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Article

Understanding the Impact of Soil Characteristics and Field Management Strategies on the Degradation of a Sprayable, Biodegradable Polymeric Mulch

by
Cuyler K. Borrowman
1,
Raju Adhikari
2,*,
Kei Saito
1,
Stuart Gordon
3 and
Antonio F. Patti
1,*
1
School of Chemistry, Monash University, Clayton, VIC 3800, Australia
2
CSIRO Manufacturing, Clayton, VIC 3168, Australia
3
CSIRO Agriculture and Food Research Unit, Werribee, VIC 3030, Australia
*
Authors to whom correspondence should be addressed.
Agriculture 2024, 14(11), 2062; https://doi.org/10.3390/agriculture14112062
Submission received: 13 September 2024 / Revised: 8 November 2024 / Accepted: 11 November 2024 / Published: 15 November 2024
(This article belongs to the Special Issue Impact of Plastics on Agriculture)
Browse Figures
Versions Notes

Abstract

:
The use of non-degradable plastic mulch has become an essential agricultural practice for increasing crop yields, but continued use has led to contamination problems and in some cropping areas decreases in agricultural productivity. The subsequent emergence of biodegradable plastic mulches is a technological solution to these issues, so it is important to understand how different soil characteristics and field management strategies will affect the rate at which these new materials degrade in nature. In this work, a series of lab-scale hydrolytic degradation experiments were conducted to determine how different soil characteristics (type, pH, microbial community composition, and particle size) affected the degradation rate of a sprayable polyester–urethane–urea (PEUU) developed as a biodegradable mulch. The laboratory experiments were coupled with long-term, outdoor, soil degradation studies, carried out in Clayton, Victoria, to build a picture of important factors that can control the rate of PEUU degradation. It was found that temperature and acidity were the most important factors, with increasing temperature and decreasing pH leading to faster degradation. Other important factors affecting the rate of degradation were the composition of the soil microbial community, the mass loading of PEUU on soil, and the degree to which the PEUU was in contact with the soil.

