Evaluation of Polyethylene Mulching and Sugarcane Cultivar on Energy Inputs and Greenhouse Gas Emissions for Ethanol Production in a Temperate Climate

: Fossil energy inputs and greenhouse gas (GHG) emissions associated with the cultivation and transport of sugarcane ( Saccharum o ﬃ cinarum ) for bioethanol production in Tanegashima, Japan, were estimated by life cycle assessment (LCA). The aim was to understand the e ﬀ ects of combined systems of polyethylene mulching treatment (mulching at planting and every ratooning, MM; mulching only at planting, MU; and untreated, i.e., no mulching at all, UU) and cultivar (a cold-tolerant genotype, NiTn18, and a conventional variety, NiF8). The mulch treatments and cultivars were combined to create six cultivation systems that were used to conduct a comparative assessment of cradle-to-gate energy inputs and emissions for bioethanol production. The LCA results showed that the energy inputs and GHG emissions resulting from the MM / NiF8 system were 6.29 MJ L − 1 and 0.500 kg CO 2 e L − 1 , which were 14% and 23% lower, respectively, than the corresponding values in the UU / NiF8 system. In contrast, the MU / NiF8 system increased the environmental loads slightly. The use of NiTn18 improved sugarcane performance and ethanol yields substantially as compared with NiF8, reducing energy inputs to 5.38, 5.24, and 5.55 MJ L − 1 and GHG emissions to 0.473, 0.450, and 0.441 kg CO 2 e L − 1 for the UU, MU, and MM treatments, respectively. The energy inputs and GHG emissions were similar among the systems, indicating that more ﬂexible mulching treatments might be acceptable in the NiTn18 systems than in the NiF8 systems. The energy inputs and GHG emissions resulting from the UU / NiTn18 system were 14% and 5% lower, respectively, than those of the MM / NiF8 system, suggesting that it may be possible to overcome the handicap of sugarcane production in cold conditions by breeding cold-tolerant cultivars.


Introduction
Because of increased concern about global warming and its impacts on human civilization, biofuels have attracted attention as a means to mitigate anthropogenic greenhouse gas (GHG) emissions caused

Field Experiment
To determine the stalk yield and yield-related components of sugarcane in different treatments of mulching and cultivars, three field experiments were conducted at the Kumage Branch (30 • Tables A1 and A2). At planting, two-bud sets of sugarcane cultivars NiF8 and NiTn18 were planted with a row space of 1.2 m and an inter-hill space of 0.3 m. The size of each plot consists of three rows of 5.1 m length (experiment 1) and four rows of 5.1 m length (experiments 2 and 3). The cultivation scheme was four times in 4 years: one plant cane and three ratoons. The soil is a well-drained soil of volcanic origin (Andosol), representative of the region. NiF8, a conventional cultivar, has been commonly grown in Tanegashima since its release in 1992 by the Tanegashima Experimental Station currently known as the Tanegashima Sugarcane Breeding Site, Kyushu Okinawa Agricultural Research Center, National Agriculture and Food Research Organization (NARO). NiTn18 is an alternative cultivar characterized by an improved yielding ability and cold tolerance during the period of early germination [28].
Except for the treatments, field practices followed the local recommendation for sugarcane production both on plant cane and ratoons [17]. Before planting, cattle manure (moisture content of 50% and 1.0% of N concentration were assumed) was applied at a rate of 20 Mg ha −1 and incorporated into the soil using a plow and harrow powered by a wheel tractor. Fertilizer was applied using bulk blending fertilizer mixing different types such as ammonium sulphate, ammonium phosphate and potassium chloride. At basal dressing, 70 kg ha −1 of N, 120 kg ha −1 of P 2 O 5 , and 60 kg ha −1 of K 2 O were applied; 80 kg ha −1 of N and 60 kg ha −1 of K 2 O were added as a top dressing. Weed management was a combination of herbicide application and mechanical weed control by inter-row tillage (hereafter, intertillage) powered by a wheel tractor. Pest control against major pests such as sugarcane wireworm (Melanotus spp.) and oriental chinchi bug (Cavelerius saccharivorus) followed the standard methods used in the region and included the application of two types of insecticides in the present study.

