Additional Greenhouse Gas Emissions and Offset Potential from Sugarcane Straw Recovery for Bioenergy Production in Southern Brazil

This study was designed to assess the additional greenhouse (GHG) emissions including measurements of soil CO 2 -C emissions in sugarcane areas plus emissions associated with the recovery and transport operations of straw bales up to the factory gate for electricity production, contrasting with leaving all straw on the soil surface. The total additional GHG emissions considering the main sources evaluated ( i.e ., soil CO 2 -C, diesel use and N 2 O from straw), was estimated at 1,465 kg CO 2 eq ha -1 , resulting an emission factor of 212. 6 kg CO 2 eq ton -1 of straw recovered. Applying the parameters cited in this study for electricity or 2G ethanol production (GHG balance of emission and offset potential), our results showed that straw-based for electricity production would result in additional GHG emissions of (+) 860 kg CO 2 eq ha -1 . In contrast, applying the same study parameters for 2G ethanol production replacing gasoline, an avoided GHG emission of (-) 2,316 kg CO 2 eq ha -1 could be achieved. The route of recovering around 27% of sugarcane straw through bale system for bioelectricity production using technical parameters and industrial eciency rate of this study could not be a sustainable option because the additional emissions can be higher than its potential to offset generated emissions, based on the emission factor of Brazilian energy matrix. Applying the same experimental parameters, the option of producing ethanol 2G with around 6.89 ton DM ha -1 of sugarcane straw could offset gasoline GHG emissions, contributing with sustainable energy systems.


Introduction
Inventorying greenhouse gas (GHG) balance related to sugarcane (Saccharum o cinarum) based byproducts is critical to assess the degree of carbon (C) neutrality of biofuels production [1] and, the advances to better support renewable energy source and the sustainability parameters, including GHG balance of these bioenergy sources should be better investigated to support public policies. The latest effort worldwide comes in the form of Law 13.576/2017 for the fostering of biofuels in Brazil, which creates the National Biofuel Policy, o cially known as RenovaBio Program, which will create a controlled market of decarbonization credits (known as CBios). However, the innovation in such mechanisms lies in the fact that the amount of CBios which an entity may emit in the Brazilian stock market will be linked to the reduction in GHG emissions associated with the production of a given biofuel in comparison to its fossil competitor [2], as investigated in this study.
Several acts and standards have been developed in different countries including rigorous requirements for biofuel producers to con rm the environment-friendly characteristics and ability to effectively reduce GHG emissions in comparison to the corresponding fossil fuels [2]. Towards this goal, the National Biofuel Policy, o cially known as RenovaBio Program comes in the form of Law 13.576/2017 for the progress of biofuels in Brazil. Bioenergy production could potentially mitigate GHG emissions and climate change by avoiding the combustion of fossil energy, therefore safeguarding energy security. However, there are issues that need to be considered, including the sustainability of practices and the e ciency of bioenergy systems [3]. The increased attention on bioenergy occurs in conjunction with the global interest in transitioning from large scale dependence upon fossil fuels to the use of renewable energy sources to curb GHG emissions [4]. Research evidence suggests that some already available and well-established options with low lifecycle emissions (e.g., sugarcane, and sustainable use of biomass residues) could reduce GHG emissions, but outcomes are site-speci c and rely on e cient integrated 'biomass-to-bioenergy systems' [3].
The impact of sugarcane straw removal on economic, environmental, and agronomic factors are well documented and discussed in terms of sugarcane growth and biomass production, nutrient recycling, soil conservation, soil biological attributes, greenhouse gas emissions, weed control and pest infestation [5][6][7][8][9]. Although the retention of sugarcane straw presents several bene ts to the soil, there is potential to withdraw part of the straw from the elds to produce bioelectricity and cellulosic ethanol (second generation-2G). However, further studies are needed to demonstrate the sustainability of this practice and the best options for industrial use of this crop residue.
Regardless of considerable potential for increased energy generation from the use of recovered straw, few studies have investigated the impact of straw removal on soil CO 2 -C emissions and additional GHG emissions associated with straw removal and transport operations [6, 10,11]. In a sugarcane area under 3 years of green mechanized harvesting, [6] showed that around 100% removal of straw from the soil surface led to an additional emission of 927.7 kg of CO 2 ha −1 to the atmosphere over a 25-day measurement period. In terms of the GHG balance, such additional emissions can annul the bene ts of banning the preharvest burning, which is responsible for an estimated net emission (CH 4 and N 2 O) of 941 kg CO 2 eq ha −1 [12].
The hypotheses tested in the present study were: (1) The recovery and transport of large amount of sugarcane straw from elds upon mechanized harvesting without burning may result in additional soil CO 2 and other GHG emissions (soil N 2 O and emissions from diesel combustion), greater than the potential to offset emissions through the use of straw for bioenergy (bioelectricity or 2G ethanol); (2) Partial removal of sugarcane straw from the soil surface can be a sustainable alternative for its use as bioelectricity source or 2G ethanol production, taking into account their additional GHG emissions and offset potential. This study was designed to assess the additional GHG emissions including measurements of short-term soil CO 2 -C emissions as affected by straw removal from the soil in sugarcane areas plus emissions associated with the recovery and transport operations of straw bales up to the factory gate, contrasting with leaving all straw on the soil surface. Speci c objective is to contrast the additional emissions of straw recoveredmanagement operations with respective GHG offset potential of straw-based bioenergy production (i.e. electricity generation or 2G ethanol), based on Brazilian energy matrix.

