccharide fillers are added (Table 5). Some mulch films reached nearly 50% of biodegradation when reinforced with 20% of starch after three months (Imam et al. 2005), as did mulches based on PVA as matrix and green algae/corn starch as fillers (Chiellini et al. 2008). Unfortunately, none of these PVA mulch films achieved 100% of biodegradation. In the case of PBAT-based mulches, the incorporation of polysaccharide impacts on biodegradation rates in different ways. Wang et al. (2015) reported that weight loss of mulches increases with starch content, while Oliveira et al. (2019) showed that weight retention was higher with 5% than 2.5 wt% of sugar cane content; this final trend changed when carnauba wax was incorporated (Table 5). Also in PVA films, PBAT mulch films did not achieve the total biodegradation under soil burial tests. In contrast, Arrieta et al. (2016) obtained the total disintegration of PLA-PHB films, independently of chitosan filler content (Table 5). Finally, Tan et al. (2016) showed a faster biodegradation rate for cotton films reinforced with starch than PVA fillers under the same biodegradation time. In this case, starch reinforcements proved to be better than PVA, but neither reached 100% of biodegradation (Table 5).

In summary, from all these findings, it seems to be important and imperative to respect crop times. In this sense, to comply with the biodegradation process, polysaccharide-based BDM biodegradability rates could be modified with the introduction of organic fillers and/or additives. However, efforts should be addressed to carry out biodegradation studies related to comparing different bio-composite BDMs with different organic filler contents on the long term.

2.3 Water barrier properties

Water barrier properties are particularly essential in mulching crop growth for reducing soil water evaporation loss and preventing hydric stress in plants (Martin-Closas et al. 2017). Polysaccharides contain several hydrophilic functional groups (Table 1), which provide good water affinity and, in consequence, poor water vapor barrier properties. PE mulch possesses considerably low water vapor permeability (WVP, i.e., the mass of water vapor transmitted through a unit area in a unit time under specified conditions of temperature and humidity, expressed hereafter in kilograms per kilopascal per square meter per second) (Table 2). For this reason, WVP values in polysaccharide mulch films can be improved through the introduction of fillers (Abdul Khalil et al. 2018). Different organic fillers were added into biodegradable matrices. Khan et al. (2012) prepared biodegradable films based on polycaprolactone (PCL) reinforced with methylcellulose fibers, with the following contents of fillers: 0, 10, 20, 30, 40, and 50 wt%. The WVP values improved progressively with increasing methylcellulose fibers inside PCL matrix was added, resulting in 1.75 × 10−11 ± 1 × 10−12 to 2.91 × 10−11 ± 1 × 10−13 kg m−2 s−1 kPa−1 with 0 and 50 wt% contents, respectively. The authors explained that this behavior is related to the large numbers of hydrophilic bonds that are provided by methylcellulose fibers, generating poor water barrier properties. Wu et al. (2018) added different contents of corn straw as organic fillers: 0, 5, 10, 15, and 20 wt% inside starch-based biodegradable films, and they studied their water barrier properties. The WVP values decreased when corn straw content increased. The starch film loaded with 15 wt% of corn straw showed the lowest WVP value: 1.83 × 10−11 ± 5.55 × 10−13 kg m−2 s−1 kPa−1. These results could be attributed to their network structures, i.e., lower WVP values were associated with denser film structures. Regarding alginate as organic fillers, Merino and Alvarez (2019) introduced them into a matrix composed of starch and chitosan varying their contents in the following amounts: 0, 0.5, 1, 2, and 10 wt%. Bio-composite films presented worse water barrier properties when the filler content increased. The lowest WVP value of 1.18 × 10−10 kg m−2 s−1 kPa−1 corresponded to the film with 0 wt% of alginate, while the highest 3.55 × 10−9 ± 5.2 × 10−10 kg m−2 s−1 kPa−1 to 10 wt% of seaweed microparticles. This fact was attributed to low association between molecules, which facilitates water penetration. Ayu et al. (2020) studied WVP of BDMs based on PBS and modified tapioca starch as matrix and empty fruit brunch fiber as reinforcements at three different weight contents: 0, 30, 40, and 50. The WVP values increased with empty fruit brunch fibers: from 5.71 × 10−14 ± 1.32 × 10−14 kg m−2 s−1 kPa−1 with 0 wt% to 7.02 × 10−14 ± 1.51 × 10−14 kg m−2 s−1 kPa−1 with 50 wt%. The authors justified this WVP values trend to hydrophilic nature of lignocellulosic fibers that promote water sorption and migration. Lower WVP values were obtained for these bio-composite mulch films than for PE (Table 2).

