Evaluating agricultural management practices to improve the environmental footprint of corn-derived ethanol

Renewable Energy 66 (2014) 454e460

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Renewable Energy journal homepage: www.elsevier.com/locate/renene

Evaluating agricultural management practices to improve the environmental footprint of corn-derived ethanol Xiaobo Xue a, YuLei Pang b, Amy E. Landis c, * a

Department of Civil and Environmental Engineering, University of Pittsburgh, 3700 O’Hara Street, Pittsburgh, PA 15260, USA Department of Mathematics and Statistics, Texas Tech University, 2500 Broadway, Lubbock, TX 79401, USA c School of Sustainable Engineering and the Built Environment, Arizona State University, 501 E. Tyler Mall, Tempe, AZ 85287, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 February 2013 Accepted 27 December 2013 Available online 24 January 2014

This study examines three agriculture management practices with the aim of improving the environmental performance of corn-derived products such as bioethanol. Corn production is energy intensive and contributes to water quality degradation and global warming, thus affecting the environmental impact of corn-derived ethanol. Life Cycle Assessment (LCA) is used to quantify and compare the environmental impacts of three management strategies: tillage, fertilizer choices and the use of buffer strips to sequester nutrients. Detailed energy, carbon, nitrogen and phosphorus inventories are compiled to represent corn production scenarios within the US Corn Belt. The LCA was developed using GREET 1.8 (Greenhouse Gases, Regulated Emissions, and Energy use in Transportation) and emission factors with statistical analyses to estimate energy consumption, associated air emissions, and aqueous nutrient runoff potentials. Results show that using manure fertilizers as opposed to synthetic fertilizers requires less energy, however the use of manure generates more CH4, N2O, CO2 and results in more variable concentrations of nitrogen and phosphorus leaching from farmlands. No tillage emits less greenhouse gas emissions, sequesters more soil organic carbon and slightly reduces nutrient runoff compared with conventional tillage practices. Building buffer strips of certain widths is an efficient way to reduce N and P discharge to surrounding waters with minimal effect on the energy or global warming profile. Based on the results of the LCA studies, replacing conventional tillage with no till, and installing buffer strips can improve environmental performances of corn derived ethanol. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Agricultural best management practices Energy Global warming Eutrophication Life cycle assessment (LCA) Ethanol

1. Introduction Currently, most ethanol production in the United States uses corn as the feedstock. Production of corn-based ethanol has grown from less than 3 billion gallons in 2003 to over 10 billion gallons in 2012. Increasing demands for biofuel and animal feed may continue to drive large expansion of corn farming [1]. The United States Department of Agriculture predicts corn acreage in United States will maintain more than 85 million acres till year 2018 [2]. However, corn farming is energy intensive and contributes to global warming and water quality degradation [3e8]. Modern agriculture relies on high chemical application and use of farming equipment. Fertilizers are used to increase crop yield and can potentially disrupt natural nitrogen and phosphorus cycles,

* Corresponding author. Tel.: þ1 480 965 4028; fax: þ1 480 965 0557. E-mail address: [email protected] (A.E. Landis). 0960-1481/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.12.026

resulting in hypoxia and eutrophication. Agriculture production in the Corn Belt of the US contributes the highest fluxes of nitrogen and phosphorus to the Mississippi River Basin and is considered one of primary contributors to the growing hypoxic zone in the Gulf of Mexico [9,10]. To minimize and control this hypoxic area, agriculture management practices in the Corn Belt will need to play an important role to reduce nutrient load. Furthermore, operating farming equipment (such as tractors etc.) and producing associated chemicals rely on the combustion of fossil fuels that release green house gases and consequently influence climate change. Previous studies show that farming equipment usage and associated processes annually consume a large amount of the total fossil energy use [11]. Syntheses of commercial fertilizers are also energy demanding processes [12]. In the US, corn production uses more than 40% of the nation’s commercial fertilizer (about 330 million pounds of nitrogen fertilizers), mainly in the Midwestern Corn Belt region in 2008 [13]. As corn farming increases, increased use of fertilizers and farming operations

