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Comparative environmental performance of three different annualenergy crops for biogas production in Northern Italy

Sara González-García a,*, Jacopo Bacenetti b, Marco Negri c, Marco Fiala b, Luís Arroja a

aCESAM, Department of Environment and Planning, University of Aveiro, 3810-193 Aveiro, PortugalbDepartment of Agricultural Engineering, University of Milan, Milan, ItalycDepartment of Plant Production, University of Milan, Milan, Italy

a r t i c l e i n f o

Article history:Received 6 July 2012Received in revised form31 October 2012Accepted 14 December 2012Available online 27 December 2012

Keywords:DigestateLife Cycle Assessment (LCA)Maize (Zea mays L.)Po ValleyTriticale (� Triticosecale Wittmack)Wheat (Triticum spp. L.)Energy crops

a b s t r a c t

Bioenergy is becoming increasingly important for agriculture. However, the use of first generation cropsfor biofuels and bioenergy production is controversial due to their competition with food and feed. Thecultivation of three different energy crops e wheat, maize and triticale e for biomass in order toproduce biogas in Lombardy, in the Po Valley (Italy), was evaluated in this study to quantify theirenvironmental profiles and identify the best biomass source from an environmental perspective. Thechoice of these cropping systems was based on the fact that they are well-known and extensivelycultivated for energy purposes in Italy. The standard framework of the Life Cycle Assessment (LCA) wasfollowed in this study and detailed inventories for these crops were designed. The environmental profilewas analysed in terms of abiotic depletion, acidification, eutrophication, global warming, ozone layerdepletion, photochemical oxidants formation, human toxicity and ecotoxicity. In addition, an energyanalysis was performed using the cumulative energy demand method. According to the results, theselection of the best biomass source depends on several factors such as the functional unit and biomassyield. Therefore, a sensitivity assessment was done in order to identify these differences. Moreover, themost critical processes throughout the life cycle of the cropping systems were identified andimprovement alternatives were proposed, specifically for the mineral fertilization (one of the mostimportant hot spots). Thus, different scenarios built on alternative nitrogen based fertilizers wereassessed in detail and discussed, resulting in the identification that the use of calcium ammoniumnitrate instead of urea should improve the environmental profile regardless of the energy crop. Finally,the combination of triticale or wheat with maize classes 300, 400 and 500 in rotation systems was donein order to achieve similar biomass yields, per ha, to the maize classes 600 and 700, which were alsoevaluated. The best results were obtained for maize classes 600 and 700 regardless of the functional unitconsidered in all the categories assessed except in GWP, where triticale with maize 400 and with maize500 were the best options.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The use of renewable sources for energy production is consid-ered a potential solution to reduce the environmental problemsderived from fossil fuels (Gomez et al., 2011; Benoist et al., 2012;Uchida and Hayashi, 2012). Since the energy crisis of the 1970s, theinterest in the production of biomass for energy purposes hasincreased considerably in Europe (Walle et al., 2007; Guidi et al.,

2009; Mizey and Racz, 2010), along with the concern for theconsequences of the combustion of fossil fuels on climate changeand oil prices. Energy crops and the corresponding derived bio-energy production are expected to bring environmental, social andeconomic benefits (Fazio and Monti, 2011). In addition, biofuelsrepresent a market opportunity for European agriculture in pros-pect of the 2012 Common Agricultural Policy reform, and a poten-tial way to improve local economies (Benoist et al., 2012).Numerous authors have reported benefits in terms of the reductionof greenhouse gas emissions, air pollution, acidification andeutrophication (Buratti and Fantozzi, 2010; Fazio and Monti, 2011;González-García et al., 2011). Nevertheless, the environmentalimpacts strongly depend on the crop cultivation (Fazio and Monti,2011; Uchida and Hayashi, 2012).

* Corresponding author. Tel.: þ351 234370349.E-mail addresses: sara.gez.garcia@gmail.com, sara.gonzalez@usc.es

(S. González-García).

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In recent years the production of bioenergy on a large scale hasfocussed on the energy efficiency associated with different agri-cultural systems (Gelfand et al., 2010). Remarkable amounts offossil energy are required in modern agricultural practices in theform of fertilizers, pesticides and fuel for agricultural equipment(Gelfand et al., 2010). Moreover, crop cultivation involves land-usechanges whose direct and indirect effects can be great enough tonegate the positive effect of shifting from conventional fossil fuelsto bioenergy (Uchida and Hayashi, 2012). In this context,improvements are being performed on the cultivation and breedingpractices (Gelfand et al., 2010; Uchida and Hayashi, 2012).

The use of first generation crops for biofuels and bioenergyproduction (e.g. maize, sugar beet, sunflower or wheat) is contro-versial due to their competition with food and feed resulting inincrements in food/feed prices as well as biodiversity losses anddeforestation (Gomez et al., 2011). Yet, they are now being usedsince second and third generation bioenergy production systemsare still under development (Gomez et al., 2011; Uchida andHayashi, 2012). Currently, all commercial production of biofuelsuses only first generation technologies and the role of the secondand third generation biofuels will depend on factors such as thederived costs, as well as agricultural and climate-change policies(Gomez et al., 2011).

The promotion of energy crops in Northern and Central Euro-pean countries began with the energy crisis of the 1970s. However,in Southern countries, such as Italy, the promotion of bioenergy ismore recent and still moderate (10% of global energy consumptionby renewable energy). Italy’s interest in increasing the contributionof agro-energy due to the demand of national energy has surged inrecent years and new incentive-providing instruments such as theRenewable Energy Certificates (Gasol et al., 2010), specificallyfocussed on electricity, have been approved.

