Initial Experimental Experience with a Sprayable Biodegradable Polymer Membrane (SBPM) Technology in Cotton

: Preformed biodegradable and next generation sprayable biodegradable polymer membrane (SBPM) formulations, which biodegrade to non-harmful products (water, carbon dioxide and microbial biomass), have been introduced as an alternative to plastic mulch ﬁlms in order to mitigate plastic pollution of the environment. In this preliminary ﬁeld study on cotton ( Gossypium hirsutum L.), a novel SBPM technology was compared to preformed slotted oxo-degradable plastic (ODP) mulch ﬁlm and no mulch control (CON) in terms of yield, crop water productivity (CWP), and soil temperature. The ﬁrst results showed higher CWP and crop yield, and increased soil water content under the SBPM cover. This study indicates that SBPM technology could perform at similar level as ODP or comparable ﬁlms under ﬁeld conditions and, at the same time, provide environmentally sustainable agricultural cropping practices. Additionally, the fully treated, non-replicated SBPM plot had a wetter soil proﬁle throughout the entire crop season. This innovative technology has shown a high potential even at this early stage of development, indicating that advances in formulation and further testing can lead to signiﬁcant improvements and thus increased use in crop production systems.


Introduction
As the world population continues to increase, the production of food and fibre will need to increase using the same (or reduced) area of agricultural land, but with less water [1]. One of the ways to increase crop productivity is to minimise water losses by soil evaporation and crop transpiration [2], which can be achieved in the field by using various mulch surface covers and/or with improved irrigation technologies. Plastic mulch films used in crop production help to control pests, increase soil and air temperature, reduce soil evaporation, minimize soil erosion, and prevent soil particles attaching to fruits or vegetables [3]. However, a major and continuing problem with the use of plastic mulch films is disposal and non-biodegradability issues [4]. Consequently, the expanding use of plastics has led to environmental pollution, which will have long-term consequences on soil quality, the environment, and possibly human health [5]. The newer oxo-degradable mulch films reduce the amount of plastic mulch currently being disposed in landfills, but are still essentially non-biodegradable [6]. The result is that they break down to smaller fragments, causing even more environmental damage [7]. Biodegradable plastics are a promising solution, but the release of microand nanoparticles from biodegradable plastic upon degradation requires long term field trials in order to confirm that either complete biodegradation occurs, or that no long-term harm to the environment is caused [8].
Recently, a biodegradable spray-on mulch based on sodium alginate was developed as a potential alternative to the thin plastic mulch films currently used in horticulture [9]. Polymeric protein-based biodegradable spray coatings were tested in greenhouse trials [10] and exhibited agronomic performances comparable to commercial low-density polyethylene mulch film, indicating a similar rate of plant growth and dry matter accumulation and a complete biodegradation (<5% residues after 2 months). Similar sprayable polymer formulations (for small scale handheld sprayers and large-scale mechanised boom sprays) designed to biodegrade have been reported previously [11,12]. An increase in crop water productivity of 20%-30% for rockmelons under drip irrigation was demonstrated using a sprayable biodegradable polymer membrane (SBPM) compared to the bare soil [13]. The use of an additional viscosity modifier reduced soil wicking (polymer adsorption into the soil) by 10%-90% without compromising the system's sprayability or the general mechanical properties of the membrane, which were similar to those of the unmodified SBPM. Soil evaporation was reduced by more than 60% at a low SBPM application rate [14]. The beneficial effect of SBPM on the soil water regime in terms of the restriction of soil evaporation was even more pronounced when the polymer application rate increased (up to 1 kg m −2 ) [15].
Using the same SBPM formulation [15], a field study was conducted to determine whether SBPM applied to irrigated cotton would be effective in conserving soil profile water and improving crop water productivity and yield. This research was conducted to give important small-scale insights, before expanding and upscaling to full field scale research. The aim was to compare SBPM with oxo-degradable plastic (ODP) mulch film and with no mulch control (CON) in terms of the soil water content, soil temperature, cotton yield, and cotton quality.