1. Introduction

Plastic (mostly polyethylene) mulch (film) is widely used to conserve water, suppress weeds, and improve crop yields and product quality. However, plastic mulches have emerged as a major source of plastic waste and soil and water pollution due to their widespread use (estimated at 2.5 million tons/year in 2021) [1]; the fact they do not biodegrade; and their sometimes-ineffective removal and disposal from the field [2,3]. Problematically, for the foreseeable future, plastic mulches will continue to be applied to maintain and improve crop yields and conserve agricultural water usage [4,5,6,7,8]. This is despite the deleterious effects on soil health and soil biota [9,10,11] associated with their persistence in the field. With a projected increase in global food demand under increasing water stress [12,13,14], it is critical that effective and biodegradable mulching materials be developed and used.
The development of biodegradable polymers that can be extruded or sprayed into a film for application as an agricultural mulch represents emerging alternatives to non-degradable plastic mulches [15,16]. This is a relatively new application for a range of biodegradable polymers, e.g., thermoplastic starch, polylactic acid, polyhydroxyalkanoates, aliphatic–aromatic copolyesters, and others, that has drawn investment and interest from manufacturers and researchers around the world. The impetus for business is largely a result of administrative authorities worldwide, and particularly in the European Union, issuing stringent guidelines for the use of plastics across all industries [17]. It is worth noting that the term ‘biodegradable’ is not consistently applied in the marketplace, and its use can be misleading. A wide range of so-called ‘biodegradable’ plastics have been introduced to the market over the last 20 years, many of which have degradation cycles much longer than the 6- to 10-month interval that crop mulches are used for. Other drawbacks include, e.g., variable degradation rates that leave fragments in the soil for long periods of time to contaminate the soil [18,19] and poorer mechanical properties, which mean that the films have less resilience in the field and are prone to tearing or puncturing [19]. Additionally, biodegradable plastic films have different gas permeability and thermal properties than non-biodegradable films and will alter the microbial life of the soil they cover [20].
A number of studies have reported on the degradation of preformed biodegradable polymer films from different substrates and combinations of substrates with different formulation parameters [21,22]. It is understood that the higher the degree of crystallinity within a biodegradable polymer, the slower the rate of biodegradation, and that typically with increasing polymer chain length and branching, the biodegradation rate will decrease. The relationship of other important polymer characteristics with biodegradation rate have also been studied, including glass transition temperature (Tg) and melting temperature (Tm). These both relate to polymer chain flexibility and conformational freedom and ultimately are proxies for crystallinity. The consensus is lower Tg and/or Tm correlates with faster biodegradation [23,24]. These factors also have a relationship with water infiltration into the polymer network, which in turn facilitates abiotic hydrolysis reactions and substrate access for relevant enzymes.
There has also been an abundance of work published on different environmental (temperature, soil type), biotic (polymer degrading microbes and enzymes) and abiotic factors (hydrolysis at varying pH, oxidation) affecting biodegradation of these polymers [25,26]. Contrary to, perhaps, an intuitive assumption, increased temperature does not always result in an increased rate of biodegradation and in fact can decrease the rate of biodegradation [27]. However, in most cases, a higher temperature does provide for a higher rate of degradation [25,28], at least until a point where microbial activity is inhibited, wherein the rate of biodegradation reduces.
Acidity and alkalinity also play an important role in controlling the rate of biodegradation, and each polymer will react differently in different pH conditions. This is both because acidic and basic conditions can catalyse abiotic hydrolysis of hydrolysable moieties and because different pHs provide optimal (or suboptimal) conditions for biological (enzymatic) reactions [25,29].
Microbial action is often considered the most important factor controlling polymer biodegradation. This occurs through polymer surface colonisation, the excretion of enzymes (exoenzymes), which can break down a variety of polymeric moieties, and via the uptake of small oligomers. Many microbes native to the soil environment have been identified as biodegradable polymer degraders, but this varies between soil types, soil health with respect to organic matter, and the incumbent soil microbiome and polymer type. However, there are only a few studies that specifically investigate the relationship between soil type and polymer degradation [30,31]. Soils with a greater proportion of organic matter are more likely to accommodate favourable conditions for degradation due to their greater abundance of microbes, given that there is adequate water, available nutrients, and optimal temperature.
To date, there have been a handful of studies describing the development and efficacy of sprayable biodegradable mulches (SBMs) for agriculture. Giaccone et al. developed a sprayable mulch based on deacetylated chitosan mixed with polyglycerol and cellulosic fibres and studied its efficacy on weed suppression [32]. Sartore et al. developed and studied the efficacy of sprayable mulches based on protein hydrolysate (PH) blended with other biodegradable polymeric components (polyethylene glycol, poly (ethylene) vinyl acetate, lignin) [33,34]. Schettini et al. have carried out work developing sprayable mulches based on polysaccharides and pH and evaluated their efficacy and material properties [35,36]. There has been some work performed on the development of sprayable polysiloxane mulches and the evaluation of their efficacy (water conservation, enhancement on crop yield, suppression of weed growth), material properties, and effect on soil temperature [37,38]. Immirzi et al. developed a sodium alginate-based sprayable mulch and conducted a thorough investigation into its material properties [39].
Of these studies, only three (Sartore et al., 2016, 2018, and Immirzi et al.) [33,34,39] have published test results relating to polymer degradation. Sartore and colleagues performed polymer degradation testing in water and measured degradation by polymer mass loss alone, whilst Immirzi and colleagues performed a standard biodegradation test (ASTM D5988) [40] in which a polymer film is buried in soil and the CO2 evolved is measured.
Sprayable polymeric mulches and preformed (film) polymeric mulches have different interactions with the soil to which they are applied. Sprayable mulches derive much of their strength from their interaction with the soil and form physical (and perhaps chemical) composite interactions with the soil, which are absent with preformed polymeric mulches. These differences are likely to cause differences in degradation behaviour between the two types/forms of mulch. Paying heed to this soil–polymer interaction, and the large variety of soil types to which a sprayable biodegradable polymer could be applied, it was of interest to gain an understanding of the relative importance certain soil characteristics play in affecting degradation behaviour and rate.
In addition to the variety of different soils to which a sprayable polymer could be applied, application strategy, land management practice, and the presence of inclement weather all could play an important role in the rate of degradation of a sprayable biodegradable polymer. The same polymer applied at different mass loadings may degrade at different rates, and the soil microbial communities’ capacity to degrade a particular polymer structure across multiple applications may change. Also, whether a sprayable polymer film has its contact with the soil disturbed by inclement weather could impact the rate of degradation.
Adhikari et al. [41], with the Commonwealth Scientific and Industrial Research Organisation (CSIRO), have developed sprayable, degradable polyester–urethane–urea (henceforth referred to as PEUU) for use as an agricultural mulch. In previous work, we have studied its degradation on the soil surface under a variety of environmental conditions and observed variability in the rate of its degradation, predominantly due to the soil type to which the polymer was applied [42]. Based on the characteristics of those soils, three parameters stood out as the possible causes for the different rates of degradation: pH, soil organic matter (SOM) content, and differences in the microscopic shape the polymer setting took on each soil type. It was of interest to deconvolute the importance of each of these factors on PEUU degradation.
Here, through a series of controlled laboratory experiments, the first systematic study showing the impacts of soil pH, soil microbial community, and the microscopic form of the set polymer upon application to soil on the hydrolytic degradation of this sprayable, biodegradable polymer is presented. The impacts of additional factors, e.g., application mass loading, repeated applications, and soil–polymer contact disturbance, which might also influence degradation rates were investigated via long-term, outdoor trials.