Treatments for Mulching
At planting, the field experiment was designed as a split-plot of two factors with cultivar being the main plot and mulch treatment being the sub plot replicated three times (experiment 1) and twice (experiments 2 and 3) (Table A1). Mulching treatment consists of three ways of applying mulch, (1) mulching at planting and every ratooning (MM), (2) mulching only at planting (MU), and (3) untreated, i.e., no mulching at all (UU) ( Table 1). At harvest, the crop was manually removed from one row of 4.5 m length (experiment 1) and two rows of 4.5 m length (experiments 2 and 3). A series of ratooning operations were conducted using a stubble shaver and a root cutting plow, details of which can be found elsewhere [25]. A 0.02 mm thick transparent film [17] was applied with a mulcher pulled by a hand tiller. The width of the film was Energies 2020, 13, 4369 4 of 17 0.45 m and 0.60 m for plant cane and ratoons, respectively. The applied polyethylene mulch was torn with a hand knife once the nursery cane grew enough beneath the film; it was then removed from the fields before intertillage prior to disposal.

Measurement and Statistical Analysis
Fresh-basis stalk yield (SY, Mg ha −1 ), brix in juice (BJ, %) and fiber yield (FY, Mg ha −1 ) were examined on harvest samples of both plant cane and ratoons. A shredder KS-MS (Matsuo Co., Ltd., Kagoshima, Japan) and a hydraulic press machine KS-OP (Matsuo Co., Ltd., Kagoshima, Japan) were used to prepare the samples for measurements, and BJ was determined with a Brix meter DBX-55 (Atago Co., Ltd., Tokyo, Japan). Total sugar (TS, Mg ha −1 ) and ethanol yield (kL ha −1 ) were estimated as follows [29][30][31]: where T c is a theoretical conversion efficiency from sugar to ethanol (51%), P c is a practical conversion efficiency (90%), and D e is the density of ethanol (0.806 kg L −1 [20 • C]). The SPSS statistical program (IBM) was used to analyze the results. Analysis of variance (ANOVA) was performed on SY, BJ, FY and TS, and effects of treatments were evaluated by mean comparisons with the Scheffé method. All of the significant results are referred to at the 5% level unless otherwise specified. Ethanol yield was treated as the key variable to link the field experiments and environmental assessment described in the next section.

Life Cycle Assessment
Data collected from the field experiment described in Section 2.1 were used to estimate fossil energy consumption and GHG emissions in the various cultivation systems.

System Description
The goal of the LCA was to estimate fossil energy consumption and GHG emissions in sugarcane cultivation systems to understand the effects of alternative treatments of mulching (i.e., treatments MM, MU, and UU) and cultivars (i.e., NiF8 and NiTn18). Six cultivation systems were generated to comparatively assess the cradle-to-gate energy inputs and emissions within the system boundary, with the functional unit being 1 L of anhydrous ethanol produced ( Figure 1). Energy inputs and GHG emissions associated with sugarcane production are also reported on an area (ha) basis following the common practice in agricultural research. Assuming that fuel consumption during harvest from the use of a self-propelled chopping harvester depended on SY (Mg ha −1 ), we calculated the diesel oil consumption for harvest, Dh (L ha −1 ): where Eh is the fuel efficiency of harvester (1.63 L Mg −1 of canes; [25]).