Materials And Methods
This study was carried out at a commercial sugar, ethanol and energy production plant located in the region of Ribeirão Preto, Sao Paulo, southern Brazil, where the company performs the removal of straw for bioenergy generation. The results of GHG emissions and offset potential for straw recovery were presented in terms of carbon dioxide equivalents (CO 2 eq) according to the global warming potentials of 1, 28 and 2.1. Short-term measurements of soil CO 2 -C emissions, soil water content and soil temperature The eld experiment for measuring soil CO 2 -C emissions was in Serrana, Sao Paulo State, Brazil (21º06' S -47º37' W, 623 m of altitude). According to the data provided by the agricultural and industrial facility, the sugarcane variety in the experimental area was IACSP 95-5000, planted at 1.5 m inter-row spacing, and with a rst cutting stalk yield of 143 tons ha −1 . The eld experiment started on August 03, 2016, immediately after the straw removal, in which all operations were carried out within 20 days of mechanized harvest without burning, following the standard procedure of the company. The regional climate is classi ed as B 2 rB´4a´ by Thornthwaite system [14], indicating a mesothermal region with rainy summers and dry winters. The soil is classi ed as a Eutroferric Red Latosol (Haplustox, USDA Soil Taxonomy).
Short-term soil CO 2 -C emissions, soil water content (SWC) and soil temperature (12 cm depth) were monitored starting 24 h after straw removal (August 4th 2016) in different treatment plots where PVC collars were previously inserted into soil (15 replicates per treatment). Measurements were taken 17 days over a 21-day period after straw removal in the morning between 8:00 to 11:00 AM on each sampling day.
Soil CO 2 -C emissions were measured using a portable LI-COR (LI-8100, NE, USA) system which monitors the changes in CO 2 concentration inside the chamber by an infrared gas analyzer. The oor chamber has an internal volume of 854.2 cm 3 with a circular area of 83.7 cm 2 . The soil temperature was monitored with a 20 cm probe (thermistor-based sensor) inserted into the soil near the PVC collars at a depth of 12 cm. This measuring sensor is an integral part of the LI-8100 system. Soil water content (%) was measured (12 cm depth) with a portable time domain re ectometer system (HydroSense System, Campbell Scienti c, Utah, USA). During the measurement period, rainfall events occurred on August 10th (02 mm), 15th (10mm), 16th (10 mm), 18th (08 mm), 19th (10 mm), 20th (15 mm) and 23rd (11 mm).