2. Materials and Methods

2.1. Soil

Soil was obtained from three locations in Victoria, Australia: a grazing paddock in Seville (latitude (lat.) −37.798 and longitude (long.) 145.488); a well-tilled, active commercial tomato farm in Echuca (lat. −36.141 and long. 144.752), and an active, well-tilled wheat farm in Ouyen (lat. −35.071 and long. 142.315). These each represent very different geological histories and therefore soil types. Soil from each location was collected from the top 30 cm and was air-dried and sieved < 2 mm prior to being loaded in pots. Table 1 lists the classification and basic properties of each soil as per Borrowman et al. [42].

2.2. Polymer Mulch

The sprayable polyester–urethane–urea (PEUU) developed by Adhikari et al. [41] and applicable as a crop mulch was used in this study. The main constituents of the PEUU are polycaprolactone (PCL, >70 wt%) and isophorone diisocyanate (IPDI, 25 wt%). The rest of the PEUU is made up of dimethylol propionic acid (DMPA) and ethylene diamine (EDA) as a chain extender (Figure 1). The formulation was prepared as an aqueous suspension (20 wt% PEUU solids) with Methocel® as a biodegradable viscosity modifier and carbon black as a biodegradable pigmentation modifier.
Specimens of the PEUU were prepared by drop casting the suspension into a film on a polytetrafluoroethylene (PTFE) plate. The resulting film’s thickness was 1 mm, and it was cut into 10 mm × 50 mm strips, each with a weight of 116 ± 19 mg. It is noted that the thickness of the test strips was an order of magnitude thicker than other agricultural films, e.g., polyethylene films. Additionally, PEUU films were formed on each field-moistened soil (Table 1) and then were cut into strips with the same dimensions.

2.3. Incubation Media

(Sigma-Aldrich) and HCl (37%, Sigma, Melbourne, Australia) were used with ultra-high purity water to form pH-adjusted incubation media, of pH 9 and 5, respectively. Ultra-high purity water was used as pH 7 incubation media. Luria–Bertani (LB) broth (pH 7), prepared by Monash University School of Biological Sciences Media and Prep Services, was used as incubation media for degradation experiments using soil microbial extracts as inoculants. All media was sterilised by autoclaving (121 °C, 15 psi for 30 min).