Taking these previous studies, many authors considered introducing modifications in order to obtain better results for the parameter WVP. For these reasons, researchers have studied the effects of different cross-linker agents on WVP properties of alginate (Liling et al. 2016) and kelp (Zhao et al. 2017) mulch films. Liling et al. (2016) investigated four different ion agents: Mn2+, Zn2+, Ca2+, and Al+3 and in all cases, WVP values decreased compared to alginate mulch film control. Mn2+ represented the best option, achieving 3.031 × 10−11 ± 0.064 × 10−11 kg m−2 s−1 kPa−1 from control: 4.58 × 10−11 ± 0.583 × 10−11 kg m−2 s−1 kPa−1. Zhao et al. (2017) obtained similar results using Ca2+ and H+ as cross-linker agents. The lowest WVP value was 8.263 × 10−11 ± 0.282 × 10−11 kg m−2 s−1 kPa−1 in the case of employing simultaneously Ca2+ and H+ as cross-linker agents. The same authors obtained worse WVP results for Ca2+/kelp and H+/kelp film formulations. This reduction on WVP values could be explained according to a decrease in the porous microstructure and the hydrophilicity of ionic alginate films, compared to neat alginate films. Alternatively, Merino et al. (2019b) chemically modified the surface of native and phosphorylated corn starch with chitosan. All of the BDMs prepared showed higher WVP values than neat native and phosphorylated starch films, achieving, in the worst of the cases, WVP values near to 5.50 × 10−9 ± 0.17 × 10−9 kg m−2 s−1 kPa−1. The increase in the hydrophilicity was attributed to cross-linking.

Another efficient way to reduce WVP values is through nanoformulations. Chang et al. (2010) were able to reduce WVP values of starch matrix by introducing chitosan nanoparticles. WVP of neat starch decreased between 18 and 30% when 1–2 wt% of chitosan nanoparticles were added. Comparable results were obtained by Han et al. (2018) and Hosseini et al. (2015). These authors used nanocellulose and chitosan nanoparticles, respectively. This significant improvement in WVP properties by nanomaterials can be attributed to the increasing of tortuosity in composite films and, consequently, low water diffusion.

Summarizing, WVP properties of BDMs can be improved with by adding organic reinforcements, cross-linker agents, and/or chemically surface modification. However, there exists a small number of original science publications related to barrier properties and bio-composite films with agronomic purposes in open literature. This constitutes a reason why research efforts should be intensified and addressed to enhance water barrier behavior of BDMs, testing different bio-based fillers, cross-linker agents, functionalized surfaces, and developing bio-nanoformulations, among several other chemical and biological alternatives.

3 Applications of polysaccharide-based bio-composite mulch films

The technological advances in agriculture have led to the search for technologies that allow greater control over the variables that interfere in crop yield. Several techniques stand out for optimizing yield capacity, including for instance, cultivation, and mulching. Weed control is the primary function of mulches, but they also conserve soil moisture, modify soil temperature, and, consequently, increase crop yields. Worldwide, the use of non-degradable plastic mulching is widespread in vegetable and fruit crop productions. The objective of using mulches can vary with the crop, the cropping system, and the production environment. While the durability of non-degradable PE-based mulches is high, there is a growing interest in developing suitable biodegradable mulching to minimize the negative environmental impacts of non-degradable materials. To expand the use of mulch films in a protected environment, bio-based polymers such as cellulose and starch have been proposed to be employed due to their easy biodegradability. In this section, we will introduce and discuss traits mainly of starch-based composite mulch films due to the wide framework of findings that highlights how these biomaterials work as biodegradable mulching in crop production systems.