Field treatments for measuring soil CO 2 -C emissions
The experiment for measuring soil CO 2 -C emissions comprised of three treatments replicated three times.
The treatments consisted of a control (T1), which are areas of 12 × 20 m plots (~240 m 2 ) where all straw was left on the soil surface after sugarcane harvest without any straw recovery, and two eld conditions (T2 and T3 explained below), according to the soil cover observed after the straw management and recovery operation in a commercial sugarcane plantation (Fig. 1).
In areas where straw is managed for recovery, straw from 6 crop rows (5 inter-row areas) are collected with each passing of a tractor and heaped in between two crop rows (one inter-row area) to be removed later by baling. After straw was baled and removed, there are still more straw left on the soil surface of the inter-row area where straw was heaped prior to baling than in the control area. This inter-row area where straw was accumulated was de ned as T2 (line plots of 1.5 × 20 m or 30 m 2 ). The area of 6 crop rows from which straw was collected has less straw on the soil surface than in the control area, and this area was de ned as T3. Overall, in a eld where straw was managed and recovered, T2 area occupies approximately 17% of the eld area and T3 area occupies about 83% of the eld area. Therefore, to calculate the CO 2 -C emissions from 1 ha eld in response to straw management and recovery, a weighted average emissions per ha was calculated using measured emissions from T2 and T3 area according to the Equation 1: Where, WAFCO 2 : weighted average CO 2 ux; FCO 2 T3: CO 2 ux from T3 area; and FCO 2 T2: CO 2 ux from T2 area. In each treatment (3 replicates), 15 PVC collars were used for measurements of soil CO 2 -C emissions.
In general, straw management and recovery operation resulted in a removal of 27% of sugarcane straw for bioenergy generation (explained below).

Statistical analysis
Data related to CO 2 -C emissions, soil temperature and soil water content (SWC) were analyzed using a completely randomized experimental design with three treatments and three replicates per treatment (de ned in section 2.1.1) and repeated measurements in time. For the analyzes, the mixed-model procedure of SAS statistical software was used [SAS Institute Inc., Cary, North Caroline]. When statistically signi cant (P < 0.05), the means were compared by the Tukey's test at a signi cance level of 5%. The variables (CO 2 -C, soil temperature and soil water content) were also analyzed by linear correlation, testing if correlation coe cient is statistically different from zero (P < 0.05).

Soil analysis, quanti cation of post-harvest straw and estimated N 2 O emissions
Soil samples were collected to a 0.20-m depth after measurements of soil CO 2 -C emissions. To obtain 1 kg of soil, 12 single samples were collected from the experimental area, and soil physical-chemical characteristics such as pH, soil organic matter (SOM) and soil texture were analyzed ( Table 1). The samples were dried and sieved through a 2 mm mesh prior to analysis. These analyzes include determination of SOM content by oxidation with sulphuric acid [15].  [20] to be compared with the results derived using the IPCC methodology [17]. Evaluating GHG emissions in response to sugarcane straw removal, authors observed no net CH 4 emissions [21], and thus, CH 4 emissions were not considered in our study.

Estimation of greenhouse gas emissions from fossil fuel used in the recovery and transport of straw
In addition to industrial plant, further investments on tractors, trucks and other heavy machines are required in the management and recovery of sugarcane straw after green mechanized harvest. For this experiment, the data collected and analysed were: Truck carry capacity of approximately 70 bales per trip; bales weighing on average of 350 kg bale −1 and 24,500 kg of straw transported per trip, with a straw moisture content of around 6 to 10%.
The technical parameters for mechanized operations considered in the straw management and recovery operation and the transport of straw until the biore nery gate are presented in Table 2. The fossil fuel typically used for sugarcane production systems is diesel oil, consumed mostly during the agricultural operations by machinery, tractors, and trucks [12]. In our study, the diesel consumption related to the nal use of sugarcane straw was measured according to all eld operations performed (Table 2.).