2.4. Hydrolytic Degradation Experiments

PEUU films were placed in sterile vessels and incubated in a variety of different media (pH 5, 7, and 9, as described above) such that they could be destructively sampled in triplicate at four time points over a period of 60 days. PEUU films formed on PTFE plates were sterilised by UV irradiation for 10 min on each side. Films were then placed in pH 5, pH 7, and pH 9 solutions and stored at room temperature (23 °C) to determine the effect of pH on PEUU degradation, and an additional set of films were incubated in pH 7 solution at 40 °C to determine the importance of temperature on PEUU degradation.
To determine the impact on degradation of the shape the formed PEUU film takes when applied to different soils, films were formed on the three soil types (as per Table 1). The films were then removed from the soil and attached soil particles were gently removed via ultrasonication for 5 min and by manual agitation. The resulting film specimens were then air dried at room temperature and sterilised by exposure to UV irradiation (10 min of exposure to each side). Gel permeation chromatography (GPC) was conducted on the specimens before and after sonication, in addition to UV irradiation exposure to ensure there was no change in polymer Mw and Mn from hydrolytic reactions or UV-induced cross-linking. Scanning electron microscopy (SEM) was used to visualise the different morphologies the polymer films took when formed on different soils.
The effect of the soil microbial community on polymer degradation was determined by incubating sterilised (by UV irradiation as described above) PEUU films formed on PTFE in LB broth (pH 7) inoculated with soil microbial extracts from each soil (Table 1). Soil microbial extractions were performed using an adapted method originally described by Riis et al. [43]. In brief, soil was agitated in sterile pH 7 phosphate-buffered saline (PBS) for 10 min followed by 5 min of ultrasonication, and this process was repeated two additional times giving a total agitation time of 30 min and a total ultrasonication time of 15 min. Soil solutions were then left undisturbed for five minutes to allow the heavy particles (sand and coarse silt) to settle, and after the five-minute settling period, an aliquot of the supernatant was transferred to the appropriate vessel containing LB broth and polymer film. No centrifugation was performed to ensure the extraction captured both bacteria and fungi. To ensure sterility and adequate oxygenation of the incubation media, vessels were topped with a cotton plug soaked in 70% ethanol and loosely capped to slow evaporation. Vessels were stored at room temperature (23 °C) in an active fume hood. Additional ethanol was added to the cotton plugs three times weekly to ensure the cotton plug was always near saturation.

2.5. Long-Term Outdoor Degradation Experiments

To determine the effect on degradation of different PEUU mass loadings, repeated applications of the PEUU, and disturbance of the soil–PEUU contact, a long-term outdoor degradation study was carried out from 24 September 2018 to 3 February 2020. Average monthly temperature and total monthly rainfall are plotted in Figure 2. The study was conducted in soil pots (24 cm inner diameter, 23 cm depth) filled with 8 kg of Seville soil (Table 1). The pots were allowed to drain freely to prevent oversaturation during periods of heavy rainfall. Soil was brought to 65% of the soil’s experimentally determined field water-holding capacity, which was essentially the soil moisture in each pot after drainage, and provided soil conditions that allowed for a PEUU film to form in a consistent manner between experimental units, and then the PEUU was applied. Thereafter, the PEUU was allowed to degrade in an outdoor environment exposed to the natural weather in Clayton, Victoria. The PEUU treatments were as follows with treatment codes given in bold; treatments were replicated 5 times:
  • PEUU at zero days: Initial.
  • 0.5 kg PEUU m−2, sampled after 275 days: 0.5.
  • 1.0 kg PEUU m−2, sampled after 275 days: 1.0.
  • 1.0 kg PEUU m−2, disturbed 1 day after application via mechanical ripping and mixing into soil: Disturb.
  • 1.0 kg PEUU m−2, reapplied with 1.0 kg m−2 loading after 275 days. Sampled at 497 days: Reapply.
  • 1.0 kg PEUU m−2, mechanically tilled into the soil after 275 days of degradation and sampled at 497 days: Till.
Pots were watered until they were draining out of the bottom during periods when there was no rain in order to ensure soil was never dry and thus to allow for continued microbial activity. It should be noted that this additional watering was not performed to maintain a particular moisture level in soils, only to maintain a moist environment. Sampled PEUU films were characterised by GPC.