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increases energy consumption and generates considerable greenhouse gases. Direct agricultural practices and indirect supporting activities lead to environmental quality impairment. Indeed, integrating and optimizing farming practices will enable the mitigation of environmental degradation and reduce biobased products’ environmental footprints [14e16]. Specifically, we focus on three agricultural management practices in this study: tillage, fertilizer type and buffer strips as possible strategies to alleviate corn derived products’ environmental impacts resulting from agriculture. Tillage is often practiced as the first step in the preparation for a soil bed to be made suitable for seed germination and seedling development. According to the amount of plow surfaces and crop residues, there are three typical tillage methods: conventional tillage, reduced tillage, and conservation tillage [17]. Conventional tillage involves plowing the entire soil surface and leaving less than 15% of crop residue to cover the soil surface after planting. Reduced tillage utilizes a chisel plow to mix soil and crop residue, leaving 15%e30% residue coverage on soil. Conservation tillage includes ridge tillage, no tillage etc. This study evaluated the environmental impacts of corn produced with conventional and no tillage practices. Compared to conventional tillage, no tillage often has more environmental advantages including surface runoff reduction and soil erosion mitigation [18]. The no tillage method uses crop residue mulch to provide a protection against raindrop impact, thereby increasing soil organic carbon and decreasing decomposition of soil organic matter and oxidization of soil organic carbon. Other possible environmental benefits include energy and emissions savings resulting from less fuel consumption for operating farming equipment and associated air emissions [19,20]. The choice of fertilizer types can affect the energy profile, greenhouse gas emissions, and aqueous emissions attributed to the environmental footprint of agricultural products [21,22]. Both synthetic fertilizers and animal manures are used to enrich soil nutrition within Corn Belt agriculture [23]. Compared to manures, most commercial synthetic fertilizers contain higher nutrient concentrations by weight, more appropriate nutrient ratios (i.e. N:P:K ratios), and are more readily available to crops when applied to the soil. High gaseous carbon and nitrogen fluxes that result from handling and applying manures are concerns for the use of manure as a source of fertilizer [24,25]. However, manure has the potential to adjust the soil carbon cycle and maintain soil fertility. Reusing manures, which are usually waste products of animal raising systems, is often explored as an economic and sustainable alternative to synthetic fertilizers [16,26]. Establishment of buffer strips is used as an important component of integrated farming nutrient management plans [27e29]. Installing riparian buffer zones is a recognized agroforestry practice that not only provides phytoremediation for nonpoint source pollutants but also increases biodiversity of terrestrial ecosystems. It can also moderate flood damage, control nutrient leaching, sequester carbon, and recharge groundwater. Riparian zones generally consist of two types: grass strips and wooded buffers. Woody vegetated strips can include shrubs and have advantages in controlling bank erosion and providing biological abundance, while grass strips might be more acceptable in keeping with the original character of the landscape. Both woody vegetation and grass strips can uptake of nutrients to improve water quality [30e32]. Alternative agricultural management strategies have been studied widely in the agricultural literature, primarily focusing on narrow aspects of agriculture management, such as their effects on yields, nutrient leaching etc. However, the overall environmental impacts from agriculture and related management strategies, and thus their implications on the environmental footprint of bioproducts have not been explored adequately. With the growth of