Anaerobic digestion and biogas production are promisingmeans of producing an energy carrier from renewable sources andof achieving environmental benefits (Berglund and Börjesson,2006). In the European Union the biogas production was esti-mated at about 5901 � 103 toe (tonne of oil equivalent) for 2007,406 � 103 toe of which were produced in Italy (Piccinini, 2008).Agricultural rawmaterials such as straw andmanure are commonlyconsidered for biogas production. Moreover, the use of the diges-tate as a fertiliser on arable land can improve its derived environ-mental profile. In Italy there are now 520 agricultural biogas plants(210 located in Lombardy) with a global power of 156 MWel.

The cultivation of three different energy crops e wheat, maizeand triticale e for producing biomass into biogas in Lombardy, inthe Po Valley (Italy), was assessed in this study in order to quantifytheir environmental profiles. The general approach of this study isto gather the inventory data for the production of these energycrops, quantify their environmental effect as well as to identify the“hot spots”, and to discuss some possible improvement opportu-nities due to the relevance of these crops in the Italian biogasproduction. Likewise, it is also to identify the best productionmanagement system in order to obtain biomass for energypurposes with the best quality and yield.

2. Materials and methods

The Life Cycle Assessment (LCA) is a quantitative procedure toevaluate the environmental burdens associated with a product andto identify opportunities to attain environmental advantages. Withthis aim, an LCA study identifies the consumption of naturalresources and the emissions to environmental compartmentsassociated with the life cycle of the product (ISO 14040, 2006). TheLCA, following the principles of the relevant ISO standards (ISO14040, 2006), was used herein to evaluate the energy crops

systems under assessment e wheat, maize and triticale e from anenvironmental perspective. Thus, the present work constitutes anattempt to determine the life cycle environmental impacts of threedistinct energy crops by trying to identify and quantify all relevantinputs and outputs.

2.1. Goal and scope definition

The goal of this study was to evaluate and compare (cradle-to-gate approach) the environmental performance of three energycrops, whose biomass should be dedicated to biogas production.Specific objectives included the identification of the most criticalstages (environmental hot spots) throughout the life cycle and theidentification of the cultivation system with higher environmentalbenefits. In order to study the impacts of these Italian crops, threeagricultural scenarios were constructed, based on records fromplantations located in the Lombardy Region in the Po Valley (45�120

long. N, 10�150 lat. E).According to ISO standards, the functional unit (FU) is defined as

a quantified performance of a product system to be used asa reference unit in an LCA (ISO 14040, 2006). In this study, the FUfor each agricultural scenario was defined as one tonne of biomassobtained under the best management practices and destined tobiogas conversion. The selection of this FU is in agreement withother related LCA studies where energy crops were studied(Brentrup et al., 2004a; Iriarte et al., 2011;Wang et al., 2007; Uchidaand Hayashi, 2012).

2.2. Description of the cropping systems under assessment

The energy crops under study included two winter crops whichwere wheat (Triticum spp. L.) and triticale (� Triticosecale Witt-mack) and one summer crop, maize (Zea mays L.). In addition, forthe maize study, five different classes (maize 300, 400, 500, 600and 700) were also considered. All cultivations are located in theLombardy and Po Valley regions of Northern Italy. The climate inthis area is a transition between the Mediterranean climate and theCentral European climate, with rainfall mainly concentrated in thefall and spring and a medium annual temperature of 12 �C. Theaverage annual precipitation in the area is 745 mm. The maintexture of the soil is: 52% sand, 30% silt and 17% clay.

The information concerning the different crops was takendirectly from three farms dedicated to their extensive production;average data were considered:

� The Muraro farm located in Maleo, in the district of Lodi, wasassessed in detail for the cultivation of triticale, wheat, maize300, maize 400 and maize 500.

� The Dotti farm also located in the district of Lodi suppliedinformation for maize 600 and maize 700.

� The Eurosia farm located in Formigara, in the district of Cre-mona, supplied information regarding triticale, wheat maize300, maize 400 and maize 500.

The analysed agricultural systems dedicated to the productionof biomass for biogas can be divided in three main stages: fieldpreparation, crop growth and biomass harvesting. All these stagesincluded different processes which have been identified in detail inthe three agricultural systems. Fig. 1 shows the scheme of triticaleand wheat systems (8 months per year) and maize systems (6months per year) with the corresponding processes involved. Someminor differences were identified in the agricultural processesinvolved for the different classes of maize under assessment (300,400, 500, 600 and 700). These differences are related to the doses of

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fertilizers and herbicides applied, the amount of seeds required,irrigation and diesel fuel requirements.

The life cycle of each agricultural process is included within thesystem boundaries. This life cycle considers rawmaterials extraction(e.g. fossil fuels andminerals),manufacture (e.g. seeds, fertilizers andagricultural machines), use (diesel fuel consumption and derivedcombustion and tyre abrasion emissions), maintenance and finaldisposal of machines and supply of inputs to the farm (e.g. fertilizersand herbicides). The agronomic inputs for each crop growth areshown in Tables 1 and 2, where the features of specific agriculturalmachines commonly used for these crops are summarized.

2.2.1. Wheat and triticale cropping systemsWheat and triticale are both winter crops which are particularly

well suited to the Po Valley. Triticale is a hybrid of wheat and rye,which incorporates a high yield potential and quality, as well as theadaptation to awide rangeof soil types andenvironments.Wheat andtriticale can be used as feed for animals (e.g. cows and poultry) eitheras source grainor fodder. Furthermore, in the cropping systemsunderstudy, the harvesting was performed before the end of the grain

ripening and all thebiomasswasharvested together. Before sowing inboth crops, soil was previously fertilized with digestate of 40 t ha�1.After that, the soil was ploughed and harrowed. The digestate camefrom a nearby biogas plant to which the biomass was delivered. Thedigestate applied presented 94.6% moisture and an average compo-sition of 0.40 kg total N 100 kg�1 digestate, 0.20 kg NeNH4 100 kg�1

digestate, 0.08 kg P 100 kg�1 digestate and 0.31 kg K 100 kg�1

digestate. The digestate was applied in September when averagemaximumtemperature is29 �Candaverageminimumtemperature is8.4 �C.Rainfall (1mm)occurred thedayafter thedigestate applicationand no rain occurred after that. All this climatic information isimportant since incorporation of organic fertilizer into the soil orrainfall after application leads to a reduction of the maximumpotential NH3 loss. In addition, air temperature is an importantparameter for the NH3 volatilization (Brentrup et al., 2000). Thesowing was performed in October at a density of 35,000 seeds ha�1.After the sowing, chemical weed control was employed by sprayingterbutilazine and alachlor at a total rate of 5 kg ha�1 once.Mechanicalweed control was also carried out. Moreover, fertilizers were appliedin two steps. The first fertilizationwas carried out themonth after the