Life Cycle Inventory
We compiled LCIs of overall fossil energy inputs, material inputs, and GHG emissions associated with each sugarcane production process. The energy inputs and GHG emissions were estimated by multiplying fuel/material consumption by energy equivalents as well as emission factors, respectively. The energy equivalents and emission factors for fuel/material consumption were adopted from the Inventory Database for Environmental Analysis [32,33], which is frequently used in LCA studies in the field of agriculture in Japan (e.g., [25,34,35]). Diesel oil and gasoline consumption (L ha −1 ) for tractor/tiller-based field operations were computed on the basis of fuel efficiency (L hr −1 ) and work efficiency (hr ha −1 ) data collected from other field experiments and supplemented by other published results (e.g., [25]).
Assuming that fuel consumption during harvest from the use of a self-propelled chopping harvester depended on SY (Mg ha −1 ), we calculated the diesel oil consumption for harvest, D h (L ha −1 ): where E h is the fuel efficiency of harvester (1.63 L Mg −1 of canes; [25]). Assuming a 15 km one-way distance from the fields to the mill, diesel oil consumption in a round-trip truck transport (total 30 km) of sugarcane (D t , L ha −1 ) was calculated as follows: where E f is the fuel efficiency (3.5 km L −1 ) of a 10 Mg (load capacity) truck [36]. Diesel oil consumption for the transport of input materials (e.g., chemical fertilizers and polyethylene film) between the farmhouse and fields by truck (load capacity of 2.0 Mg) were also considered. The amount of chemical fertilizer, agrochemicals (e.g., herbicides and pesticides), and agricultural machinery were adopted primarily from the local recommendations for sugarcane production [17], supplemented by information obtained in interviews with sugarcane farmers and breeders. The amount of polyethylene film used for mulching (m 3 ha −1 ) was calculated on the basis of the length, width, and thickness of the material applied. The total length of the film was calculated to reflect a row space of 1.2 m (Section 2.1.1). The energy consumption and GHG emission amounts related to seed cane were taken from Nakashima and Ishikawa [25].
Soil-associated emissions of N 2 O resulting from the application of fertilizers and manure under aerobic conditions were also considered. The level of N inputs was 150 kg N ha −1 year −1 for chemical fertilizers, and the N concentration of the manure applied before planting was assumed to be 1.0% on a fresh matter basis. Emission factors for soil emissions were taken, as in the previous study [25], from the 2006 IPCC Guidelines for National Greenhouse Gas Inventories [37], except for direct N 2 O emissions from nitrification-denitrification processes. For readers' convenience, the emission factor is listed here to be 0.62% (the percentage of N 2 O-N out of the applied N) for direct N 2 O emission. This is the average value reported for upland soils in Japan [38,39]. Indirect emissions of N 2 O considered in the present study supposedly occurred through atmospheric deposition, N leaching, and run-off to water [37].

Life Cycle Impact Assessment and Sensitivity Analysis
Life cycle impact assessment (LCIA) was performed to evaluate potential environmental impacts from cultivation system involving mulching and cultivar. The inventory data were sorted and assigned to the two midpoint impact categories of cumulative fossil fuel demand and global warming; they were then multiplied by characterization factors to be aggregated in each group. The characterization factors of global warming were adopted from the CO 2 -equivalent global warming potential (GWP) for a 100-year time horizon. For example, the GWP 100 of N 2 O and CH 4 were reported to be 298 and 25 times greater than that of CO 2 , respectively [40]. After performing the LCIA, we conducted a sensitivity analysis to test the effects of variation in two parameters (number of ratoons and the direct N 2 O emission factor for mulched soil) on the comparative LCA results.