3.1 Crop production

Among polysaccharides, starch is the most abundant and available low-cost renewable natural biopolymer which has been widely attempted as raw material for the development of biodegradable film as packaging material and mulch films (Medina Jaramillo et al. 2016; Merino et al. 2018b). Although starch confers poor mechanical properties for mulching, it is a polymer capable of forming thermoplastic films by the processing methods currently used for petroleum-derived polymers. In addition, to improve the mechanical property of the films, starch often needs supplementation with other biopolymers, additives, and fillers (Merino et al. 2018a). Starch for films is mostly obtained from corn, potato, and rice crops. Starch-based mulch films are impermeable to water but permeable to water vapor and degrade into harmless products (CO2 and water) when placed in contact with the soil humidity and microorganisms (Chandra and Rustgi 1998). The controlled-release formulation of agrochemicals using native starch has not provided satisfying results because of the lack of suitable thermoplastic properties. However, in the presence of plasticizers, high temperature, and shear, starch can start to exhibit thermoplastic properties required for its application as an extruding material (Liu et al. 2011). Urea is a well-known natural plasticizer used for chemical modification of starch (Wang et al. 2014) and it is also one of the major nitrogenous fertilizers in agriculture. These facts confer it optimum properties to be used as a plasticizer in mulch films to cultivated vegetable species. Urea has also been successfully applied in controlled-release systems for agricultural purposes (Azeem et al. 2014). A marketable potato (Solanum tuberosum L.) tuber yielded production on silt loamy soil over 2 years in Tai’an City, Shandong Province, and Feicheng City, Shandong Province. Potato quality and yield were promoted by the effect of controlled-release urea (CRU) including polymer coated urea (PCU) and polymer coating of sulfur-coated urea (PSCU). The CRU treatments increased total tuber yields by 8.77–19.88% and 14.36–26.46% in 2012 and 2013 respectively, in comparison with the urea treatment during the same year. The marketable tuber yield was also highly promoted by the application of PCU and PSCU in both years. Urea as a potential plasticizer of mulch and fertilizer was also assayed in oat (Avena sativa) and common radish (Raphanus sativus L.) seeds (Rychter et al. 2016). Extruded starch-based films plasticized with increasing amounts of urea (1%, 5%, and 10%) revealed that film containing the highest amount of the fertilizer was the most promising for delayed release system and the promotion of growth in both seedlings. In the case of 100% starch/glycerol used as control, the lowest values of plant growth parameters were obtained. These findings highlight the advantages of the use of urea-modified starch films, such as reduction of water evaporation, weed control, reduction in doses of nitrogenous fertilization, and relatively easy biodegradability. Although the high costs could limit its uses, beneficial results were also obtained combining CRU and normal urea to improve the nitrogen use efficiency and yield under wheat (Triticum aestivum L.) and maize (Zea mays L.) double cropping systems (Zheng et al. 2016). In this sense, lignin as a biopolymer is easily available in industrial waste and it was also evaluated as a natural plasticizer. The effect of 5 to 20% alkaline kraft lignin (AL) loadings on the biodegradation and nitrogen release in urea-modified tapioca starch was investigated in reduced soil condition (Majeed et al. 2014). Starch biodegradation in a lignin-modified slow-release urea depended on the film thickness. Interestingly, the slow-release fertilizers by slowing biodegradability of urea-cross-linked starch mediated by the lignin reinforcement could counteract the high biodegradability of starch (Majeed et al. 2017). These studies on the effects of natural plasticizers in starch composite mulch films are very interesting but apparently, their effects on productive crops remain to be evaluated. At productive scale to produce high strawberry fruit quality, mulching and irrigation are essential practices. For this reason, there are numerous studies about the effects of biodegradable mulch film in strawberry production worldwide. Particularly, in Southern Europe, strawberry crop production is a very high consumer of agricultural plastics. Then, it is very important to assess the effectiveness of their biodegradability in this region. For this reason, white-on-black biodegradable mulch films were compared with PE mulch film during autumn–winter cycle of strawberry production in the Ribatejo Region, Portugal (Andrade et al. 2014). The biodegradable mulches were of starch-based raw material supplied by Polivouga enterprise (M1-Biomind) and by Silvex (M2 and M3 Mater-Bi®) and conventional PE mulch films (M4). Mater-Bi® is a commercially know mulch film composed by PCL and starch (Sarkar et al. 2018). These authors investigated soil warming, lifetime, and the effects of the films on fruit yield. Soil temperatures showed differences among treatments during summer period under open field conditions and autumn-winter period under tunnel. The temperatures under biodegradable films, especially M2 treatment were always higher than those under PE mulch. However, after this period, soil temperatures were very similar among treatments. Fruit yields from plants grown under all biodegradable mulches were significantly lower than those grown under 100% PE resulting in 20% for M2 and M3 and 37% for M1 (Table 8). Otherwise, no significant differences were observed among bio-based mulch films on fruit yield. The authors assumed that the highest fruit yield on PE may be related to the associated to the lowest soil temperature, mainly during the beginning of the trial, when the air temperature was very high. In the same geographical region, five other mulches produced with raw material Mater-Bi® were also tested on strawberry production over 2 years (Costa et al. 2014). The biodegradable mulch films had similar traits than PE, with no significant differences in productivity or quality probably due to adequate soil temperature and water volume content. The different treatments based on starch-based biodegradable mulches have shown good results of quality and crop yields (Tables 6, 7, and 8). However, these authors suggested that it would be convenient to recommend a gradual replacement of the PE mulch film due to the cost of BDM is still high.

Table 6 Impact of starch mulches on tomato yield.
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Table 7 Impact of starch mulches on different vegetables, cereal, and cotton yield.
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Table 8 Impact of starch mulches on different fruits and legume yield.
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Black and white biodegradable films by extrusion from cassava starch and PBAT blend were developed and applied on a commercial strawberry plantation in the city of Londrina, Parana, Brazil (Bilck et al. 2010). Weed growth was observed in beds covered with white biodegradable film, probably due to its transparency. However, PBAT film provided efficient mulching for strawberry production because fruit production was qualitatively and quantitatively very similar in fresh weight to PE mulch film.