Fig. 1. Scheme of the system boundaries for the cultivation systems of wheat, triticale and maize under assessment, dedicated to the production of biomass for bioenergy.

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sowing. It consisted on the application of ammonium nitrate at a rateof 60 kg ha�1. The second fertilizationwas in February with urea anda dose of 60 kg ha�1. Then the biomass (straw and grain) was har-vested and chopped simultaneously in May by a self-propelledforager that loaded the biomass into trucks to be delivered up to thebiogas plant gate. Although the cultivation system was exactly thesame for both crops, the biomass yield and moisture content arelightly different being 14 tdry basis ha�1 (62% moisture) and 12 tdrybasis ha�1 (68% moisture) for the triticale and wheat, respectively.

2.2.2. Maize cropping systemsMaize is a summer crop which yields grain and stover. It can be

used as animal feed and nowadays plays an important role asapotential candidate for ethanolproduction (Oleskowicz-Poliel et al.,2008) not only by converting the grain (starchymaterial) but also thestover (lignocellulosic biomass) into ethanol (Luo et al., 2009). In this

study, five different classes of maize were cultivated in the Po Valleyand like thewheat and triticale, themaize silage (i.e. the whole cropof the maize plant harvested at about 10e20 cm above the groundincluding the stem, leaves and cobs) was harvested.

Soil was previously fertilized with digestate at a rate of 45 t ha�1

for the maize classes 300, 400 and 500 and at a rate of 85 t ha�1 forthe maize classes 600 and 700. After that, the soil is ploughed,harrowed, sown and controlled with herbicide (Lumax, 4 kg ha�1).All these processes were performed inMay. The rate of seeds sowedranges from 70,000 to 77,000 seeds ha�1, depending on the classesof maize (Table 2). In addition, a mineral fertilization with potas-sium and phosphorous based fertilizers was required for maizeclasses 300, 400 and 500 between the ploughing and harrowingsteps. In June, therewas chemical weed control with s-metolachlor,hoeing and amineral fertilizationwith urea. Moreover, an irrigationstep was required prior to the hoeing for the cultivation of maize

Table 2Field operations timeline and inventory data (per ha) for the cultivation of maize (classes 300, 400, 500, 600 and 700) in plantations located in the Lombardy Region, Po Valleyarea (Italy).

Operation Time(month)

Tractor (A) Implement (B) A þ B Input rates

Weight andpower

Tillage item Weight(kg)

Power(kW)

Effectiveworkcapacity(ha h�1)

Fuel dieselconsumption(kg ha�1)

Organicfertilization

May 6500 kg 120 kW Manure spreader 2500 40 2,5 4,71 Digestatea

2,2 5,36 Digestateb

Ploughing May 8500 kg 150 kW Ploughshare 2000 80 0,8 22,64 e

MineralFertilizationc

May 5050 kg 90 kW Fertilizer spreader 350 50 3,0 3,17 100 kg P ha�1 100 kg K ha�1

Harrowing May 5050 kg 90 kW Rotary harrow 1000 55 0,5 24,24 e

Sowingd May 6500 kg 120 kW Precision seed drill 900 70 1,5 10,27 Seeds m�2

Herbicide control May 4580 kg 80 kW Spraying machine 500 45 3,0 3,32 Lumaxe 4 kg ha�1

Herbicide control June 4580 kg 80 kW Spraying machine 500 45 3,0 3,32 Dual (S-metolachlor) 1 kg ha�1

Herbicide controlf June 4580 kg 80 kW Spraying machine 500 45 3,0 3,32 Dual (S-metolachlor) 1 kg ha�1

Irrigationc June e e e e e e 400 m3 ha�1

Hoeing June 4580 kg 80 kW Hoeing machine 550 25 3,0 2,75 e

MineralFertilization

June 5050 kg 90 kW Fertilizer spreader 350 50 3,0 3,17 Ureac 200 kg ha�1 7 60 kg ha�1

Irrigation July e e e e e e 2 � 800 m3 ha�1

Irrigation August e e e e e e 800 m3 ha�1

Harvestingh September 14,000 kg 363kW

Combine harvester e e 2,5f 27,17f e

2,0g 33,96g

a Maize Classes 300, 400 and 500: 45 t ha�1; Digestate composition: 5.36% dry matter; 0.4% total N, 0.2% NeNH4, 0.080% P, 0.314% K.b Maize Classes 600 and 700: 85 t ha�1; Digestate composition: 5.36% dry matter; 0.4% total N, 0.2% NeNH4, 0.080% P, 0.314% K.c Maize Classes 300, 400 and 500 cultivation.d Maize Classes 300, 400 and 500: 7.7 seeds m�2; Maize Class 600: 7.5 seeds m�2; Maize Class 700: 7.0 seeds m�2.e Lumax is a selective herbicide with S-metolachlor, atrazine and mesotrione as main active ingredients.f Only on Maize Classes 600 and 700 cultivation.g Maize Classes 300, 400 and 500 cultivation.h Maize Class 300 yield: 44 twet basis ha�1 (15 tdry basis ha�1); Maize Class 400 yield: 35 twet basis ha�1 (17 tdry basis ha�1); Maize Class 500 yield: 50 twet basis ha�1 (19 tdry

basis ha�1); Maize Class 600 yield: 81 twet basis ha�1 (29.3 tdry basis ha�1); Maize Class 700 yield: 90 twet basis ha�1 (26.0 tdry basis ha�1).