3. Characterisation

3.1. Gel Permeation Chromatography

Gel permeation chromatography was performed on a Shimadzu system (Kyoto, Japan) equipped with a CMB-20A controller system, an SIL-20A HT autosampler, an LC-20AT tandem pump system, a DGU-20A degasser unit, a CTO-20AC column oven, an RDI-10A refractive index detector, and 4× Waters Styragel (Milford, MA, USA) columns (HT2, HT3, HT4, and HT5, each 300 mm × 7.8 mm2, providing an effective molar mass range of 100–4 × 106). Samples were dissolved at a concentration of 1–2 mg mL−1 in dimethylacetamide (DMAc) containing 4.34 g L−1 LiBr. Columns were calibrated with low-dispersity polystyrene (PS) standards ranging from 575 to 3,242,000 g·mol−1. DMAc containing 4.34 g·L−1 LiBr was used as an eluent at a 1 mL·min−1 flow rate and 80 °C. Mn and Mw were evaluated using Shimadzu LC Solution software (LC-20A series).

3.2. Scanning Electron Microscopy (SEM)

SEM micrographs were obtained using the secondary electron detector in a ThermoScientific FEI Quanta 3D FEGSEM (Waltham, MA, USA). The SEM was operated under low vacuum imaging conditions to mitigate sample charging issues. Operating conditions were as follows: 6 nA beam current, 20 kV accelerating voltage, 50 Pa chamber pressure (ultra-pure H2O), ~5 mm working distance.

3.3. Statistical Analysis

Statistical analyses were performed in Microsoft Excel 2016, and IBM SPSS Statistics 25. Excel was used for data organisation and processing, preliminary clean-up, outliers testing (Grubbs’ test), normalisation, and visualisation. SPSS and excel was used for conducting single factor ANOVAs to determine the presence of statistical differences between treatment groups with significance level set at α ≤ 0.05, and post hoc testing was used to determine statistically different groups through pairwise comparisons using the Least Significant Difference (LSD) post hoc test.