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biofuels and biobased products, Life Cycle Assessments (LCAs) applied to agricultural activities can be a useful method to quantify environmental impacts. LCA is a systematic approach to analyze and quantify the environmental impacts of a product or process over its entire life cycle. Comparative LCAs among possible products or processes can help to determine the environmentally preferable alternative. Guidelines for performing LCAs are delineated by American National Standards Institute (ANSI) and International Organization of Standardization’s (ISO) 14040 series [33,34]. LCA is an iterative four-stage process including goal and scope definitions, life cycle inventory (LCI) analysis, life cycle impact assessment (LCIA), and interpretation. Currently, agricultural LCAs have been mainly carried out for either single crops or individual production processes [3,35e39]. The comparative analysis of farming practices, which is important to identify environmentally preferred practices and reduce negative environmental impacts resulting from corn farming, is still lacking from life cycle perspectives. This study aims to quantify energy consumption, air emissions, and aqueous nutrient emissions under different corn farming management scenarios. The inventory includes global warming emissions, aqueous nutrients (N, P) and energy usage. Comparative LCA results of the three farming practices discussed above are presented. Moreover, this article provides detailed and comparative analysis of environmental impacts to identify farming practices that improve environmental performances of corn derived ethanol. 2. Materials and methods 2.1. System boundary The agricultural system boundary, material and energy flows accounted for in the LCA are depicted in Fig. 1. The system boundaries include on-field production practices (tillage practices and fertilizer application), integrated farming practices (buffer strips), associated equipment and chemical manufacturing, transportation processes, and also power generation. Energy usage, atmospheric and aqueous emissions are calculated during every stage. Geographically, this system reflects the farming scenarios in US Corn Belt states, which produced more than 75% of total U.S. corn in 2008. US Corn Belt states include Iowa, Illinois, Nebraska, Minnesota, Indiana, Ohio, South Dakota, Wisconsin, and Missouri. Data was collected from literature that included experimental data, onfield survey data, and geological modeling estimation results between 1990 and 2007. 2.2. Models A description of the models used to develop the LCI is discussed in this section. Energy flows and associated EPA criteria air emissions are calculated with GREET 1.8 (Greenhouse gases, Regulated Emissions, and Energy use in Transportation) model while nutrient outputs are estimated through the development of LCA models using emission factors. Variability of agricultural systems is accounted for using statistical analyses and Monte Carlo Analysis (MCA). 2.2.1. GREET 1.8 model GREET 1.8 was developed by Argonne National Laboratories and was utilized within this study to compile an LCI of on-farm and upstream energy use and air emissions. GREET consists of a comprehensive spreadsheet-based database that calculates energy consumption and emissions of criteria air pollutants (VOC, carbon monoxide, nitrogen oxides, particular matter, sulfur oxides, carbon

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, etc.

Raw materials

Farming machines production

Operating machines

ill

Producing synthetic fertilizers

Aqueous N, P emissions

Fuel and electricity production

Animal housing

System Boundary *Shaded areas are compared farming practices in this study. Fig. 1. System boundaries for the agriculture life cycle inventory (LCI). Energy flow and material flows (carbon, nitrogen, phosphorus) under three types of agriculture practices are compared (tillage practices, fertilizers practices, and integrated buffer strips). Air emissions are modeled by modified GREET (Greenhouse Gases, Regulated Emissions, and Energy use in Transportation model version 1.8 developed by Argonne National Labs). Nutrient models explained below are employed to determine aqueous emissions.

dioxide, methane, and nitrous oxide based on EPA emissions factors). These emissions and energy consumption are available for corn growing as well as for transportation of agricultural chemicals and fuel production and usage. GREET calculates the total energy, fossil energy, and petroleum consumption for various agricultural processes. Based upon emission factors from EPA databases, GREET delineates the associated material and energy flows of petroleum recovery, refining, transportation, and use in the agricultural sector [40]. GREET lacks description on handling manures as fertilizers, on-farm application of agricultural machines, and installing and maintaining buffer strips. Therefore, these aspects of agricultural management strategies were added into the study to complement data available from GREET. 2.2.2. GREET Modification Three major modifications were made to GREET to enhance the level of detail of fertilizer types and farming equipment usage; (a) addition of energy consumption and emissions for producing, handling, and transporting manure, (b) detailed description of farming equipment used during different tillage practices, and (c) installing and managing buffer strips. These three agricultural management strategies were modeled independently, as described below, while the energy and fuel inputs for the additional activities and associated emissions were estimated by GREET. To model manure, energy consumption and GHG emissions related to manure production and application were added to GREET. IPCC equations and suggested input values for North American region were used to estimate GHG emissions in the animal raising and manure management systems [41]. GHG emissions after manure application on corn farm lands were determined according to publications [24,25,42,43]. Detailed data sources are reported in Supporting information. The distance of transporting manure from storage room to farmland was assumed to 50 miles by light trucks based on literature [23,44]. To model the effects of different types of tillage, detailed inventories of energy consumption and air emissions for operating farming equipment were incorporated. Energy consumption for operating farming equipment (including tractors, field cultivators, plowers, combines, irrigators, and sprayers etc.) was quantified based on publications [45]. Air emissions generated by operating farming equipment were estimated by GREET model based on energy consumption. Most current results show crops rotation, soil properties, and weather conditions can complicate the effects of tillage practices on