Table 1Field operations timeline and inventory data (per ha) for the cultivation of wheat and triticale in plantations located in the Lombardy Region, Po Valley area (Italy).

Operation Time (month) Tractor (A) Implement (B) A þ B Input rates

Weight and power Tillage item Weight(kg)

Power(kW)

Effectivework capacity(ha h�1)

Fuel dieselconsumption(kg ha�1)

Organic fertilization September 6500 kg 120 kW Manure spreader 2500 40 2,5 4,71 Digestatea 40 t/haPloughing September 8500 kg 150 kW Ploughshare 2000 80 0,8 22,64 e

Harrowing September 5050 kg 90 kW Rotary harrow 1000 55 0,5 24,24 e

Sowing October 6500 kg 120 kW Seeder 800 50 0,7 8,53 350 seeds$m�2

Herbicide control October 4580 kg 80 kW Spraying machine 500 45 3,0 3,32 Terbutilazine þ Alachlor 5 kg ha�1

Mineral fertilization November 5050 kg 90 kW Fertilizer spreader 350 50 3,0 3,17 Ammonium nitrate 60 kg ha�1

Mineral Fertilization February 5050 kg 90 kW Fertilizer spreader 350 50 3,0 3,17 Urea 60 kg ha�1

Harvestingb May 14,000 kg 363 kW Combine harvester e e 2,5 27,17 e

a Digestate composition: 5.36% dry matter; 0.4% total N, 0.2% NeNH4, 0.080% P, 0.314% K.b Triticale yield: 37 twet basis ha�1 (14 tdry basis ha�1); Wheat yield: 38 twet basis ha�1 (12 tdry basis ha�1).

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classes 300, 400 and 500. After the fertilization in summer months(July, August), irrigation was done (2400 m3 ha�1). The harvestingof the maize silage was performed in September with self-propelled forages. Large differences were obtained on thebiomass yield and moisture content among the different classes ofmaize, being 15 tdry basis ha�1 (66% moisture), 17 tdry basis ha�1 (51%moisture), 19 tdry basis ha�1 (62% moisture), 29.3 tdry basis ha�1 (64%moisture) and 26 tdry basis ha�1 (71% moisture) for the maize classes300, 400, 500, 600 and 700, respectively.

2.3. Data acquisition

The Life cycle inventory data for the foreground system (directagricultural inputs) such as primary and site-specific data (infor-mation concerning tractors and implements, labour hours andinput rates) for all case studies were mainly collected directly onthe farms previously mentioned by means of surveys and inter-views with growers (Tables 1 and 2). Secondary data regarding theproduction of the different agricultural inputs, such as nitrogenbased fertilizers (urea and ammonium nitrate), phosphorus andpotassium based fertilizers, and herbicides (terbutilazine, alachlor,lumax and S-metolachlor) were taken from the Ecoinvent database(Althaus et al., 2007).

Digestate production was not included within the systemboundaries, only its application and diffuse emissions derived fromits application. The digestate, a very useful organic fertilizer, is a co-product derived from the anaerobic digestion of a biodegradablefeedstock where the main product is the biogas. Thus, all theenvironmental burdens derived from the production process wereallocated to the biogas.

Field emissions from fertilizers (mineral and organic) wereestimated according to Brentrup et al. (2000). The nutrient emis-sion rates vary according to the soil type, climatic conditions andagricultural management practices (Brentrup et al., 2000). Nitrogenbased diffuse emissions were calculated according to the emissionfactors proposed by Audsley et al. (1997), Brentrup et al. (2000) andthe EMEP/CORINAIR (2006). Phosphate emissions into the waterthrough run-off were also taken into account at a ratio of 0.01 kg PePO4

�3 kg�1 of applied P (Iriarte et al., 2011).Regarding herbicides, no direct emission from these chemicals

application was taken into account due to the absence of datarequired to calculate the fractions of applied herbicides that reach theenvironmental compartments (air, surface water and groundwater).

Combustion emissions from diesel fuel used in the agriculturalmachines (ploughing, harrowing, sowing, organic and mineralfertilizers application, hoeing, irrigation, harvesting, and loadingthe biomass), as well as from agricultural inputs supply up to theplantations (3.5 t van and 16e32 t trucks) were also considered andacquired from the Ecoinvent database (Nemecek and Käggi, 2007;Spielmann et al., 2007).

Carbon stored by the biomass was estimated considering thecarbon content in the dried biomass. The average carbon content(expressed in drymatter) assumed for the different cropswas 40.2%for wheat, 42.1% for triticale and 41.5% for maize.

Zero changes in the overall soil carbon content were assumed inthis study for these cropping systems regime, due to its specificdependency on factors such as management practices, climate,previous cropping regime and the limitation of available data forthese crops and Italian conditions.

Allocation of the environmental burdens in multi-outputprocesses is one of the most critical issues in the LCA. In thesecradle-to-gate assessments, allocation was not required since onlythe production of the total biomass or silage (stem, leaves and cobs)to be delivered to an Italian biogas plant was considered. Therefore,all environmental effects were allocated to that amount of biomass.

3. Impact assessment results and discussion

3.1. Environmental results for the cropping systems

Only the classification and characterization steps defined withinthe life cycle impact assessment stage of the standardized LCAmethodology were undertaken here (ISO 14040, 2006). Normali-zation and weighting were not conducted as these are optional(and, to some extent, subjective) elements considered to provideadditional feeble information for the objectives established in thisstudy. The characterization factors reported by the Centre ofEnvironmental Science of Leiden University (CML 2001 method)were used (Guinée et al., 2001). The impact potentials (or impactcategories) evaluated according to the CML method were: abioticdepletion (ADP), acidification (AP), eutrophication (EP), globalwarming (GWP), land competition (LC), ozone layer depletion(ODP) and photochemical oxidant formation (POFP). Furthermore,an energy analysis was carried out based on the cumulative non-renewable fossil and nuclear energy demand (CED) computedaccording to Hischier et al. (2009) as an additional impact category.