Stalk Yield and Yield-Related Components
The mean values of SY, BJ, FY, and TS are presented in Table 2. An interaction was observed in SY and TS between cultivar and mulching for the total (plant cane plus the three ratoons averaged on a yearly basis). Mulching treatments significantly improved SY and TS of NiF8, but the effect was less apparent with NiTn18. SY of NiF8 was the greatest in the MM plots, confirming that there were beneficial effects of mulching both at planting and ratooning. The plots treated with mulch at planting performed rather poorly in the following ratoon unless mulch was reapplied; this was especially true with NiF8, where SY was smaller in the MU plots on ratoons than it was in the UU plots ( Table 2). The under-performance of the crop in the MU plots might be an example of "stress imprint effects", a term explained by Bruce et al. [41] as "a genetic or biochemical modification of a plant that occurs after stress exposure that causes future responses to future stresses to be different". Even though a question arises about how long the priming effects could last, it is interesting that Bruce et al. [41] mentioned cases of seed priming and even transgenerational stress imprint effects. As to BJ and FY, no interaction was observed between cultivar and mulching ( Table 2). FY was significantly affected by cultivar and mulching, whereas although BJ did increase in the plots treated with mulch, none of the differences were statistically significant. With plant cane, Ebata et al. [12] also observed an increased BJ for the plots treated with mulch as compared to untreated plots at some locations in Kagoshima Prefecture, Japan. They attributed the increased brix of mulched plots to a greater share of early sprouted stalks to harvested stalks as compared to the share in untreated plots. Unlike SY and FY, it is likely that one cannot always expect significant and positive effects of mulching on BJ.

Energy Inputs and GHG Emissions from Fuel and Agricultural Materials
On-farm energy inputs during sugarcane cultivation and transportation totaled 7.77−11.6 GJ ha −1 year −1 (Table 3), and the corresponding GHG emissions were 0.561-0.837 Mg CO 2 e ha −1 year −1 ( Table 4). As is shown in Table 3, the greatest amount of the on-farm energy was required for the harvest (3.28−5.57 GJ ha −1 year −1 ), followed by transport of sugarcane to the mill (1.73−2.93 GJ ha −1 year −1 ); the sum of these two amounted to 64.0-73.6% of total on-farm energy consumption, indicating energy intensiveness of the processes. The rest of the energy was spent in tillage (0.812 GJ ha −1 year −1 ), intertillage (0.727 GJ ha −1 year −1 ), and ratooning operation (0.531 GJ ha −1 year −1 ), and the related GHG emissions were incurred correspondingly. Field operations for mulching required a relatively small amount of fuel energy consumption (0.0813 GJ ha −1 year −1 for MU and 0.325 GJ ha −1 year −1 for MM). However, it is obvious that the mulch treatments increased energy consumption during sugarcane harvest and transportation by increasing SY (Table 3).
Soil-associated N 2 O emissions owing to the application of fertilizers and manure were estimated to be 0.909 Mg CO 2 e ha −1 year −1 (Table 4), a comparable but slightly higher level than the on-farm GHG emissions from fuel energy consumption. When both the on-and off-farm processes and soil-associated emissions are accounted, energy inputs from fuels and agricultural materials totaled 32.0-41.4 GJ ha −1 year −1 and total GHG emissions were 2.85-3.29 Mg CO 2 e ha −1 year −1 . Table 4. Greenhouse gas emissions (Mg CO 2 eq. ha −1 year −1 ) estimated for the combined cultivation systems of three polyethylene mulch treatments and two cultivars. NiF8, a conventional cultivar; NiTn18, a cultivar characterized by improved yielding capacity and cold tolerance. MM, mulching at planting and every ratooning; MU, mulching only at planting; UU, untreated, i.e., no mulching at all. 1 Includes plowing and harrowing. 2 Includes furrowing, basal dressing, insecticide application, soil covering, and herbicide application to control sugarcane wireworm. 3 Includes two types of intertillage, one for ridge breaking with top dressing and the other for crop banking. 4 Includes stubble shaving and a root cutting.