corn yield rate. The differences in corn yields can be neglected after long-term application of tillage practices [18]. In Corn Belt states, corn is usually rotated with soybeans. This study mainly focused on corn rotated with soybean and assumed that the corn yield rate under conventional tillage was as the same as no tillage. Buffer strips are usually installed at the edge of farmlands. According to government guiding documents [46,47], buffer strips’ width was modeled as 30 m to calculate pertaining energy consumption and coupled air emissions. Farming equipment used during installation and maintenance (i.e. tractors, mowers, cultivators) was assumed to be the same as those used within the tillage practice. In addition, fertilizer application rates, average farmland acreage and corn yield rates were adjusted within GREET to match recent US Corn Belt levels. A detailed description is presented in Table 8 of the supporting information. 2.2.3. N/P model Nutrient outputs were calculated by developing emission factors for N and P flows from the different agricultural management activities. The emission factor for total nitrogen (TN) and total phosphorus (TP) are defined based on a fraction of the applied nutrient load [48,49]. Eq. (1) shows that fertilizer input amounts are normalized by corn yield rates. Eqs. (2) and (3) describe the amount of TN and TP lost from fields in runoff LrunoffN, LrunoffP, which are calculated as the fraction of chemical lost with respect to the total mass of chemical applied to a crop. The nitrogen model is amended to account for the subsequent loss of nitrate via the denitrification process. While there are complex interactions among N/P species, plants and surrounding environments, this model does not attempt to model such interactions but rather to utilize LCI in conjunction with MCA to represent the possible range of N/P in runoff. It was assumed no soil P was available from the previous year as an input. Phosphorus inputs from the air (deposition), rain, and wind erosion were not accounted for. Table 1 2.2.4. Model uncertainty and variability Agricultural processes contain high degree of system variability which depends on geography, weather patterns and soil type etc. The use of MCA has been incorporated into LCA in previous studies and has been shown to provide an appropriate representation of agricultural variability [3,49]. Statistical software (Crystal Ball7.0 and Minitab 11.5) were used to model the variability and calculate the uncertainty of the

X. Xue et al. / Renewable Energy 66 (2014) 454e460 Table 1 Description of equations and parameters in the LCA emission factor models. Eq.

Equation/variable

Description

(1)

1 Lin X ¼ RX *Y

Input load

(2)

* Lrunoff ¼ Lin x *fem ð1  fTN;NO  3*fde Þ N

N Load in runoff

(3) (4) (5)

¼ Lin Lrunoff N P *fem L R

(6) (7)

Y fem,x

(8) (9)

fTN,NO3 fde

P Load in runoff Mass load for per kg crop grain Rate of application (kg  synthetic fertilizers or manure per ha crop) Yield (kg crop grain per ha) N,P runoff coefficient, this coefficient reflects the nutrient discharge potential of fertilizers and tillage practices. Ratio of nitrate to total nitrogen. Fraction of denifrication

emission factor model. Chi-squared and AndersoneDarling tests were used to determine the best fit distribution for the models’ input parameters and then MCA was employed to develop cumulative probability curves of parameters at confidence level of 90%. Crystal Ball and Minitab complemented each other’s capabilities to provide optimized distribution of nutrient runoff. Independent observation values were collected to calibrate equations’ parameters and verify modeling results. Detailed procedures and datasources are explained in SI. 2.3. Life cycle impact assessment Life cycle impact assessment (LCIA) aims at describing the environmental consequences of the environmental emissions quantified in the inventory analysis. The impact assessment is achieved by translating the environmental loads from the inventory results into environmental impact using LCIA tools. The U.S. EPA has developed TRACI 2.0 (Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts) model to assist environmental impacts assessment for LCAs [50]. TRACI was used to facilitate the characterization of stressors that may have global warming potential and eutrophication potential in this study. Global warming and eutrophication potential categories were calculated by multiplying TRACI’s characterization factor (CF) corresponding to each LCI emission to the LCI output emission value. The CF represents the equivalent effect of individual compounds with respect to a reference substance. CO2 equivalent and nitrogen equivalent were used as common units to aggregate and compare environmental impacts of LCI emissions. The global warming potentials of CH4 and N2O are, respectively, 23 and 296 times as that of CO2. The eutrophication potential of phosphorus (P) is 7.4 times as that of nitrogen (N).