In addition, toxicological impact categories (human toxicity(HP), fresh water aquatic ecotoxicity (FEP), marine aquatic eco-toxicity (MEP) and terrestrial ecotoxicity (TEP)) were analysed. Thesoftware SimaPro 7.3 was used for the computational imple-mentation of the inventories (Goedkoop et al., 2008).

3.1.1. Global analysisFig. 2 shows the comparative environmental characterisation

results for the different scenarios under assessment.According to these results, the maize class 400 showed the

highest impacts per tonne of wet biomass harvested among thedifferent energy crops assessed in almost all the categories, exceptfor the GWP, where it showed the best profile, and for the TEP andLC where the triticale and wheat dominate the contributions. Thegood result achieved by the maize class 400 in the GWP wasaffected mainly by the carbon dioxide sequestered by the biomassduring its growth, and is fixed in the biomass. The field emissionsderived from the application of fertilizers and greenhouse gas(GHG) emissions derived from the use of diesel fuel in the agri-cultural labors and inputs supply. Maize class 700 showed the bestresult in almost all the impact categories (ADP, AP, EP, ODP, POFP,HP, TEP, LC) with improvements in comparisonwith themaize class400 ranging from 68% to 82% depending on the category. Triticaleand wheat are also good alternatives in terms of FEP, MEP and CED.The satisfactory result achieved for both crops in terms of the CED(improvement of 76% in comparison with maize class 400) can beexplained by the lower agronomic input requirements in compar-ison with maize cropping systems. Maize class 700 also showed

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ADP AP EP GWP ODP HP FEP MEP TEP LC POFP CED

Co

mp

arative resu

tls (%

)

Maize-Class 300

Maize-Class 400

Maize-Class 500

Maize-Class 600

Maize-Class 700

Triticale

Wheat

Fig. 2. Comparative environmental results for the energy crops under assessment:wheat, triticale and maize (classes 300, 400, 500, 600 and 700).

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a reduction of 72% in terms of energy requirements (in comparisonwith the maize class 400) which is mainly due to the high biomassyield in comparison with other maize classes.

The cropping management system was exactly the samebetween wheat and triticale. However, the yield is a little lower fortriticale than for wheat (37 twet basis ha�1 vs 38 twet basis ha�1). It isslightly influenced by some factors (e.g. AP, ODP or ODP) wherefaint differences (around 1%) were identified between the findingsfor triticale and wheat systems. In contrast, in terms of dry basis,the triticale presents the highest yield (2 tonnes more per ha). Inthis study, the FU was established as one tonne of wet biomassharvested and ready to be delivered to the biogas plant. Thus, theselection of the FU (1 ha, one tonne of dry biomass and one GJ ofenergy content) can influence factors and sensitivity assessments ofthe environmental results for the various cropping systems underevaluation; they have been performed and discussed in this study.

3.1.2. Maize classes 300, 400 and 500 analysisFig. 3a displays the contributions from the different factors

involved throughout the life cycle of these crops. As shown inTable 2, the agricultural activities and the agrochemical require-ments are the same for all of themwith the only difference attainedin the biomass yield and corresponding moisture content. Themaize class 400 presented the lowest yield (in wet basis) and, forthis reason, the global environmental profile is the worst.

Fig. 3a shows the importance of the agricultural activities to besignificant specifically in terms of ADP, ODP, POFP, CED and alltoxicity categories with contributions of up to 85%. These results aremainly due to the diesel fuel requirements in the agriculturalmachineries and derived combustion emissions. Within thedifferent activities, the irrigation steps dominate the operation. Insitu field emissions, derived from the application of the fertilizersinto the soil, contribute considerably to the AP and EPmostly due tothe ammonia (77% of total acidiphying emissions and 67% of totaleutrophying emissions) and phosphate emissions (19%). In relationto the GWP, the uptake of carbon dioxide by photosynthesis offsetsthe GHG emitted throughout the cultivation system. Finally, theoccupation of arable land for themaize cultivation is responsible for94% of contributions to the LC. The production of mineral fertilizers,specifically urea, also had a critical role in categories such as ADP,ODP, POFP and CED due to the associated high energy require-ments. Seed production contributed to 8% of the TEP due tocypermethrin derived emissions.

3.1.3. Maize classes 600 and 700 analysisFig. 3b shows the contributions to each category for the different

processes involved in the cropping systems of maize classes 600and 700. As with the previously assessed crops, the management ofmaize 600 was similar to that of maize 700. According to Table 2,distinctions are evident in the amount of seeds sown and thebiomass yield. Concerning this last point, maize class 700 yields 11%more biomass (wet basis) per ha than the class 600. However, theyield is 13% lower if it is expressed in dry basis.

Regarding the identification of the main hot spots, once again,the agricultural efforts are the main cause of the results for cate-gories such as ADP, ODP, POFP, CED and toxicity. They are alsoimportant in AP and EP due to nitrogen oxides and sulphur dioxideemissions derived from the combustion of diesel fuel. Within thedifferent activities, irrigation and harvesting operations were themain hot spots. Again, field emissions derived from fertilizersapplication were important in AP and EP (mostly ammonia,nitrogen oxides and phosphate). Maize seed production was a hotspot in TEP (10%).

A remarkable difference was identified between maize classes600 and 700, and maize classes 300, 400 and 500: the contribution

from fertilizer production. Maize classes 600 and 700 only requirethe application of urea and in a lower dose (60 kg ha�1 vs200 kg ha�1). For this reason, its impact on the global environ-mental profile was considerably minor (Fig. 3a and b).