Very recently, an innovative cost-effective reutilization of carbon waste ashes as reinforcing fillers of thermoplastic maize starch-based film has been developed for agriculture applications (Stasi et al. 2020). Ashes at different amounts ranging from 7 to 21 wt% were added to starch and glycerol used as a plasticizer. Carbon waste ashes allowed improving thermal and durability performances of the thermoplastic starch films and also reduced the water absorption of starch matrix and deterioration. In comparison to those of neat starch, the waste carbon ashes/maize starch films can biodegrade and release the plant nutrients contained in the ashes into the soil. Even though this result is very promising, their effects on plant growth were not assayed in this study. In tomato (Solanum lycopersicum L.) crop, the critical period of weed control is 4–5 weeks after transplanting and plastic mulch is a common component of tomato production systems. In Central Spain, the substitution of PE film by starch-based film for tomato crop was proposed by Moreno and Moreno (2008). These authors conducted a 2-year study to determine the response of a tomato crop to seven mulch materials including PE and biodegradable films. These films underwent early decomposition but in general remained functional during use and did not affect yield (Table 6) or fruit quality attributes including total soluble solids, firmness, dry weight, juice content, and shape. Next year, Moreno et al. (2009) evaluated the effect of three mulches, black polyethylene, black biodegradable corn starch plastic, and aluminized photodegradable plastic in an open field of tomato. Different attributes such as mulch deterioration, soil temperature, and tomato yield and fruit quality were quantified. Black biodegradable and PE films were similar without significant differences between them in tomato yield (Table 6). Aluminized photodegradable film exerted the lowest yields (30%) with respect to the biodegradable mulch probably attributed to its early breakage which did not avoid the weed growth. According to Martín-Closas et al. (2008), the type of mulch employed had no effect on the fruit quality parameters. In support of this fact, PE and two Mater-Bi®-based mulch films showed similar weed suppression during the tomato production season, despite 15% of the soil becoming exposed because of biodegradable mulch deterioration (Minuto et al. 2008). Also, Minuto et al. (2008) studied the replacement of PE mulch films by starch-based BDMs, with similar results in crop yield of tomato, lettuce, and zucchini (Tables 6 and 7). The use of mulch has also proved to be suitable for the worldwide production of lettuce (Lactuca sativa L.). Compared to conventional PE mulch film, starch-based mulches were effective in increasing lettuce yield (Table 7). That suggests that the biodegradable mulch film can result in an alternative to PE mulch for lettuce cultivation.

The perennial wall rocket Diplotaxis tenuifolia (L.) is currently cultivated in several agricultural areas and, in particular, on about 4800 ha in Italy (Caruso et al. 2019). The crop cycles of the cultivated perennial wall rocket plant are carried out in sequence from autumn to spring or from spring to summer. Its transplantation is usually practiced on mulched ridges for a reduction of crop duration, and an improved weed management and product quality. In order to assess the effects of three crop cycles (autumn–winter, winter, spring), Caruso et al. (2019) studied three soil mulching types in a factorial combination: starch-based mulch named as Mater-Bi® black film, a brown photoselective low-density PE (LDPE) film, black-standard LDPE film, and a non-mulched control. Then, they analyzed leaf yield, quality, and antioxidants. The spring cycle of this cultivated plant was the most benefited by the presence of the mulch. Mater-Bi® biodegradable black film did not differ from both LDPE and the photoselective plastic mulch in terms of crop yield (Table 7). However, the biodegradable mulch film resulted in the best performances with regard to leaf quality, mineral, and antioxidant composition.

To compare their effects on soil temperature and moisture as well as peanut yield, six types of treatments were tested. These treatments included four different ratios of starch/PBAT biodegradable mulch films, containing 0% (B1), 10% (B2), 15% (B3), and 20% (B4) starch, respectively, and controls with PE and no mulching. The trial was performed in a randomized complete block design with three replications (Sun et al. 2018). Compared to other treatments, peanuts mulched with B3 showed a higher leaf area index, chlorophyll content, and net photosynthetic rate at late growth stages. Soil moisture of B3 was similar to that under the PE revealing that suitable biodegradable film can satisfy the changing needs of soil conditions in different seasons and improve plant yield (Table 8). Similar results were obtained for other crops: melon (Vetrano et al. 2009; Iapichino et al. 2014), tomato (Miles et al. 2012; Sekara et al. 2019), maize (Yin et al. 2019), and cotton (Wang et al. 2019). All this comparative information is summarized in Tables 6, 7, and 8. A summary of agronomic impacts of biodegradable mulches used in several crop plants including fruits and vegetable production systems was performed by Martin-Closas et al. (2017).

In view of potential agricultural applications, the resistance of natural occurring materials represents an important feature to consider, especially when protection is needed for perennial trees such as fruit and vineyard crops. It is well-known that some characteristics of polysaccharide-based films, mainly low mechanical properties, slow down their applications for long-duration crops. Hence, functionalization of polysaccharides could be necessary to achieve chemical structures of interest for fruit crops (Nešić et al. 2019). However, nowadays biodegradable plastic films are scarcely used in fruit production (Martin-Closas et al. 2017). Otherwise, to obtain more favorable characteristics for use in long-duration crop, films might include combination of two or more biopolymers or combination with some other component. In this sense, bacterial exopolysaccharides (EPSs) have been used in high-valuable applications because they are mostly appreciated for their stabilizing, thickening, and basically for their structure-creation agents (Chaabouni et al. 2014). Nanoparticles such as nanoclays can also enhance technical properties, such as mechanical and thermal stability. Likely, a combination of more than two materials is sometimes necessary to provide the best matrix for certain crops in particular (Merino et al. 2018a). In agreement with Martin-Closas et al. (2017), we argue that although horticultural species is the main application of biodegradable films, they should also be implemented for other crops including fruit production and in extensive crops.