Quanti cation of energy and cellulosic ethanol (2G) production from straw recovered
The use of sugarcane bagasse as a fuel for combined heat and power (CHP) systems to meet energy needs of the mills for bioenergy generation is a common practice in Brazilian sugarcane industry [26]. In this study, we present an additional energy generation potential of sugarcane straw recovered after green mechanized harvesting as per the technology used by the company. According to the information from the company's industrial sector, the straw used for energy generation is added to the sugarcane bagasse at a proportion of 5 to 7% of the volume, presenting a potential value for electricity generation of approximately 0.6 MWh ton −1 DM, with humidity between 6 and 10% and the technology being used is high pressure boiler/condensation and steam extraction turbine.
Analysing the use of sugarcane straw for power generation, some authors reported a higher heating value of cane straw of 17.1 MJ kg −1 (DM) and lower heating value of 12.15 MJ kg −1 , without considering of the losses through the process [27]. With respect to industrial productivity, the premise to produce 240 L ethanol ton −1 LM (lignocellulosic material on dry basis) in the short-term was reasonably conservative by companies [28]. For some companies, it is already feasible to reach 300 L ethanol ton −1 LM, or even 350 L ethanol ton −1 LM in the long-term. On the other hand, electric power demand for 2G process is depicted as 48 kWh ton −1 LM over a short, medium, and long-term basis. However, the energy demand for 2G ethanol production from the straw can be supplied by the residual material (residual cellulignin) from the 2G process, and thus there is no need to burn additional straw to supply this energy demand [28]. of gasoline [23] with EF of 2.312 kg CO 2 eq L −1 (207 g CO 2 km −1 , 0.009 g CH 4 km −1 and 0.023 g N 2 O km −1 ).
Therefore, when replacing 1 liter of fossil gasoline per 1 liter of hydrated ethanol, an avoided emission of 2.26 kg CO 2 eq considering only CH 4 and N 2 O emission from ethanol was assumed in this study when considering 2G ethanol production from sugarcane straw.