3.1.4. Triticale and wheat analysisFig. 3c displays the contributory factors from the different

processes involved in the cultivation of both triticale and wheat.

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According to Table 1, though the cultivation system is the same forboth crops, the only difference is in the biomass yield as the triticaleis a hybrid species derived from wheat and rye which affects itsyield. The trend of the results is similar to the other studied systemsand agricultural activities; mineral fertilizers production and in situfield emissions were the main responsible for the environmentalweight. In wheat and triticale, two different nitrogen based fertil-izers were applied: ammonium nitrate and urea. Their production,considerably influenced categories such as ADP, ODP, POFO, CEDand other toxicity related categories. Both fertilizer productionprocesses are intensive energetic systems. In addition, these cropsdo not require irrigation and thus, the most important agriculturalactivity is the harvesting with forager. As regards to the fieldemissions originating from the application of mineral fertilizers,ammonia was responsible for 76% and 64% of the AP and EP,respectively. In these crops, the contribution from seeds productionto the TEP (70%) is important mainly because of isoproturonemissions.

3.2. Discussion of the alternative scenarios

The replacement of fossil fuels with biomass for energyproduction is an important strategy promoted by the EU tomitigatefossil CO2 emissions, as well as to preserve the future availability offossil fuels (von Blottnitz and Curran, 2007; Mizey and Racz, 2010).Within the implementation of this European bioenergy programmefor large-scale power and liquid biofuels generation, it is essentialthat the production of the feedstock is carried out under sustain-able conditions. The LCA is a methodology which is not free ofuncertainty and variability, but it is a useful and comprehensivetool for assessing and characterizing the environmental impactsderived from biofuels and energy crops production. Thus,numerous authors have used the LCA methodology in studieswhere a huge range of bioenergy systems had to be evaluated indetail (Luo et al., 2009; Fazio and Monti, 2011; González-Garcíaet al., 2012a).

As reported by González-García et al. (2012b), over the last fiveyears numerous hectares of arboreous energy plantations has beenestablished in Italy (specifically in the Po Valley), the region underassessment in this paper. Although for these plantations the mainspecies grown is poplar, annual crops such as wheat, triticale andmaize are more widespread, and are the focus of special attentionfor bioenergy production.

3.2.1. Selection of the functional unitIn accordance with different authors (Kim and Dale, 2006;

González-García et al., 2009), there are different ways to define theFU and the influence of its choice on the environmental results isparticularly significant, especially when the results are going to beused as a decision support tool. The function of the croppingsystems under study is defined as producing biomass to be con-verted into biogas. Therefore, the selection of the FUwill depend onthe goals and must satisfy the function of the study. According to it,four different FUs can be proposed. As reported before, the FUchosen as base for the calculations was 1 tonne of biomass har-vested in the fields and ready to be delivery to the biogas plant. Thisbiomass is on a wet basis since the biomass was not dried at thefarm, only harvested and chopped. Fig. 2 shows the comparativeenvironmental results for the different energy crops and, accordingto these results, the maize class 400 is the worst option in all thecategories, except in terms of GWP, where it presented the bestenvironmental credit and in terms of TEC and LC, where the triticalepresented the highest environmental burdens.

As, three additional FUs were proposed in order to assess theirinfluence on the results. The alternative FUs handled were: one

tonne of dry basis biomass (a), 1 ha (b) and one GJ of energy content(c), measured in terms of the lower heating value (LHV). All of themare FU proposed in other related studies (Buratti and Fantozzi,2010; Fazio and Monti, 2011) and play important roles, mostlywhen crops with different purposes are compared (e.g. food cropsand energy crops) (Fazio and Monti, 2011). As displayed in Tables 1and 2, the moisture content and biomass yield are differentbetween maize, wheat and triticale and also between the differentclasses of maize (300e700). The yield ranges from 35 twet basis ha�1

up to 90 twet basis ha�1 for maize class 400 and maize class 700,respectively. Expressed on a dry basis, the biomass yield rangesfrom 12 t ha�1 to 29.3 t ha�1 for triticale and maize class 600respectively. Concerning the LHV, 15.91 Gt t�1 were for all thedifferent classes of maize and 17.17 Gt t�1 for the triticale andwheat(Ecoinvent database, 2012).

Fig. 4 displays the comparative environmental results for thedifferent alternative FUs. Taking into account the results presentedin Fig. 4a, the maize class 300 showed the highest environmentalburdens in almost all the categories.

The triticale should be the best option in terms of GWP, mainlyinfluenced by lower energy requirements. Once again, this crop hadthe highest contributions in TEP and LC due to the low dry basisbiomass production, in comparison with the remaining crops. Theresults should be considerably different if an FU based on landrequirement had been considered (Fig. 4b). In this case, the maizeclasses 300, 400 and 500 would be the worst options due to theirhigh contribution to all the categories except in GWP where themaize class 600 should present the highest environmental creditand in TEP and LC, where again the triticale should dominate thecontributions (followed close up by the wheat). Fig. 4c displays theresults per GJ of potential energy content in the biomass. Theresults tend to becomemore similar when expressed on the basis ofton of dry biomass (only slight differences can be found in ADP,GWP, TEP and LC and mainly in triticale and wheat croppingsystems). Thus, the choice of the best source of biomass to biogasproduction purposes from an environmental point of view dependson the FU assumed for the calculations.

3.2.2. Alternative to mineral fertilizerIn accordance to the results shown in Fig. 3, the production of

the mineral fertilizers and in situ field emissions derived fromfertilizers application are important environmental hot spots. In thecultivation of all these crops, an organic fertilization with digestatesupplied from the biogas plant was used and also considered in thesame amounts for all the proposed scenarios. The fertilizing wascompleted with nitrogen based fertilizers such as urea andammonium nitrate depending on the crop.