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was treated as a coproduct of animal husbandry systems. Energy usage of producing manures was determined according to total energy consumption in animal raising systems and the energy based ratio of animal excretion/animal feed. 30% of energy consumed by animal husbandry system and associated emissions were allocated to manures. This ratio is estimated based on energy flows on cattle and dairy farms in Midwest states [51]. This research conducted nutrient flow analysis of corn production. When fertilizers as major N and P sources are applied in farmlands, these nutrients will be shared between corn and other rotation crops. The TN/TP leaching share of corn fields was determined by area weighed average values [48]. Thus, 51% of TN and 61% of TP was allocated to corn. The influences of changing farming practices in Corn Belt on environmental performances of corn-derived ethanol were estimated based on previous studies [7,52]. These studies quantified the contributions of corn production to overall environmental impacts of corn-derived ethanol. Based on the previous ethanol LCA analyses, we calculated the reduction of corn derived ethanol’s life cycle energy consumption, global warming and eutrophication potentials by adopting various farming practices in corn production stage. 3. Results and discussion 3.1. Comparing the use of synthetic fertilizers to manure Fig. 2 presents the environmental impacts of alternative fertilizers. Energy consumption during fertilizer manufacturing, processing and transportation was determined using average values of ammonium nitrate, urea and ammonium. From a life cycle perspective, ammonium nitrate is the most energy demanding product among the four types of fertilizers evaluated (2.1 MJ/kg corn), followed by urea (1.7 MJ/kg corn), ammonium (1.4 MJ/kg corn), and finally manure (as coproduct in animal raising system, 0.2 MJ/kg corn), shown in detail in Supporting information. If farmers choose synthetic fertilizers as nutrient sources to produce 1 kg corn, around 1.7 MJ energy is required to transport raw materials, produce fertilizers, deliver final products to farmlands, and support all upstream activities as well. Compared with synthetic fertilizers, the manure requires much less energy than synthetic fertilizer over its life cycle. If manures are considered as coproducts of animal raising systems, energy embedded in manure practices

2.4. Functional unit and allocation Functional units aim to provide a reference level for comparison. One kg corn was used as the functional unit to compare the effects of farming practices on corn production in this study. Allocation within LCA allows for products from corn farming to be attributed an appropriate percentage of environmental impact from the corn farming process. In this study, allocation was conducted on a mass basis to corn crops normalized per year. All energy use and associated emissions are allocated 100% to the corn grain on a mass basis, because the boundaries of LCI end before the milling phase. Environmental impacts from manure practices were allocated in two manners for comparison: Firstly, manure was treated as a waste, thus emissions associated with animal feeding operations were not allocated to manure as fertilizer. And secondly, manure

Fig. 2. Environmental tradeoffs of fertilizer practices. Environmental release profiles of synthetic fertilizer and manure (as coproduct from animal raising systems) were shown. Median values and 95% confidence intervals of energy use, greenhouse gas emissions, and aqueous nutrients outputs for fertilizers practices are shown. Energy use of fertilizer practices includes the production, transportation, and application stages.