CO 2 emission factors for electric energy production in Brazil
The CO 2 emission factors of electric energy production used in life-cycle inventories in Brazil estimate the amount of CO 2 associated with the generation of a particular type or source of electric energy. These The CO 2 emission factors calculated according to the methodological approach "Tool to calculate the emission factor for an electricity system, version 04.0 and earlier" approved by the Clean Development Mechanism (CDM), Executive Board are intended to estimate the contribution, in terms of CO 2 emission reductions, from a project that generates electricity for the grid [30]. Brie y, the interconnected system CDM emission factor is a combination of the operating margin emission factor, which re ects the intensity of CO 2 emissions of the energy dispatched at the margin, with the build margin emission factor that re ects the intensity of CO 2 emissions from the last plants built [30]. Highest temporal emissions were observed in T2, except for day 20 (P < 0.05). This could be explained by the straw management and accumulation of large amount of straw between two crop lines (inter-row), which could promote higher soil aeration, higher O 2 availability for microorganism's activity and higher C accessibility (straw carbon) to be decomposed through mineralization process. In contrast, keeping sugarcane straw on the soil surface without management led to a lower soil CO 2 -C emission [6], probably due to higher soil bulk density and lower soil aeration [31], showing that in a condition where soil and straw were managed, the mineralization of SOM (soil organic matter) increased signi cantly with higher aeration, thus supporting the ndings observed in T2.
The impacts of straw management on soil temperature can be observed in Figure 2b, when comparing T1 (straw maintained), with T3 (27% of straw recovery). There was signi cant difference between these two treatments only on days 07 and 09 (P < 0.05), in which soil temperature was higher in T3, showing that recovering 27% of straw had a slight impact on soil temperature.
The higher amount of straw accumulated in T2 (44.3 tons DM ha −1 ) could explain this higher soil water content (SWC), with straw mulching contributing to soil protection against evaporation. Moreover, when comparing the management of removing 27% of straw (T3) with the management of keeping all straw on the soil surface (T1), during the dry period, there was no signi cant difference for soil water content (P > 0.05), showing that higher amount of straw could keep higher available soil water content.
In terms of accumulated short-term soil CO 2 -C emissions (Fig. 2d), lower emissions were observed in T1 with 788 kg CO 2 -C or 2,889 kg CO 2 ha −1 . However, comparison of the accumulated soil CO 2 -C emissions in the management of recovering 27% of straw (WAFCO 2 , 963 kg CO 2 -C or 3,531 kg CO 2 ha −1 ) with the management of keeping all straw on the soil surface (T1), did not indicate signi cant difference between the two managements, showing that recovering 27% of straw in the experiment conditions did not impact the accumulated short-term soil CO 2 -C emissions. Therefore, the impact of straw removal (27% or 6.89 ton DM ha −1 ) on accumulated soil CO 2 -C emission over 21 days (WAFCO 2 ) was 175.5 kg CO 2 -C ha −1 or 643.5 kg CO 2 ha −1 , which is equivalent to 25.5 kg CO 2 -C ton −1 or 93.4 kg CO 2 ton −1 of straw (Fig. 3). It is important to note that the additional short-term soil CO 2 -C emissions in sugarcane areas may be higher soon after straw management, and then these emissions tend to stabilize to similar levels compared with emissions from areas with no straw management, as observed for T1 and T3 at the end of the experimental period. Exploring the GHG responses to sugarcane straw removal, a rate of 6 ton DM of straw ha −1 resulted in a total CO 2 -C emissions of 82.2 g m −2 or 3,014 kg CO 2 ha −1 in a period of 180 days [21]. In a scenario with straw removal and possible SOC losses, we need to consider no-tillage operations during sugarcane crop renovation, mainly if it is combined with other best management practices, such as reduced tillage accompanied with vinasse and lter cake application, which could replace the SOC losses after residue removal [6, 32, 33].