Numerous studies have identified nitrogen mineral fertilizationas an intensive process and one of the most energy demandingprocesses all over the cycle of energy dedicated biomass planta-tions (Di Candilo et al., 2010; Gasol et al., 2009; Brentrup et al.,2001). Thus, the usage of manure instead of mineral fertilizerscould contribute to the reduction of the environmental profile of allthese crops (González-García et al., 2012b). Alternative nitrogenbased fertilizers should be proposed with low ammonia volatili-zation rates (e.g. ammonium nitrate instead of urea). Thus, thecontributions to AP and EP could be considerably reduced(Brentrup et al., 2001, 2004a,b). By other hand, according to theresults from Di Candilo et al. (2010) for the poplar plantations, iforganic fertilization with manure is compared to industrial fertil-izing, the biomass yield can be increased up to 6%. Brentrup et al.(2001) assessed the production of sugar beet under differentforms of nitrogen based fertilization considering urea, calciumammonium nitrate (CAN) and urea ammonium nitrate (UAN).According to this study, the application of CAN involves higher

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biomass yield than when urea and UAN are used. Taking intoaccount these results and under the same nitrogen application ratio(kg N ha�1), the use of CAN showed the lowest environmentalimpact being the highest for the urea, regardless the biomass yield.Brentrup et al. (2004b) also assessed from the LCA perspective theproduction of winter wheat considering the application of differentnitrogen rates from ammonium nitrate which involved changes onthe environmental results and on the biomass yield. Herman andLal (2007) studied the production of maize comparing the use ofcattle manure and mineral fertilizer, reporting an increment of 68%of the maize yield under mineral fertilization.

Therefore, several “hypothetical” alternative fertilizationscenarios were evaluated in order to determine the possibility ofachieving reductions on the environmental profiles for the differentcrops under assessment.

The first hypothetical scenario was based on the use of cattlemanure instead of the nitrogen mineral fertilizers in the differentcrops and assuming a reduction on the biomass yield of 68% (worstcase). Table 3 displays the new dose of organic fertilizers requiredin order to supply the nitrogen requirements. The production ofthis cattle manure was excluded from the system boundaries as itwas considered a waste from farms.

In situ field emissions derived from the manure application (aswell as from the digestate) were also taken into account. Fig. 5displays the comparative results between the current agriculturalcropping systems and the hypothetical scenarios based on cattlemanure fertilization. According to these results and regardless ofthe crop under assessment, the use of manure does not improve theenvironmental profiles.

The main reason for these poor results is the reduction on thebiomass yield, since a reduction of 68% in the total biomassproduced was assumed for all the scenarios. Thus, the absence ofcontributions from the mineral fertilizers production (a remarkablehot spot, as previously mentioned) is almost negligible. It is alsoimportant to point out the highest ammonia volatilization due tothe use of cattle manure in comparison with the current scenarios.

Moreover, the cover fertilization with manure (and also withdigestate) presents notable problems: the nitrogen is not alwaysimmediately available for the crops. The amount of manure appliedwas really high because the nitrogen content was low and themachines for its distribution were completely different from thoseused for spreading out mineral fertilizer. Moreover, these machinescannot always be used without damage to the crop (soilcompressions, plants destruction, leaf contamination, etc.).

The second hypothetical scenario was based on the use ofalternative mineral fertilizers instead of urea, since this fertilizerhas a high NH3 volatilization rate e 15% (Brentrup et al., 2000) aswell as being one of the nitrogen based fertilizers with the highestenergy requirements for its production (Ecoinvent database, 2012).The maize classes 300, 400 and 500 requires 200 kg N ha�1 in theform of urea; the maize classes 600 and 700 requires 60 kg N ha�1

and the triticale and wheat requires 60 kg N ha�1 in the form ofurea and 60 kg N ha�1 in the form of ammonium nitrate. Changeson the ammonium nitrate were not considered here as it is one ofthe nitrogen fertilizers with the lowest NH3 volatilization rate (2%).

From an environmental point of view, the substitution of ureawith other nitrogen fertilizers, like CAN and UAN, could be inter-esting due to their low volatilization rate. It is worthy of note that,for the present, it is unlikely that farmers will substitute urea withother nitrogen fertilizers with a lower nitrogen percentage and, theItalian agriculture, these are little known.

The alternatives proposed for the urea were:

� Scenario A: use of ammonium nitrate (AN) instead of urea andmaintenance of the biomass yields.

� Scenario B: use of CAN (volatilization rate of 2%) instead of ureaand increment of 8% on the biomass yield with regard to thecurrent scenarios for all the crops (Brentrup et al., 2001).

� Scenario C: use of UAN (volatilization rate of 8%) instead of ureaand reduction of 2.7% on the biomass yield with respect to thecurrent scenarios (Brentrup et al., 2001).

Table 4 displays the comparative results obtained in the hypo-thetical Scenario A for each cropping system.

The use of AN instead of urea involves improved results,regardless of the crop, in all categories except on the toxicity relatedones. These results are related to the lowest energy requirements inthe AN production process which is reflected on the ADP, CED andODP results, as well as due to the lowest nitrogen emissions derivedfrom fertilization which is reflected on the AP and EP. Moreover, onthe GWP there is a lower environmental credit when AN is applied.Although the AN presents a lower ammonia volatilization rate, itsproduction involves up to 165% more GHG emissions than ureamainly in form of N2O emissions, a potential GHG. Concerning thetoxicity related categories, the AN production encompasses thehighest toxicity contributing substances such as nickel and vana-dium derived from the ammonia requirement. It is also remarkablethat the reductions achieved in the environmental categories formaize classes 300, 400 and 500, as well as for triticale and wheat,are higher than for maize classes 600 and 700 (Table 4). This ismainly due to the highest contribution to these categories fromfertilizer production (Fig. 3), as well as the dose of nitrogen fertil-izer applied, which is also higher for these crops.