Total Energy Inputs and Total GHG Emissions to Produce 1 L of Ethanol
In the sugarcane cultivation system where NiF8 was combined with treatment UU (hereafter referred to as the UU/NiF8 system), an estimated 4.39 kL ha −1 year −1 of ethanol could be generated from the harvested crop ( Figure 2). The ethanol yields were higher for all the other systems, but varied widely, ranging from 4.44 kL ha −1 year −1 in the MU/NiF8 system to 7.46 kL ha −1 year −1 in the MM/NiTn18 system.
Total energy inputs and total GHG emissions during sugarcane cultivation and transportation to produce 1 L of ethanol are presented in Figure 3a,b, respectively. In the UU/NiF8 system, 7.29 MJ of energy was input and 0.649 kg CO 2 e of GHG emission was incurred per 1 L ethanol produced. Total energy inputs and total GHG emissions from fuel and agricultural materials were considerably higher in the MM/NiF8 system, mostly for the harvest and transport of sugarcane as well as the synthesis of polyethylene film used for mulching (Tables 3 and 4), but the increase in ethanol yield was more pronounced (Figure 2). As a consequence, total energy inputs and total GHG emissions to produce 1 L of ethanol were 6.29 MJ L −1 and 0.500 kg CO 2 e L −1 ; these values are 14% and 23% lower, respectively, than those of the UU/NiF8 system (Figure 3a,b). This same result was not found for the MU/NiF8 system. Because of the poor performance of ratoons (see Section 3.1), the increase in ethanol yield was insufficient to compensate for the increased total energy inputs and GHG emissions from fuel and agricultural materials. The cradle-to-gate energy inputs and GHG emissions were very similar (7.48 MJ L −1 and 0.651 kg CO 2 e L −1 , respectively) to those of the UU/NiF8 system. The adoption of NiTn18 improved ethanol yields substantially for all three mulch treatments as compared with NiF8, by 45%, 53%, and 17% for treatment UU, MU, and MM, respectively ( Figure 2). This led to 5.38, 5.24, and 5.55 MJ L −1 of energy inputs and 0.473, 0.450, and 0.441 kg CO 2 e L −1 of GHG emissions in the respective UU/NiTn18, MU/NiTn18, and MM/NiTn18 systems (Figure 3a,b).
(R1, R2, R3) to facilitate the understanding of sensitivity analysis. Sugarcane cycle, PC-R1-R2 required less energy inputs for tillage and planting compared to PC-R1, implying some merits of taking ratoons twice instead of once. The performance of the third ratoon, however, was not good enough ( Table 2) to improve the energy performance of sugarcane cycle PC-R1-R2-R3 compared to PC-R1-R2 (Figure 4a). This is in line with observations often made with sugarcane where yield of ratoons tends to decline with time [3] (pp. [22][23]. Meanwhile, the comparative LCA results were valid, except for the case that the environmental advantage of the UU/NiF8 system over the MU/NiF8 system disappeared when the number of ratoons decreased to one or two as compared with the default value of three. The comparative LCA results for GHG emissions were sensitive to the direct N2O emission factor for mulched soil ( Figure 5). This issue is addressed in Section 3.4. Figure 2. Ethanol yields estimated for the combined cultivation systems of three polyethylene mulch treatments and two cultivars. MM, mulching at planting and every ratooning; MU, mulching only at planting; UU, untreated, i.e., no mulching at all. NiF8, a conventional cultivar; NiTn18, a cultivar characterized by improved yielding capacity and cold tolerance.