Experimental Field Set-Up
Field experiments were conducted during the 2014/2015 season at the Australian Cotton Research Institute (ACRI), Narrabri (149 • 40 E, 30 • 10 S), New South Wales, Australia. The soil type was classified as a grey self-mulching Vertosol with 59% clay, 28% silt, a pH of 7.27 (0.01 M CaCl 2 ) and 0.77% Organic Carbon on average [16]. Nitrogen fertiliser was applied as urea at the rate of 180 kg N ha −1 prior to planting. Weeds and insects were managed as per Bollgard ® II protocol and plots were irrigated according to the station's schedule when an approximate soil profile water deficit of 70-80 mm was detected. Cotton (Gossypium hirsutum L. cultivar Sicot 74 BRF, germination percentage 96%) was sown (10 seeds per m of row) on 26 October 2015.
Three treatments, laid out as a completely randomised block design with three replicates, were tested: a thin slotted oxo-degradable polyethylene clear plastic (ODP) film, a new spray-on biodegradable polymer membrane (SBPM_gap) was applied on both sides of the plant row using a handheld pressure sprayer (Figure 1), and no mulch control (CON). The sprayable polymer formulation contained 20 wt% polymer content with 100-500 µm particle size and viscosity in the range of 50-100 mPa s. The polymer formulation that showed the best combination of film formation, water barrier and mechanical properties in the previous pot trial [13] was selected for the field trial. An unsprayed area (12 cm) between the spray lines was left to ensure that cotton seedlings would emerge. In one additional plot, SBPM was diluted by half to 0.5 L m −2 and applied over the plant line to determine whether cotton would emerge through the polymer membrane (SBPM_full). An additional plot with spray-on SBPM applied over the plant line (SBPM_full) was not replicated (due to the limited amount of available SBPM) and therefore was not included in the statistical analysis.

Field Sensor Installation and Crop Data Collection
Sensors were installed prior to applying the preformed film and the spray-on polymer. The soil profile water content and seedbed soil temperature were monitored below the plant line in each plot, respectively, using multi-depth soil capacitance sensors (at 10, 20, 40, 60 and 80 cm soil depth, Odyssey, http://odysseydatarecording.com/), and with thermistor probes (at 10 cm depth, Tinytag provided by Hastings Data Loggers, www.hdl.com.au). Data logging was commenced two days after the installation to allow soil moisture to re-equilibrate. Crop water consumption (CWC, mm) was determined using a simple mass balance expression: initial profile water (IPW) + irrigation (IR) + incrop rainfall (R) -final profile water (FPW) (i.e., CWC = (IPW + IR + R) -FPW) and crop water productivity (CWP) (kg lint mm −1 ha −1 ) = lint yield (kg ha −1 ) / (CWC -100 mm). The threshold of 100 mm was used to account for runoff, evaporation, and plant growth at the early growing stage, as proposed in previous studies under the same climatic conditions [17,18]. This approach was recommended given the local experience in New South Wales and Queensland cultivation systems, where 60%-80% of rainfall is lost as runoff and evaporation during the initial crop growing stages.
Crop establishment was determined by counting the number of plants when no further seedlings had emerged at 51 days after sowing (DAS), and crop height was determined by measuring the height of all plants 122 DAS, when no further increase in height was observed. Crop yield was determined by picking all cotton bolls at maturity and the fibre quality was assessed using a Hi Value Instrument (HVI -Uster Technologies and https://www.cottoninfo.com.au/fibre-quality). Field crop quality parameters (CWP, yield, plant establishment, and fibre quality) were analysed by the ANOVA procedure at a level of significance of P < 0.05 using Genstat 16 software [19].