Emissions from fossil fuel used to recover and transport of straw
Speci c management options practiced by sugarcane mills for straw recovery systems certainly have distinct impact on the economic and technical parameters. Taking into account of all additional mechanized eld operations for straw recovery and their respective machinery use and diesel consumption (Table 2), the total amount of diesel consumed to recover 1 ton of straw under the experimental parameters was 9.9 L ton −1 of straw or 68.2 L of diesel ha −1 (6.89 ton ha −1 , dry matter-basis), resulting in an emissions of 31.6 kg CO 2 eq ton −1 of straw or 217.1 kg CO 2 eq ha −1 (Fig. 3). These emissions from mechanized operations to recover straw is considered as additional emissions compared with the management of leaving all straw in the eld and should be included in assessing the GHG mitigation potential of straw for bioenergy generation. Contrasting the results estimated here by other methodology [17], some authors reported higher accumulated N 2 O emissions from sugarcane straw left on soil (15 ton DM ha −1 ), with 2.28 kg N 2 O ha −1 or 604.2 kg CO 2 eq ha −1 over 270 days-period [20], which was used in our GHG balance for the potential of straw for bioelectricity or 2G ethanol production (Fig. 3  The RenovaBio Program presents itself as a promising and innovative policy in many aspects (biofuels demand planning and boosting energy security in Brazil), which every single biore nery in the country will pursue the production of biofuels with an ever-higher environmental responsibility [2]. The characteristics of each country's energy matrix, based on fossil fuels or in renewable sources, as found in Brazil [30] will determine the potential for each country to develop and use straw-based bioenergy sources to reach a lowcarbon energy matrix.
The 2G ethanol produced from lignocellulosic biomass (LM) has been considered as the biofuel with the greatest potential to replace fossil fuels [39,40]. Based on our experimental results, with the production potential of 240 L 2G ethanol ton −1 LM [26], it would be possible to produce an additional of 1,654 L of ethanol ha −1 (6.89 ton DM straw ha −1 ), which could promote an avoided GHG emissions of 2.26 kg CO 2 eq L −1 of 2G ethanol by replacing 1 liter of gasoline or 3,738 kg CO 2 eq ha −1 considering only CH 4 and N 2 O emissions from 2G ethanol combustion (Fig. 3).
Taking into account of all mechanized operations to recover around 27% of sugarcane straw, the total additional GHG emissions, considering the main emission sources (i.e., additional soil CO 2 -C, diesel use and N 2 O from straw), contrasting with leaving all straw on the soil surface was estimated at 1,465 kg CO 2 eq ha −1 (6.89 ton DM ha −1 ), resulting an emission factor of 212. 6 kg CO 2 eq ton −1 of straw recovered.
With an avoided N 2 O emissions of 42 kg CO 2 eq ha −1 , compared with higher N 2 O emissions with leaving all straw on soil, 27% of sugarcane straw removal would therefore result in a total additional emissions of 1,422 kg CO 2 eq ha −1 (Fig. 3).
Applying the parameters cited in this study for electricity or 2G ethanol production, our results for their respective GHG balance (emissions and offset potential) showed that, straw-based electricity would result in an additional GHG emissions of (+) 860 kg CO 2 eq ha −1 (Fig. 3), which could not be a sustainable route for bioenergy production. In contrast, applying the same parameters for 2G ethanol production replacing gasoline, an avoided GHG emission of (-) 2,316 kg CO 2 eq ha −1 could be achieved, showing that the route of 2G ethanol production could be the best strategy to use sugarcane straw as a by-product to reduce GHG emissions from the agricultural sector, offsetting emissions from fossil fuel and contributing to mitigate global climate change using a renewable bioenergy source.
The results from this research can support deriving environmental indicators to assess the sustainability of 2G ethanol by regulatory and certifying bodies, such as the CARB (California Air Resources Board) and the most recent State Program "RenovaBio", whose objective is to stimulate the e ciency of the biofuels industry, in the agricultural and industrial sectors, with an emphasis on total energy demand and the mitigation of GHG emissions (lower C intensity) associated with each stage of the life cycle of bioenergy production.

Conclusions
Bioelectricity and cellulosic ethanol production using sugarcane straw recovered may not be a sustainable management strategy to mitigate GHG emissions when considering the associated impacts on sugarcanecultivated soils and related additional emissions from soil and straw recovering operations. The main additional GHG sources from straw management and agricultural processes include soil CO 2 -C emissions, soil N 2 O emissions and emissions from mechanized eld operations (diesel use), which should be accounted for deployment of new sugarcane bioenergy projects using straw as a raw material.
The route of recovering around 27% of sugarcane straw from the soil surface through bale system for bioelectricity production using technical parameters and industrial e ciency rate of this study, could not be a sustainable option because the additional GHG emissions from these processes can be higher than its potential to offset generated emissions, based on the emission factor of Brazilian energy matrix. On the other hand, using the same parameters for straw recovery at the rst cutting cycle in a clay soil, the option of producing ethanol 2G with around 27% of the total sugarcane straw recovered (6.89 ton dry matter ha −1 ), could offset greenhouse gas emissions once replacing fossil gasoline, which can be an option to contribute for a better sustainability of sugarcane straw recovery, supporting renewable and sustainable bioenergy systems, and for reducing the impact of Global Climate Change.  Weighted average of soil FCO2 (weighted average of emission in T2 and the T3). Means followed by the same letters comparing straw management types did not differ by the Tukey test (P < 0.05).