(a) Figure 2. Ethanol yields estimated for the combined cultivation systems of three polyethylene mulch treatments and two cultivars. MM, mulching at planting and every ratooning; MU, mulching only at planting; UU, untreated, i.e., no mulching at all. NiF8, a conventional cultivar; NiTn18, a cultivar characterized by improved yielding capacity and cold tolerance.
Energies 2020, 13, x FOR PEER REVIEW 10 of 17 (R1, R2, R3) to facilitate the understanding of sensitivity analysis. Sugarcane cycle, PC-R1-R2 required less energy inputs for tillage and planting compared to PC-R1, implying some merits of taking ratoons twice instead of once. The performance of the third ratoon, however, was not good enough ( Table 2) to improve the energy performance of sugarcane cycle PC-R1-R2-R3 compared to PC-R1-R2 (Figure 4a). This is in line with observations often made with sugarcane where yield of ratoons tends to decline with time [3] (pp. [22][23]. Meanwhile, the comparative LCA results were valid, except for the case that the environmental advantage of the UU/NiF8 system over the MU/NiF8 system disappeared when the number of ratoons decreased to one or two as compared with the default value of three. The comparative LCA results for GHG emissions were sensitive to the direct N2O emission factor for mulched soil ( Figure 5). This issue is addressed in Section 3.4.   The results of the sensitivity analysis illustrated that the energy inputs and GHG emissions from sugarcane production were lower with two ratoons than with one ratoon (Figure 4a,b), partly because tillage and planting were conducted only once in the first year owing to the ratooning of sugarcane. Here, sugarcane cycle is defined as a sequence of plant cane (PC) and first, second, and third ratoons (R1, R2, R3) to facilitate the understanding of sensitivity analysis. Sugarcane cycle, PC-R1-R2 required less energy inputs for tillage and planting compared to PC-R1, implying some merits of taking ratoons twice instead of once. The performance of the third ratoon, however, was not good enough ( Table 2) to improve the energy performance of sugarcane cycle PC-R1-R2-R3 compared to PC-R1-R2 (Figure 4a). This is in line with observations often made with sugarcane where yield of ratoons tends to decline with time [3] (pp. [22][23]. Meanwhile, the comparative LCA results were valid, except for the case that the environmental advantage of the UU/NiF8 system over the MU/NiF8 system disappeared when the number of ratoons decreased to one or two as compared with the default value of three. The comparative LCA results for GHG emissions were sensitive to the direct N 2 O emission factor for mulched soil ( Figure 5). This issue is addressed in Section 3.4. Figure 3. Total energy inputs (a) and total greenhouse gas (GHG) emissions (b) to produce 1 L of ethanol. MM, mulching at planting and every ratooning; MU, mulching only at planting; UU, untreated, i.e., no mulching at all. NiF8, a conventional cultivar; NiTn18, a cultivar characterized by improved yielding capacity and cold tolerance.