4. Results and Discussion

4.1. Hydrolytic Degradation

Noted here is that the PEUU will degrade abiotically via hydrolysis of the repeating ester bonds in the PEUU’s PCL soft segment and the repeating urea and urethane moieties in the hard segment. According to the literature, it can be expected that the esters will hydrolyse an order of magnitude faster than urethanes and urea moieties, and urea groups will hydrolyse faster than urethane groups [44,45,46]. Given the preponderance of ester links (prevalent in 70 wt% of the PEUU, refer to Section 2) and their enhanced rate of hydrolysis relative to the urea and urethane moieties, it was assumed that these would hydrolyse in the greatest quantity, and especially so in the early stages of degradation. In previous work investigating the PEUU, we observed the formation of degradation products that resulted from hydrolysis of, primarily, ester groups in the PCL soft segment as well as the hydrolysis of urethane groups in the IPDI-rich hard segment [47]. In terms of biotic degradation, it is understood that fungi are the primary microbes responsible for degrading polyurethanes via excretion of ureases, esterases, and proteases [48,49]. The sum of these abiotic hydrolytic reactions and enzymatically catalysed hydrolytic reactions (where applicable) will be the primary cause of the reduction in the molecular weight of the PEUU.
As the polymer backbone is lysed, there will be an increasing abundance of carboxyl and amino groups, produced from the hydrolytic reactions of esters, ureas, and urethanes. These can be susceptible to enzymatic deamination [50] and decarboxylation [51], although these reactions will have only a minor effect on the PEUU molecular weight in comparison to the main chain scissions.
Soil pH across Australia varies greatly according to soil type, organic content, and weather [52]. Due to this variation, it is important to understand how pH impacts the degradation of the PEUU.
Single factor ANOVAs were conducted to evaluate differences between treatment groups at each time point, for each characterisation technique performed. Where statistically significant differences were identified (i.e., p < 0.05), Fishers Least Significant Difference (LSD) post hoc testing was performed to identify which specific groups were significantly different (i.e., where the absolute mean differences between treatment groups was greater than relevant LSD value).
It was found that the degradation rate of PEUU was increased in acidic conditions (Figure 3). Alkaline conditions slowed the rate of degradation compared to a neutral pH. Given the PEUU structure, it is most likely that the abiotic hydrolysis of the ester bonds was acid catalysed, while conversely the alkaline conditions had somewhat of a protective effect on abiotic hydrolysis. These differences were observed to be statistically significant (p = 0.0002), as characterised by the average molecular weight (Mw), from the first time point with the exception of the PEUU incubated in pH 7 and pH 9 conditions. At the second time point, all treatment groups were determined to be statistically significantly different. Dispersity (Đ) and polymer mass were not found to be statistically significantly different at any time point. While it appears that mass was lost at a greater rate for PEUU films incubated at 40 °C, there may not have been sufficient statistical power in the study to identify a statistically significant difference.
Evidently, temperature played an important role controlling the rate of PEUU breakdown (as per Figure 3), with PEUU films incubated at 40 °C showing the fastest rate of degradation.
There was little evidence of PEUU mass loss in all treatment conditions except for those incubated under elevated temperature (Figure 2). At 40 °C, the PEUU film had an Mw of 30 kDa and 15 kda after 28 and 60 days of degradation, respectively. These Mws correlated with a mass loss of ~5% and ~40% (which was not determined to be statistically significantly different), and so it can be surmised that PEUU oligomers are not small enough to become water soluble and diffuse away from the PEUU film until some threshold molecular weight less than 30 kDa but greater than 15 kDa is achieved.
By incubating PEUU film strips in a buffered nutrient broth inoculated with soil microbial extracts, the effect of the soil microbial community on PEUU degradation was demonstrated (Figure 4). After 60 days of incubation, there was no difference in the extent of PEUU degradation between any of the soil microbial extracts, and this was verified with a single factor ANOVA (p = 0.804). It is noted that any soil microbial extraction method does not extract the entire microbial community, but the method used here has been previously validated as highly effective compared to other methods [42]. Interestingly, the PEUU film incubated in the presence of the microbial extract from Echuca soil degraded at a faster rate over the first 28 days (though not statistically significantly so, p = 0.0562). It is possible this trend would have continued if the nutrient broth had been replaced throughout the study to ensure adequate nutrient availability to the microbes because it is likely that after 60 days, the microbial community had consumed most of the resources available in the nutrient broth. This would be an interesting follow-up study. Regardless of this, however, it can be concluded that the soil microbial community plays a role in controlling the rate of PEUU degradation, although the effect in this experimental situation was less, although not significantly less, than that of acidity (pH).
The mass of the PEUU film appears to have increased over the course of the degradation study (see Figure 3 and Figure 4), but this might be attributed to the colonisation of the PEUU film by microbes or perhaps adhesion of small, suspended soil particles.
The last group of hydrolytic experiments conducted were to determine the importance of the microscopic shape of the PEUU film on the rate of degradation. Soils will vary widely in their mineralogy and particle size distribution. It is commonly understood that clay particles are <0.002 mm in diameter, silt particles range between 0.002 mm and 0.06 mm diameter, and sand particles are greater than 0.06 mm in diameter [53]. The soils used here had large variation in the distribution of these three classes of particles (Table 1). The films formed on these different soils did have slightly different microscopic shape (Figure 5), but evidently that did not play a role in controlling the rate of PEUU degradation after 60 days incubation (i.e., results were not statistically significantly different, p = 0.568) (Figure 4). An unexpected finding here was the difference in PEUU molecular weight immediately after application (Figure 6, Mw and Mn). The molecular weight (Mw) of the PEUU formed on PTFE, Seville soil, Echuca soil, and Ouyen soil was 180 ± 4 kDa, 113 ± 4 kDa, 130 ± 1 kDa, and 123 ± 3 kDa, respectively, and this was determined to be statistically significant (p = 0.000054). A possible explanation for these differences is that upon application to the soil, the PEUU film is immediately hydrolysed to different extents.
Regardless, after 14 days of degradation, the PEUU film formed on each soil had degraded to the same extent, and over the 60-day study, the soil-formed PEUU films collectively degraded at a rate of 800 ± 140 Da day−1, which was slower than the rate of the PEUU film formed on PTFE (1300 ± 20 Da day−1). Some mass loss was observed, but this is more likely attributed to soil particles being freed from the PEUU matrix during degradation than actual PEUU film mass loss.