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(including building and operating animal raising systems, transporting manures) is around 0.2 MJ energy/kg corn. The energy used to haul manure to farmlands is minimal compared with energy allocated from the animal husbandry system, thus energy consumption for manure as waste is negligible. These results are consistent with previous research about fertilizers. The synthesis of inorganic nitrogen fertilizer was reported as a very energy demanding process, typically consuming around 25e35 GJ to produce 1 ton ammonia through the steam reforming process (Kongshaug 1998). Natural gas, as both raw materials and possible energy source, is the dominant source of energy to produce fertilizers. Energy consumption of corn farming was investigated using life cycle assessment methods in previous studies [3,19]. To estimate total energy input for corn farming, these studies accounted for energy for planting, fertilizing, spraying, harvesting, as well as the energy required to manufacture synthetic fertilizers, pesticides, and limes. These studies show the total energy use varies from 1.5 to 3.4 MJ/kg corn depending on boundaries and allocation methods; Synthetic fertilizers accounts for a large portion of the energy consumption. Fig. 2 also shows greenhouse gas emissions, while the Supporting information provides resultant life cycle air emissions attributed to synthetic fertilizer and manure practices. CO2, CH4, and N2O gases are the dominant emission species for both synthetic fertilizers and organic manures. Results show organic manure practices emit more CH4 and N2O than synthetic fertilizers. Higher amounts of N2O emit from manure storage rooms and handling processes. The high release rate of CH4 is due to biological degradation reaction during storing and handling manure [53e55]. In addition, significant amount of on-farm N2O emission after applying manures also increases the global warming potential of manures. Statistical analysis shows the nutrient discharge capabilities of fertilizer practices are different. The nitrogen amounts mainly vary from 4 to 10 g N/kg corn, from 0.01 to 10 g N/kg corn for synthetic fertilizers and manure respectively. Probability distributions of aqueous nitrogen and total phosphorus runoff values are illustrated in SI. The distribution curves representing cumulative percentages of nutrient runoff were estimated via MCA. The possible runoff concentrations of manure practices have a broader range and stronger tendency to approach high concentrations than synthetic fertilizers. While discharge concentrations lower than 4 g N/kg corn and higher than 10 g N/kg unlikely exist for synthetic fertilizers, those concentrations happen frequently for manure fertilizers. High uncertainty of nutrient contents and nutrient availability rates results in variability of nitrogen leaching for manure practices [56,57]. For phosphorus discharge, there is no remarkable difference of distribution ranges between synthetic fertilizers and manure. However, organic manure has slightly higher possibility to leach more phosphorus. This phenomenon was also observed in previous studies [58e60]. Because of the imbalance of N and P ratios of manure, the P fraction is usually overloaded to farmlands in order to ensure crops’ N nutrient requirements, consequently generating higher P leaching potential for manure practices. 3.2. Comparing conventional tillage with no tillage practices As Fig. 3 shows, the use of no tillage practices could result in a large reduction of fuel consumption compared to conventional tillage. No tillage practices require much less energy for soil preparation and cultivation processes. There are no distinct differences in energy consumption resulting from pesticide application, irrigation, storage and transportation between conventional tillage practices and no tillage practices. Theoretically, conventional tillage practices require lower amounts of pesticides because moldboard

Fig. 3. Environmental tradeoffs of tillage practices. Median values and 95% confidence intervals of energy use, main greenhouse gas emissions, and aqueous nutrient leaching for tillage practices are shown. Energy use and associated air emissions of tillage practices includes the equipment production, transportation, and application stages.

or field cultivator serves the function of killing and removing weeds. However, no significant differences are discovered within this study, which is possibly due to the system boundary definition which includes energy consumption of spraying pesticides and excludes manufacturing pesticides. Other published reports show that more than 40% energy savings from reduced agricultural machinery operation can be obtained, when conventional tillage practices are switched into no tillage practices [19]. The results of this study are lower than previously reported values because broader farming categories were incorporated into farming inventories, including planting, spraying, harvesting, storing and transportation. Accordingly, the amounts of air emissions are higher in conventional tillage practices than no tillage practices. Air emissions are released through manufacturing and operating farming machines. The main species quantified through GREET are CO2, CO, CH4, SOx, and NOx. These emissions in part result from steel production and machine assembly processes, from vehicle usage, and from transportation processes. CO, SOx, and CH4 emissions are generated from incompletely oxygenated combustion during producing steel. Furthermore, no tillage practices contribute considerably to sequestration of soil organic carbon (SOC), which can offset carbon emissions from agricultural inputs and machinery [20,61,62]. Tillage practices expose soil organic carbon in the soil surface, increasing the rate of biomass decomposition and mineralization, and thus increase the release of carbon dioxide into atmosphere. The Center for Research on Enhancing Carbon Sequestration in Terrestrial Ecosystems within the US Department of Energy estimates that changing from conventional tillage to no tillage will result in sequestration of 31 kg C/(ha*year) in agricultural soils, to a depth of 30 cm. Although SOC is expected to change in response to a change in management practices, the change will be finite and the concentration of SOC will approach a new steady state after 10e20 years [62]. Tillage practices also influence aqueous nitrogen and phosphorus emissions from corn farmlands [21]. A slight increase in nutrient discharge was observed when conventional tillage is compared with no tillage practices. However, according to the distribution and range of probable values shown in Fig. 4 in SI, no tillage nutrient leaching is highly variable, thus the advantages to conventional tillage with respect to nutrient leaching are unclear. Tillage activities, by loosening soil structure and enhancing nutrient uptake rate, may result in lower nutrient leaching.