3.2 Soil protection and pest management

Since the 1990s, biodegradable plastic mulches represent a potential alternative to PE mulch but the source of polymers and/or additives often also limit their use of some biodegradable mulch in organic crop productions (Goldberger et al. 2019). One important issue is that the incorporation of polysaccharide-based BDMs into soil can result in enhanced microbial activity including bacterial and fungal microorganisms. For this reason, despite the fact that total carbon input from these biodegradable materials could be very low, the effect on the dynamic of microbiota should also be studied in deep. Thus, to contributed to their adoption and reinforce the current knowledge gaps, impacts of BDMs on environmental sustainability are still scarce (Bandopadhyay et al. 2018). A novel study included the comparison between four BDMs (three commercially available and one experimental film), a biodegradable cellulose paper mulch, a non-biodegradable PE mulch, and a no-mulch plot over 2 years on the soil microbiota of two geographical locations, Knoxville and Mount Vernon, USA (Bandopadhyay et al. 2020). The bacterial community structure was determined using 16S rRNA gene sequencing and revealed differences by location and season. However, differences in soil bacterial by mulch treatment were not significant for any season in either location. In the fall of 2015, and in one of these locations (Mount Vernon), differences were observed between biodegradable mulch and no-mulch plots. All biodegradable mulch films had comparable effects to PE in soil microbial communities. On the other hand, European standard EN 17033 (European Committee for Standardization 2018) was the first to regulate the biodegradation requirements for biodegradable mulches. This standard indicates that > 90% of the organic carbon in the plastic polymers must be converted to CO2 in a controlled laboratory test within 2 years or less but it does not establish protocols for measuring in field biodegradation of plastic mulches. However, biodegradable mulches are tilled after uses and consequently their incomplete degradation could potentially impact on soil and crops. In addition, the wide variability associated with climatic conditions and rainfall could eventually produce changes in soil moisture and biodegradable properties, making this a highly complex fact. Although reliable methods are not available to measure mulch degradation post soil-incorporation, a sampling method for macro- and microplastic quantification has been very recently developed (Ghimire et al. 2019). However, further studies are also needed to determine the amount of time needed for complete biodegradation of biodegradable plastic mulch under field soil conditions. Nevertheless, the new formulations of polysaccharide biodegradable films proved to be promising according to the major objectives of pest control and enhanced crop yield. In the particular case of inorganic fillers and native and oxidized corn starch as matrices, Merino et al. (2018b) evaluated these novel formulations as biodegradable mulch films. Polysaccharide-based nanocomposites were prepared by extrusion using bentonite clay (Bent) and chitosan-modified bentonite (Bent-CS) fillers. In this mulch film, the incorporation of nanoclays improved water resistance but did not produce a significant effect in water vapor permeability and mechanical properties. However, the nanocomposites containing Bent-CS exerted antibacterial activity against the phytopathogen bacterium Pseudomonas syringae pv tomato DC3000 reinforcing the agronomical properties of this biodegradable mulch film. The antibacterial effect on this same phytopathogen was assayed for chitosan-coated starch mulch films (Merino et al. 2018b). In agreement with these findings, a chitosan/hydroxypropyl methylcellulose pesticide mulch film allowed protection against Phytophthora root rot caused by the phytopathogen fungus Phytophthora sojae on soybeans (Glycine max L. Merr.) (Liang et al. 2020). In this sense, starch-based mulch film supplemented with linear polyvinyl alcohol and sodium propionate as an antimicrobial compound was developed (Sen and Das 2018). This mulch film underwent ≈ 90% biodegradation within a period of 28 days and also enhanced soil nutrients, but its effect on biological targets was not assayed. The same authors suggested that the antimicrobial compound in the film did not hamper its biodegradation. Indeed, results on these types of functionalized materials may vary substantially with product formulation, soil type, vegetable crops, and geographical regions. We assume that in terms of research on efficacy of functionalized mulch films for the controlled-release of pesticides, the evidence is relatively recent. Likewise, the protective effects of these types of films would avoid the use of pesticides and therefore reduce the environmental impact they have on the soil and the environment. Worldwide, it is very well-known that there is an urgent claim to develop new alternatives to reduce the amount of traditional agrochemicals into terrestrial and aquatic environments.

4 Commercial insertion and intellectual property rights

4.1 Commercially biodegradable mulch film challenges and limitations

By the end of the 1990s, the first commercial BDMs appeared in scientific publications in order to evaluate their performance and biodegradability with other biodegradable polyester films (Solaro et al. 1998). In this way, Mater-Bi® became the first polysaccharide-based commercial multiphase material to be analyzed as BDMs. Mater-Bi® is manufactured by Novamont as biodegradable/compostable bioplastic, to provide a solution with low environmental impact (Hayes et al. 2012). This product, prepared from PCL and thermoplastic starch, offers high performance for different crops (Section 3.1) and, in addition, presents satisfactory mechanical and biodegradability properties for this specific agricultural application (Briassoulis 2006). Successful Mater-Bi® formulation could be attributed to the introduction of PCL that overcame the thermoplastic starch neat weakness: low resilience, moisture sensitivity, and high shrinkage at low PCL content (Averous et al. 2000).