Results and Discussion
The preliminary results of this SBPM study indicate that there was no statistically significant difference between the SBPM_gap treatment and ODP or CON for almost all the tested parameters. However, with respect to water consumption and crop water productivity, the plot with the SBPM_gap had a tendency for higher crop water productivity (+12% and +8%) compared to the ODP film treatment and CON (Table 1). Cotton grown on the SBPM_gap treatment plot used 13 and 23 mm less water and produced 0.4 and 0.6 kg mm −1 ha −1 more lint compared with the CON and ODP film, respectively; a similar result to that achieved was reported in other studies using thin polyethylene (PE) film [20]. Using the same SBPM product, researchers reported a reduced soil evaporation rate of 10%-50% (in pot experiments) and a larger crop productivity (in rockmelons) of up to 30% [13]. Plant establishment (51 days) was within the target range of 8-12 plants m −1 , with a

Field Sensor Installation and Crop Data Collection
Sensors were installed prior to applying the preformed film and the spray-on polymer. The soil profile water content and seedbed soil temperature were monitored below the plant line in each plot, respectively, using multi-depth soil capacitance sensors (at 10, 20, 40, 60 and 80 cm soil depth, Odyssey, http://odysseydatarecording.com/), and with thermistor probes (at 10 cm depth, Tinytag provided by Hastings Data Loggers, www.hdl.com.au). Data logging was commenced two days after the installation to allow soil moisture to re-equilibrate. Crop water consumption (CWC, mm) was determined using a simple mass balance expression: initial profile water (IPW) + irrigation (IR) + in-crop rainfall (R) − final profile water (FPW) (i.e., CWC = (IPW + IR + R) − FPW) and crop water productivity (CWP) (kg lint mm −1 ha −1 ) = lint yield (kg ha −1 ) / (CWC − 100 mm). The threshold of 100 mm was used to account for runoff, evaporation, and plant growth at the early growing stage, as proposed in previous studies under the same climatic conditions [17,18]. This approach was recommended given the local experience in New South Wales and Queensland cultivation systems, where 60%-80% of rainfall is lost as runoff and evaporation during the initial crop growing stages.
Crop establishment was determined by counting the number of plants when no further seedlings had emerged at 51 days after sowing (DAS), and crop height was determined by measuring the height of all plants 122 DAS, when no further increase in height was observed. Crop yield was determined by picking all cotton bolls at maturity and the fibre quality was assessed using a Hi Value Instrument (HVI -Uster Technologies and https://www.cottoninfo.com.au/fibre-quality). Field crop quality parameters (CWP, yield, plant establishment, and fibre quality) were analysed by the ANOVA procedure at a level of significance of p < 0.05 using Genstat 16 software [19].

Results and Discussion
The preliminary results of this SBPM study indicate that there was no statistically significant difference between the SBPM_gap treatment and ODP or CON for almost all the tested parameters. However, with respect to water consumption and crop water productivity, the plot with the SBPM_gap had a tendency for higher crop water productivity (+12% and +8%) compared to the ODP film treatment and CON (Table 1). Cotton grown on the SBPM_gap treatment plot used 13 and 23 mm less water and produced 0.4 and 0.6 kg mm −1 ha −1 more lint compared with the CON and ODP film, Agronomy 2020, 10, 584 4 of 8 respectively; a similar result to that achieved was reported in other studies using thin polyethylene (PE) film [20]. Using the same SBPM product, researchers reported a reduced soil evaporation rate of 10%-50% (in pot experiments) and a larger crop productivity (in rockmelons) of up to 30% [13]. Plant establishment (51 days) was within the target range of 8-12 plants m −1 , with a significantly lower establishment on the CON (Table 2). This might be the result of slightly elevated soil temperature in treated plots compared to the no mulch plot at the initial growing stages (e.g., up to 51 DAS; Figure 2). In this study, we did not find significant differences among the treatments in lint yield or fibre quality (Table 2). However, the lint yield was higher by 5%-7% in the SBPM_gap treatment, showing that this new product may have potential benefits for cotton yield. Similar novel polymeric protein-based biocomposites applied using the spray technique resulted in adequate agronomic performance (plant growth and dry matter content) [10]. Table 1. The comparison of crop water consumption and crop water productivity of cotton grown on plots treated with sprayable biodegradable polymer membrane (SBPM), oxo-degradable plastic (ODP) mulch film, and a control plot with no mulch (CON) during the 2015/16 season at the ACRI field site (n = 9 plots).