4.2. Long-Term Degradation

The impact of PEUU mass loading, repeated applications and a disturbance of the soil–polymer interface was investigated in an outside environment over a period of nearly 500 days (Figure 7). After 275 days of soil degradation, PEUU applied at 0.5 kg m−2 degraded more extensively than PEUU applied at 1.0 kg m−2 (4500 ± 1000 Da and 25,500 ± 8400 Da, respectively). It took twice as long for the 1.0 kg m−2 application to degrade to the same extent as the 0.5 kg m−2 loading, and this was an expected finding. An unexpected finding was that the PEUU film, when mechanically disturbed immediately after curing (Disturb), degraded faster than the PEUU left undisturbed on the soil surface (Disturb vs. 1.0, Figure 7). It was hypothesised that disturbing the soil–polymer interface would slow the colonisation of the PEUU by soil microbes, but evidently that was not the case. The disturbed PEUU film had two surfaces directly exposed to the soil medium, which could explain the faster degradation rate.
The molecular weight of tilled PEUU (Till), that is PEUU which was treated in the same manner as 1.0 kg m−2 during the first 275 days of degradation and was then thoroughly mixed through the soil to further degrade, was reduced by the same amount as the 0.5 kg m−2 PEUU.
It is interesting that the reapplied PEUU degraded to a greater extent in a shorter time than PEUU applied to previously unmulched soil (Reapply vs. 1.0, Figure 7) despite exposure to similar temperature and rainfall conditions (Figure 2). This finding suggests some kind of priming effect, where the soil’s capacity to degrade the PEUU increased due to its previous presence in the soil.

5. Conclusions

A series of laboratory-based hydrolytic experiments, complemented with outdoor soil degradation studies, have revealed several important factors controlling the degradation rate of a sprayable, biodegradable PEUU mulch. It was determined that temperature had the largest effect on the rate of hydrolytic degradation, followed by pH (acidity), with increasingly acidic conditions yielding faster degradation rates.
The soil microbial community composition affected the rate of PEUU degradation, but this effect was limited. Follow-up studies are necessary to properly understand the importance of nutrient availability on microbial degradation of PEUU and the many interactions associated with temperature, moisture, soil type (physical and chemical) and organic matter. The shape or profile of the PEUU film when applied and set onto the surface of different soils had no effect on its rate of degradation.
The PEUU mass loading was a significant factor controlling the rate of degradation, with a greater mass of PEUU leading to slower degradation. Degradation was faster when the PEUU was mixed through the soil rather than applied undisturbed on the soil surface. This difference was likely caused by the greater contact between the PEUU and soil and the additional PEUU surface contact created by mechanical tearing. This could be important for PEUU applications in the field where environmental factors, e.g., gusting winds, heavy precipitation and soil ped size, will affect the PEUU–soil contact points to increase its rate of degradation.
The factors studied here can be used to help inform application rates and to predict the rate at which the PEUU will degrade in different environments using information easily available to a grower (soil pH, seasonal temperatures). Further studies incorporating a wider variety of soil types and better resolution of the investigated environmental factors (i.e., investigating pH at more and smaller intervals, investigating a larger range of degradation temperatures) could be used to develop a simple, practical model for use by growers. Synergistic effects between these factors were not investigated, and this would be an important area for further study.