Although the BDM crop productions are the same in comparison with PE (Marí et al. 2019), the wide range of commercial BDMs available in the market, and the continued global rise in plastic mulch film consumption (Briassoulis and Giannoulis 2018), biodegradable plastic has not reached large-scale commercialization (Sander 2019).

The main reason why BDMs do not enter this huge market is related to an economical limitation. In the work of Marí et al. (2019), they evaluated the economic profitability of different biodegradable mulches commercially available for a particular crop production. In the same work, the authors considered three possible scenarios of PE waste management: (1) absence of residues management, (2) landfill accumulation, and (3) total recycling. The researchers explained that these represent a significant increase of the total PE mulch film cost. Finally, they concluded that to adopt BDMs as alternatives to PE mulch, the government should increase the budget for subsidies, near 50%. Similar results were obtained by Velandia et al. (2020). The authors studied the economic feasibility of BDMs in pumpkin production and they concluded that the disposal fee has a small impact on the economic evaluation of adopting BDMs in short time. Additionally, they suggested extending the time of evaluation in order to analyze if other benefits of using BDMs appear in the long-term.

Goldberger et al. (2013) performed a sociological research to complement and explore experiences and/or views related to BDMs and their adoption from specialty crop growers, agricultural extension agents, agricultural input suppliers, mulch manufacturers, and other stakeholders. In that study, the researchers found that another challenge that BDMs face is the lack of knowledge. The authors found that insufficient knowledge related to BDMs is a huge barrier for their adoption/implementation for crop production systems. They suggested that this difficulty could be overcome with the diffusion of innovations by all participants of productive stages: farmers, agricultural extension agents, and agricultural input suppliers, among others ( 2013). Furthermore, they remarked that the process of innovation adoption is not an instantaneous act but a process that happens over time.

4.2 Commercially available polysaccharide bio-composite mulch films

Following Mater-Bi®’s commercial trend, different companies began to manufacture commercial BDMs and put them into the market. In this sense, Sarkar et al. (2018), Hayes et al. (2012), and Cowan et al. (2016) have already summarized some companies and their products, some examples are Biomax® TPS, a biodegradable agricultural mulch developed by DuPont, which consists of starch and thermoplastic starch (TPS); Biopar®, manufactured by BIOP Biopolymer Technologies, elaborated a commercial BDM based on starch and a biodegradable aliphatic co-polyester; TPS, PLA, PBS, and PBS-co-adipic acid (PBSA) were used as biodegradable polymers by Showa Denko Europe in order to prepare Bionelle, a marketable BDM; Xinfu Pharmaceutical Company elaborated Biosafe™, biodegradable agricultural mulch composed of PBAT and starch; Bioplast® BDM consists of TPS and PBAT and it was elaborated by Group Sphere Ibérica Biotech; Novamont (BDM leader) not only created Mater-Bi®, which consists mainly of PCL and starch but also prepared another commercial BDM: Eastar Bio™ prepared with PBAT and starch; BASF Company manufactured two types of commercial BDMs: Eco-Flex®, composed of starch and PBAT; and Envio®, which biodegradable polymers are starch, PBAT, PLA, and polyhydroxyalkoanote (PHA). On the other hand, Cortec Corporation elaborated EcoWorks as commercial BDM made of TPS and PBAT. Ingeo®, created by Nature Works Company, prepared a marketable BDM based on starch and PLA. Starch and polyester were used to create Naturecycle, a commercial BDM manufactured by Custom Bioplastics. Paragon was elaborated by Avebe Company using TPS and starch in its formulation. Finally, WeedGuardPlus is a cellulose-based BDMs manufactured by Sunshine Paper. All this information is summarized in Table 9.

Table 9 Commercial polysaccharide bio-composite mulch films available in the market.
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4.3 Patent evolution of polysaccharide bio-composite mulch films

As we mentioned before, innovations and inventions are crucial for agricultural progress. There exist different ways to transfer technology from academic centers to the private sector: (1) original publications, (2) research supported by private industry, and (3) the formation of emerging companies (Van Norman and Eisenkot 2017). In all cases, innovations and inventions are secured through IP rights: patents, copyrights, and trade secrets.

Particularly for mulch films, patent protection for technologies has been observed since the early development of this technology, in its origins mostly related to PE-based films. More than 2000 patents can be identified for these already mature commercial products, mostly protecting polymer chemistry, manufacturing film processing, and additives to improve functional or mechanical properties.