Treatment
Crop  For the seedbed temperature measurements (at 10 cm), an additional plot with spray-on SBPM applied over the plant line (SBPM_full) was also included. Seedbed temperatures were slightly elevated under the ODP film and SBPM early in the season, a similar response to that observed in previous studies using thin plastic mulch film, which enables earlier planting or planting in cooler regions in Australia [21]. The SBPM_gap increased soil temperature compared to the control, but only between 21 and 38 days. It was observed that the soil temperature was lower in the treatments that took place later in the season, which was probably the result of increased soil moisture during the treatments. Similar results have been reported for cotton grown under PE film when compared with planting on non-mulched soil [6]. These studies used either non-degradable plastic or oxo-degradable plastic films which, contrary to the SBPM technology, could pose a threat to the environment in terms of the retrieval of the plastic film and soil and water pollution [22]. This preliminary study looked at the potential to incorporate SBPM technology into the existing Australian cotton (and similar) production systems where cotton is mechanically planted on raised beds and irrigated with flood-furrow irrigation. Implementing SBPM to conserve soil moisture may improve crop water productivity, with the ease of spray application and no environmental pollution or disposal costs [11,12]. The SBPM_gap treatment showed that it may have the potential to improve CWP, but the application rate and coverage width still need to be optimized to minimize production costs, while enhancing growing benefits. Research Agronomy 2020, 10, 584 5 of 8 is ongoing to achieve this result. As the plant line had to be left uncovered for the seeds to emerge, the SBPM_gap application technique probably limited the full agronomic potential of SBPM cover.   Additionally, it was noted that in the (non-replicated) SBPM_full treatment, where the SBPM covered the plant line, the soil profile remained wetter (15-20 mm) throughout the season (Figure 3). This result is highlighted in Figure 3b, where water content in the soil profile under the SBPM_full treatment was wetter than the control plot. It is important to note that the soil water content in the SBPM_full treatment was higher from the start of the experiment, which also may indicate different soil hydraulic properties (water retention capacity) or different initial soil water status. However, both SBPM treatments (_gap and _full) indicated higher moisture content at various soil depths (not shown) and during multiple measurements taken throughout the growing season, which was probably associated with the presence of the SBPM layer at the soil-atmosphere interface (which limited soil evaporation). It was also evident that the gaps above the plant line in the slotted ODP film contributed to water loss from the soil. Both the slotted film and SBPM treatments did not impede infiltration of rainfall or irrigation water, which suggests that the technology could be used to harvest water under rain-fed conditions in a similar manner to the plastic mulch [23]. Previous research on using thin plastic mulch film in cotton was largely focussed on developing agricultural farming systems to extend cotton production to drier regions where mulch is used to harvest and conserve meagre rainfall and water under drip or saline water irrigation [6,24]. Four types of biodegradable plastic were tested as alternatives to polyethylene mulch and they all showed similar influence on soil temperature and water content, indicating that biodegradable products present viable replacement options [8]. However, this information should be treated with caution, as some authors [3] have raised concerns due to possible micro-and nano particle residues in the soil. The new SBPM technology was declared to be completely in line with the strict biodegradability and nontoxicity standards [12,13]. However, it needs to be stressed that the expectations of creating a biodegradable product which has similar properties and as low production costs as plastic mulch may be somewhat challenging (at least in its initial development phase). Nevertheless, significant improvements are currently being made with SBPMs and similar technologies in order to overcome these and any other issues [25].

Conclusions
This preliminary field study shows that a novel sprayable biodegradable polymer membrane (SBPM) technology has the potential to replace preformed slotted oxo-degradable plastic (ODP) mulch films by providing a similar performance in terms of yield, crop water productivity (CWP), and soil temperature. The field testing of the new SBPM technology was limited by the availability of the product, which is still under development. A small-scale field-testing phase was undertaken to validate the agronomic performance of the new technology in order to move on to a larger-scale experimental trial. Nonetheless, results highlighted a tendency to achieve higher CWP and crop yield, increased soil temperature at the initial cotton growing stage, and increased soil water content with the SBPM cover. New large-scale field trials should focus on the application of improved SBPM formulations and their management. The SBPM technology is a promising solution with which to replace the vast amount of plastics that originate from crop production, due to its agronomic performance in comparison with plastic mulch films, simplicity of application and biodegradability into non-toxic compounds.