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3.3. Effects of buffer strips Life cycle energy consumption and air emissions of building and maintaining the buffer strip are low compared to the entire life cycle impacts of corn farming. Planting, cultivation and mowing practices were incorporated to calculate energy flows of buffer strips. Generally, farmlands’ area is much bigger than surrounding buffer strips’ area, so the values of energy demand and associated air emissions resulting from buffer strips are very low when corn product related units are used to represent energy consumption and associated air emissions of managing buffer strips. However, the nutrient removal efficiency of buffer strips is significant. As Fig. 4 shows, the nitrogen removal rate of buffers is estimated to be 75% while the phosphorus removal rate is 67%. Therefore, buffer strips are effective management tools for controlling nutrient leaching when GWP, energy, and eutrophication impacts are of concern for agricultural feedstocks. 3.4. Improving environmental performances of corn derived ethanol by altering farming practices Environmental performances of corn derived ethanol could be improving through utilizing environmentally begin farming practices, as shown in Fig. 5. Previous studies have quantified the life cycle energy [7], global warming potential [7] and eutrophication potential [52] for corn derived ethanol. Their results found that the corn farming stage was the biggest contributor for eutrophication potential category and shared around 32.8%e77.2% of overall eutrophication potential. Meanwhile, the corn farming stage consumed less than 1.6% of total energy and generated around 1.5%e6.3% of global warming potential. Based on the results from this study, the influences of changing farming practices in Corn Belt on environmental performances of corn-derived ethanol were estimated. Replacing synthetic fertilizers by manure could reduce around 5.4% of total energy consumption during entire ethanol life cycle. Installing buffer strips could mitigate ethanol’s global warming potential and eutrophication potential, respectively, by 43.1% and 68.2%. Shifting conventional tillage to no tillage practice could decrease the eutrophication potential by around 43%. The impact of altering fertilizer types and tillage practices on total global warming potential of ethanol is less than 4.4%, because corn farming stage has minimal impacts on global warming potential of ethanol. The results indicated that adopting no tillage practices and installing buffer strips could significantly reduce ethanol’s eutrophication potential, while reusing manure from animal raising

Fig. 4. Environmental impacts of buffer strips. Environmental impacts of buffer strips. Median values of buffer strips’ carbon sequestration, nutrient removal capabilities are estimated through nutrient models.

Fig. 5. The potential of improving environmental impacts of corn derived ethanol by shifting farming practices in Corn Belt states. The reduction of corn derived ethanol’s life cycle energy consumption, global warming and eutrophication potentials via adopting no tillage, manure and buffer strip in biomass production stage.

system could decrease energy requirement for producing ethanol and increase the net energy output of corn derived ethanol. Best management farming practices could result in the reduction of the environmental impacts caused by the agriculture stage of corn ethanol production and consequently improve the environmental profiles of corn-derived biofuels and biobased products. However, there are practical difficulties to apply these environmentally preferred practices. Corn farmlands are tiled in the Midwest, and buffers tend to be ineffective in removing nutrients in these areas [21]. Potential economic loss resulted from retiring farmlands and installing buffer strips may discourage farmers to use buffer strips. Although the price of manure is relatively cheap, transporting manure from storage room to farmlands is relatively expensive [44]. Besides expensive transportation fee, difficulties related to manure handling are also reported as barriers of manure application. In addition, this article only investigated the influences of three types of farming practices (tillage types, fertilizer types, the use of buffer strips) on environmental impacts of corn farming. The impacts of other important farming practices (such as crop rotation, fertilizer application techniques, etc.) should be researched to aid policy decision in the future.

4. Conclusion This study represents comparative environmental impacts of alternative farming practices. The farming practices analyzed included fertilizer types, tillage practices, use of buffer strips. Furthermore, this study assessed the influences of changing fertilizer types, switching tillage practices, installing buffer strips on environmental impacts of corn. Altering conventional tillage to no tillage and using buffer strips can lower both global warming potential and eutrophication potential of corn from life cycle perspectives. Although shifting synthetic fertilizers to manures can lower energy consumption, this shift may increase eutrophication potential of corn. Integrated application of farming practices are highly needed to mitigate climate change and eutrophication impacts resulting from agricultural production. Optimized farming management methods are recommended to adjust agricultural C, N, P cycles and improve environmental profiles of corn derived ethanol. The life cycle environmental footprint of corn derived products can vary depending on the method of cultivation. This paper provides a comprehensive life cycle inventory for several agricultural management practices that can be used in life cycle assessment of agricultural products.

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