In order to identify patent documents disclosing mulching films based on biodegradable materials and analyze their evolution over time, a targeted patent search has been performed using Derwent Innovation repository. This commercially available database is a market-leading patent research and analytic platform delivering access to globally trusted patents and scientific literature. Searching strategy has been limited to patent documents, either granted or patent applications, published from 1995 to the end of 2019, using a combination of key words and codes from the International Patent Classification. Major jurisdictions such as the USA, Europe, Japan, and China are included in this mapping. This basic searching strategy has been developed to get a representative number of patent documents to perform a statistical evaluation, but a more comprehensive search should be performed in a product clearance or freedom to operate analysis.

A total of 158 patent documents claiming property over bio-composite mulch film technologies have been identified. The counting corresponds to patent families; this means that patent documents related to the same invention filed in different countries have been weighted as one record in the final count. The time evolution shows a marked increase in the number of patent documents published during the last years, mostly from 2015 onwards (Fig. 3). The evolution of patents not only shows the total number of patents applied but also exhibits the evolution of each polysaccharide analyzed in this review. Starch was the most requested among polysaccharide-based BDMs, followed by cellulose, chitosan, and alginate (Fig. 3). Documents were classified according to the specific material involved into the film composition. To do this, key words have been identified in document claims (cellulose, starch, chitosan, and alginate). From the 158 documents: 89 for starch, 30 for cellulose, 27 for chitosan, and 12 for alginate. A complete list with numbers, title, and assignee can be seen in the supplementary material.

Fig. 3
figure 3

Evolution of number of patents published according to the polysaccharide over time.

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Finally, an analysis for optimized assignees has been performed, showing that Chinese applicants are the most active. We can observe a significant increase in the number of patent documents from universities and/or academic centers as applicants in the last 5 years, including several patent documents assigned to Chinese universities. Furthermore, it is important to highlight two points associated with the number of patent documents collected: (1) the large amount is related to many small applicants and (2) high heterogeneity among patent applicants. The most frequent applicant is bioplastics world leader Novamont, followed by Jining Mingsheng New Material Co. Ltd., Anhui Delin Environmental Protection Development Group Co. Ltd., Shandong Tianye Biodegradable New Material Technology Co. Ltd., Dow Chemical Co., Chongqing Jiangjin Sende Family Farm, Chinese Academy of Science, Zhejiang University, University of Tsukuba, and Shandong Agricultural University (Fig. 4).

Fig. 4
figure 4

Number of patents assigned to different companies/universities.

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In summary, academic centers are the major source of inventions and innovations related to BDMs within agronomy field, leading by starch patent applied.

5 Polysaccharide-based BDMs and their impacts on soil quality and cultivated plants

As we mentioned before, the BDM incorporation into soil can help to enhance bacterial and fungal activity (Bandopadhyay et al. 2018). However, the effects of BDMs on soil microorganisms and the cultivated plant on the short- and long-term are still under research (Serrano-Ruiz et al. 2021). Moreno and Moreno (2008) studied the effects of seven mulch treatments (four BDMs and three non-biodegradable mulches of different colors) on soil properties in open fields in Central Spain. The authors reported that the soil temperature under the seven mulch treatments were affected by the material employed. Biodegradable and non-biodegradable mulches increased soil temperature in comparison with bare soil in all experimental dates. Nevertheless, the highest values were obtained under PE mulches, reaching a mean soil temperature difference between BDMs and PE of 1.6 °C. Similar results were obtained by Gu et al. (2017). On the other hand, differences near 5 °C in soil temperature BDM and PE mulches was found by Zhang et al. (2020) when cellulose-based mulch film was used. In the same direction, Ghimire et al. (2018, 2020) reported quite similar soil temperature for PE mulch film compared to starch-based BDM, but higher as compared to cellulose-based BDM. Sun et al. (2018) compared temperature soil with six different mulch treatments (four BDMs containing different ratios of starch/PBAT: 0%, 10%, 15%, and 20% of starch, PE film, and bare soil) on a peanut crop. In line with the above, the authors reported similar values of temperature soil for these different mulches, except bare soil, until 60 days after sowing. After that, the warming effect of BDMs decreased at late growth stage of peanut crop, due to the degradation process. In consequence, the thermal insulation effects were reduced. Similar results were obtained for oilseed rape crop using a starch/PCL biodegradable film (Gu et al. 2017).

Li et al. (2014) evaluated the impact of five treatments (two starch-based mulches, PLA-based mulch, cellulose-based mulch, and bare soil) on soil quality at three different locations: Knoxville, Lubbock, and Mount Vernon, USA. After 18 months, the authors reported that soil pH with BDM treatments does not show significant modifications as compared to bare soil for the three locations, regardless of BDM composition. Barragán et al. (2016) compared six mulch treatments (three starch-based mulches: potato TPS/co-polyester, maize TPS/co-polyester, and cereal flour/co-polyester; PLA-based mulch, PHB-based mulch, and PE mulch) for 6 months at laboratory scale in order to, among other things, monitor soil microbial activity. After 150 days, their results demonstrated that all biodegradable materials enhanced soil microbial activity in comparison with non-biodegradable PE film. PHB-based film showed the best results for soil microbial activity followed by the three starch-based mulches, PLA-based film, and non-biodegradable PE film. The authors pointed that PE mulch film does not increase soil microbial activity, like the other biodegradable plastic mulches, due to its innocuousness into the biological environment. These results are consistent with Sintim et al. (2021). BMDs have a positive impact on soil moisture according to Sun et al. (2018). The authors studied this effect with the mentioned six mulch treatments on a peanut crop at two different years. After 20 days of sowing, there are no significant differences in soil moisture content under the different mulch treatments. However, 120 days after sowing, soil moisture content for starch/PBAT mulches was lower than PE film, but higher than no mulching treatment. Other effects regarding soil properties like suppressing weed growth and improving nutrient availability were detailed in Section 3.