Comparison of Sugarcane Cultivation Systems and Other Perspectives
The comparative LCA of sugarcane cultivation and transportation for bioethanol production in Tanegashima, Japan, showed that the cradle-to-gate energy inputs and GHG emissions from the Figure 5. Sensitivity analysis of total greenhouse gas (GHG) emissions to the direct N 2 O emission factor from mulched soil. The emission factor is the percentage of N 2 O-N out of the applied N; 0.62% was the default emission factor value in the present study. MM, mulching at planting and every ratooning; MU, mulching only at planting; UU, untreated, i.e., no mulching at all. NiF8, a conventional cultivar; NiTn18, a cultivar characterized by improved yielding capacity and cold tolerance.

Comparison of Sugarcane Cultivation Systems and Other Perspectives
The comparative LCA of sugarcane cultivation and transportation for bioethanol production in Tanegashima, Japan, showed that the cradle-to-gate energy inputs and GHG emissions from the MU/NiF8 system were higher than those of the UU/NiF8 system (Figure 3a,b), although the differences were minor and more than offset by decreasing the number of ratoons to one or two (Figure 4a,b), as is frequently practiced in the study area [25]. In contrast, the energy inputs and GHG emissions resulting from the MM/NiF8 system were 14% and 23% lower, respectively, as compared with the UU/NiF8 system (Figure 3a,b), indicating that the local recommendation for mulching both at planting and ratooning is not only vital for sugarcane growth but also effective in reducing the required amount of energy and the corresponding GHG emissions from the entire process of sugarcane production. Only a small percentage ratoons are mulched in the region (15% of the sugarcane fields were mulched for ratoons [11]), so it is likely that the local recommendation for mulching is perceived as too costly for producers to implement without any support. Policymakers might need to create an economic incentive for producers so that they would more readily accept the local recommendation for mulching and thereby reduce both energy inputs and GHG emissions per unit of ethanol produced. To help policymakers design an effective policy measure, efforts have been made to extend this type of LCA to link it with other socio-economic modelling [42,43]. The cold-tolerant genotype (NiTn18) improved ethanol yields considerably as compared with NiF8 ( Figure 2), which also reduced the energy inputs and GHG emissions for ethanol production (Figure 3a,b). The improved ethanol yield was apparent with the UU and MU treatments ( Figure 2). As the result, total energy inputs and total GHG emissions to produce 1 L of ethanol were comparable among all three mulching systems with NiTn18 (Figure 3a,b), indicating that more flexible mulching treatments might be acceptable for NiTn18. Furthermore, the energy inputs and GHG emissions resulting from the UU/NiTn18 system were 14% and 5% lower, respectively, than those of the MM/NiF8 system (Figure 3a,b), suggesting that the handicap of sugarcane production in cold conditions could be overcome by using cold-tolerant cultivars. An attempt of obtaining traits related to excel early phase growth is often brought from wild species (Saccharum spontaneum) through breeding to overcome cold stress [26,27]. It should be noted, however, that this approach tends to be accompanied by unwelcoming traits of thinner stalks and poor defoliation [44]. The fact that NiF8 is still the most grown cultivar in Tanegashima [11] is showing the difficulty to develop an all-round cultivar that is characterized by excel early phase growth, thick stalk, and easy defoliation at once, the traits earned for by growers and sugar millers. Unlike thin stalks and poor defoliation that are hard to be controlled by cultivation techniques, one can make up for poor early phase growth to some extent by means of polyethylene mulching.
The sensitivity analysis showed that the GHG emissions from sugarcane production were sensitive to the direct N 2 O emission factor for mulched soil, with the parameter initially set at 0.62% (the percentage of N 2 O-N out of the applied N) ( Figure 5). The GHG emissions resulting from the MM/NiF8 system were even higher than those from the MU/NiF8 system at an emission factor of 1.8% and higher than the UU/NiF8 system when the emission factor exceeded 3.0%. In their recent review of direct N 2 O emission from mulched soil, Steinmetz et al. [45] stated, "under oxidising conditions, increased N 2 O fluxes were observed predominantly during and after solarisation and disinfection measures, or when the soil was fertilised substantially with inorganic nitrogen (300-1600 kg N ha −1 year −1 )" [46][47][48], but "plastic mulching for its original, yield-increasing purpose together with moderate fertilisation (<180 kg N ha −1 year −1 ), in contrast, mostly led to N 2 O emissions comparable to those of non-mulched soil" [49,50]. Apparently, polyethylene mulching for sugarcane production in Tanegashima, Japan, falls within the latter group. Polyethylene film is applied just after planting and/or ratooning during the cold season with moderate fertilization (150 kg N ha −1 year −1 ) to increase yields; it is then removed before intertillage in spring [17]. Nevertheless, monitoring N 2 O fluxes from the mulched and non-mulched soil for sugarcane production might be needed, coupled with an uncertainty assessment of other parameters, to improve the accuracy of the comparative LCA for GHG emissions from sugarcane production with and without polyethylene mulching. Assessing other impact categories (e.g., endpoint indicators related to microplastics in soils) and exploring the effects of alternative material use for mulching (e.g., biodegradable plastic) warrants further research to deepen and broaden our understanding of the feasibility of sugarcane-based bioethanol production in cold conditions. Author Contributions: Conceptualization, T.N. and S.I.; methodology, T.N., K.U. and E.F.; software, T.N. and S.I.; validation, T.N. and S.I.; formal analysis, T.N. and S.I.; data curation, K.U. and E.F.; writing-original draft preparation, T.N. and S.I.; writing-review and editing, T.N., S.I. and K.U.; visualization, T.N.; supervision, T.N. and S.I.; funding acquisition, T.N. and S.I. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the NARO (320c0) and the Japan Society for the Promotion of Science (Grant-in-Aid for Scientific Research C, No. 20K06267).