Author Contributions

Conceptualization, C.K.B., R.A., A.F.P. and K.S.; Methodology, R.A., A.F.P. and C.K.B.; Investigation, C.K.B.; Writing—original draft, C.K.B.; Writing—review and editing, R.A., A.F.P. and S.G.; Funding acquisition, C.K.B. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by a Monash Graduate Scholarship and Monash International Postgraduate Research Scholarship. It was also supported by funding from the Commonwealth Scientific and Industrial Research Organisation.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The datasets generated for this study are available upon request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Representative structure of the PEUU.
Figure 1. Representative structure of the PEUU.
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Figure 2. Average monthly temperature and total monthly rainfall over the study’s duration. Error bars are ±1 standard deviation.
Figure 2. Average monthly temperature and total monthly rainfall over the study’s duration. Error bars are ±1 standard deviation.
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Figure 3. The effect of pH on PEUU degradation. Incubations carried out at 23 °C unless otherwise specified. Lines are shown only to guide the eye and do not represent lines of best fit. Error bars are ± one standard deviation.
Figure 3. The effect of pH on PEUU degradation. Incubations carried out at 23 °C unless otherwise specified. Lines are shown only to guide the eye and do not represent lines of best fit. Error bars are ± one standard deviation.
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Figure 4. The effect of the soil microbial community on PEUU degradation. Lines are shown only to guide the eye and do not represent lines of best fit. Error bars are ± one standard deviation.
Figure 4. The effect of the soil microbial community on PEUU degradation. Lines are shown only to guide the eye and do not represent lines of best fit. Error bars are ± one standard deviation.
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Figure 5. SEM micrographs of the PEU film formed on each soil type.
Figure 5. SEM micrographs of the PEU film formed on each soil type.
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Figure 6. The effect of PEUU film surface shape/profile on degradation. Lines are shown only to guide the eye and do not represent lines of best fit. Error bars are ± one standard deviation.
Figure 6. The effect of PEUU film surface shape/profile on degradation. Lines are shown only to guide the eye and do not represent lines of best fit. Error bars are ± one standard deviation.
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Figure 7. Molecular weight of PEUU after outdoor degradation under a number of different degradation scenarios. Letters above columns indicate statistically different means with a significance level set at α ≤ 0.05, as determined by ANOVA.
Figure 7. Molecular weight of PEUU after outdoor degradation under a number of different degradation scenarios. Letters above columns indicate statistically different means with a significance level set at α ≤ 0.05, as determined by ANOVA.
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Table 1. Soil characteristics.
Table 1. Soil characteristics.
SevilleEchucaOuyen
Soil Classification DermosolVertosolTenosol
Electrical Conductivity, dS/m 0.430.16560.06164
pH 5.537.016.87
% Organic Matter 6.72.00.2
C:N 17.869.182.97
Sand, % 56.431.696.1
Silt, % 28.510.80.2
Clay, % 8.455.63.5
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Borrowman, C.K.; Adhikari, R.; Saito, K.; Gordon, S.; Patti, A.F. Understanding the Impact of Soil Characteristics and Field Management Strategies on the Degradation of a Sprayable, Biodegradable Polymeric Mulch. Agriculture 2024, 14, 2062. https://doi.org/10.3390/agriculture14112062

AMA Style

Borrowman CK, Adhikari R, Saito K, Gordon S, Patti AF. Understanding the Impact of Soil Characteristics and Field Management Strategies on the Degradation of a Sprayable, Biodegradable Polymeric Mulch. Agriculture. 2024; 14(11):2062. https://doi.org/10.3390/agriculture14112062

Chicago/Turabian Style

Borrowman, Cuyler K., Raju Adhikari, Kei Saito, Stuart Gordon, and Antonio F. Patti. 2024. "Understanding the Impact of Soil Characteristics and Field Management Strategies on the Degradation of a Sprayable, Biodegradable Polymeric Mulch" Agriculture 14, no. 11: 2062. https://doi.org/10.3390/agriculture14112062

APA Style

Borrowman, C. K., Adhikari, R., Saito, K., Gordon, S., & Patti, A. F. (2024). Understanding the Impact of Soil Characteristics and Field Management Strategies on the Degradation of a Sprayable, Biodegradable Polymeric Mulch. Agriculture, 14(11), 2062. https://doi.org/10.3390/agriculture14112062

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