On the other hand, it is important to highlight comprehensive studies related to the impacts caused by BDMs on plant growth and development. In this direction, Gu et al. (2017) compared the effects of three different treatments: traditional PE mulch film, starch-based biodegradable film, and no film mulching (bare ground) on root growth and seed yield of rape (Brassica napus L.) at three sequential years. Regarding root growth, only at maturity, the differences were significant. The taproots in starch-based mulch film were longer than PE mulch film, but were shorter than no mulching film for the three seasons. In addition, starch-based and PE films gained larger taproot diameters at the surface and subsurface than non-covering material for the 3 years. In the same trend, the mass density of roots was higher for PE and starch mulches than bare soil for 0–20 cm of soil depth, but for 20–30 cm of soil depth was higher in bare ground than two mulch films at both maturity and seeding stages for the 3 years. Seed yield did not show significant differences between starch and PE mulches and were substantially higher than bare soil over the three seasons. In the work of Ghimire et al. (200), the authors showed similar results for PE-based film in comparison with BDMs related to dry root biomass, as well as the root-to-shoot ratio. Zhang et al. (2019) analyzed the effect of six mulch treatments (two PLA + PBAT mulches, two starch + PBAT mulches, PE film, and bare soil) on leaf nutrients over 2 years. The results obtained prove that there are no differences across treatments for leaf tissue nitrogen, potassium, and phosphorous content during the 2 years. Caruso et al. (2019) studied over 2 years the impact of four soil mulching types (Mater-Bi® black film, photoselective brown PE film, black PE film, and non-mulched control) on leaf quality of perennial wall rocket. They obtained better results with Mater-Bi® in relation to values of dry residue than other treatments. As regards values of mineral composition, biodegradable mulch film increased magnesium, sodium, and potassium concentrations in the leaves.

Comparatively, biodegradable and PE mulches reached quite similar results in the majority of properties analyzed, widely surpassing bare soil properties. Figure 5 summarizes the advantages of using polysaccharide-based BDMs in contrast with bare soil from the soil, plant, and mulch film perspectives.

Fig. 5
figure 5

Advantages of the usage of biodegradable mulch films in comparison with bare soil.

Full size image

In the last years, the research related to polysaccharide mulch films and their effects on different crop plants, soil quality, water management, and soil microbial activity has been intensified and specified investigations on materials used like biodegradable films, types of similar plants, and different locations covering several years. However, efforts should be made to carry out original studies related to the comparison of different mulch treatments and their impacts on agroecosystem organisms on the short and long term. Furthermore, significant scientific works could be addressed to study the effects of different additives and/or fillers and their chemical release from films to the agricultural system.

6 Conclusions and perspectives

This review covers the major requirements related to polysaccharides and bio-composites that were developed as formulations of biodegradable mulch films. Polysaccharide-based composites, as the matrix and/or fillers, showed that they are able to improve mechanical and biodegradability properties to comply with international standards and with harvesting times. To meet these multiple demands, researchers have applied different methodologies: surface modification, optimization of compatibility between polymer blends/composites, and/or nanostructured formulations. Unfortunately, despite the large numbers of scientific reports regarding mechanical and biodegradability behaviors, there exists a reduced number of water barrier properties and bio-composite biodegradable mulch films, making the topic very promising to study.

Another area that polysaccharide-based multiphase materials have been highlighted is the productivity and crop yield. Commercial and emerging biodegradable composite films have demonstrated to be efficient to achieve similar performance values and, in some cases, better than traditional PE mulch films for different crops. Beside of well-known associated effects, novel polysaccharide-based mulch films allow farming protection against pests through the introduction of bioactive agents in their formulations. In this sense, more knowledge is needed on the effects of the addition of organic active ingredients, on pest and disease management as well as on its release behavior and microclimate, cultivated plants, and soil modifications.

Despite the polysaccharide’s advantages, two main limitations are still present for use of them in commercial formulations: economic aspects and the time farmers need for adopting BDMs. In the medium/long term, one way to overcome these two problems is through economic grants and promotion of BDMs by the government and agricultural public entities/agencies. Simultaneously, BDM business sector could analyze different methodologies in order to reduce costs, through the development of emerging polysaccharide-based BDMs. Regarding this last issue, a high number of patents have been recently requested by different academic centers and private companies, which indicates the strong interest in this promising type of product.