Table 5 summarizes the comparative results between thecurrent cropping systems and the hypothetical scenario B, based onthe substitution of urea by CAN and assuming an increment of 8% inthe biomass yield for all the crops.

Once again, the results for the hypothetical cropping systemsare displayed in terms of the percentage of change relative to thecorresponding current scenario. According to the results, theshifting from urea to CAN involves environmental reductions inalmost all the categories under assessment regardless of the crop-ping system. Crops such as maize classes 300, 400 and 500, as wellas triticale and wheat presented minor increments (lower than 1%),on the toxicity related categories (in HP, FEP and MEP for the firstthree ones, and in TEP and MEP for the last ones). Although theamount of nitrogen fertilizer remained the same, the contributionto the environmental profiles (specifically in categories such asADP, ODP and CED) from fertilizer production is low due to the CANproduction requiring less energy than urea. Concerning the in situemissions derived from the CAN application, these are also reduceddue to the lowest volatilization rate in comparison with urea.Therefore, the contributions to categories considerably affected bythese emissions such as AP and EP are lowaswell. Nevertheless, theproduction of CAN involves more acidifying and eutrophyingemissions throughout the production process (mainly of ammonia)

Table 3Fertilization rates for the assessed cropping systems.

Crop Mineral fertilizer Rate Cattle manurea

Maize class 300 Urea 200 kg N ha�1 44.4 tMaize class 400 Urea 200 kg N ha�1 44.4 tMaize class 500 Urea 200 kg N ha�1 44.4 tMaize class 600 Urea 60 kg N ha�1 13.3 tMaize class 700 Urea 60 kg N ha�1 13.3 tTriticale Ammonium

nitrate þ Urea120 kg N ha�1 26.7 t

Wheat Ammoniumnitrate þ Urea

120 kg N ha�1 26.7 t

a Density : 400 kg m�3, composition: 4.5 kg N t�1 (http://www.aqua-calc.com/page/density-table/substance/Manure [accessed May, 2012]).

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due to the production of the required nitric acid. Therefore,although there is an increment on the biomass yield (8%), there isa small reduction (0.4e4.2% depending on the crop) on the envi-ronmental credit in terms of GWP.

Table 6 shows the comparative results considering the hypo-thetical scenario based on the substitution of urea by UAN.

The application of this alternative crop involves lower rates of insitu nitrogen based emissions since the volatilization rate for UAN is8% (15% for urea). In the biomass yield a reduction of 2.7%, relativelyto the current scenarios, was assumed. This reduction value wastaken from Brentrup et al. (2001), where different nitrogen fertil-izers were applied for a sugarbeet production system. The envi-ronmental results are considerably different to those obtainedwhen AN and CAN are applied instead of urea. There is an incre-ment on the environmental burdens in categories such as ADP, CED,POFP, LC, GWP and in all the toxicity categories. The only reductionswere achieved in AP, EP and ODP. The results are considerablyaffected by the reduction of the biomass yield since the agricultural

activities and fertilizers doses are exactly the same as in the currentsystems. The UAN involves less energy requirements than urea but,higher acidifying and eutrophying emissions derived from itsproduction.

3.2.3. Rotation systemsThe main objective of this paper was to identify the biomass

source for biogas production purposes with the best environmentalprofile. The choice could depend on the biomass yield or on theavailability of field for its production. Thus, some of the studiedcropping systems were applied in rotation between a spring-summer crop (such as the maize classes 300, 400 or 500) andwinter crops (triticale or wheat), in order to increase the biomassyield per hectare and year since all of them are crops with lowbiomass yield when compared to maize classes 600 and 700.Consequently, the cultivation of maize classes 600 and 700 wascarried out without rotation with triticale and wheat. Therefore,eight scenarios per hectare were managed and proposed:

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Fig. 5. Comparative environmental results for the energy crops under assessment (current scenarios) and the hypothetical substitution of mineral fertilizers by cattle manure.

Table 4Comparative environmental results of the assessed energy crops (current scenarios) and the hypothetical alternative based on the substitution of urea by ammonium nitrate(AN) e Scenario A. The results are displayed in terms of the variation (%) in relation to the corresponding current scenario.

ADP AP EP GWP ODP HP FEP MEP TEP LC POFP CED

Maize class 300 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 300-AN �3.2% �58.2% �49.9% �5.9% �7.7% þ7.0% þ4.4% þ6.3% þ8.4% þ0.2% �0.1% �2.2%Maize class 400 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 400-AN �3.2% �58.2% �49.9% �5.1% �7.7% þ7.0% þ4.4% þ6.3% þ8.4% þ0.2% �0.1% �2.2%Maize class 500 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 500-AN �3.2% �58.2% �49.9% �4.5% �7.7% þ7.0% þ4.4% þ6.3% þ8.4% þ0.2% �0.1% �2.2%Maize class 600 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 600-AN �1.5% �37.9% �30.3% �0.8% �3.6% þ2.5% þ1.6% þ2.4% þ3.6% þ0.1% �0.1% �0.9%Maize class 700 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 700-AN �1.5% �37.9% �30.3% �0.9% �3.6% þ2.5% þ1.6% þ2.4% þ3.6% þ0.1% �0.1% �0.9%Triticale 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Triticale-AN �2.7% �44.5% �36.9% �2.0% �3.9% þ5.4% þ5.5% þ6.2% þ2.2% �0.0% �0.1% �2.6%Wheat 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Wheat-AN �2.7% �44.5% �36.9% �2.3% �3.9% þ5.4% þ5.5% þ6.2% þ2.2% �0.0% �0.1% �2.6%

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Table 5Comparative environmental results of the assessed energy crops (current scenarios) and the hypothetical alternative based on the substitution of urea by calcium ammoniumnitrate (CAN) e Scenario B. The results are displayed in terms of the variation (%) in relation to the corresponding current scenario.

ADP AP EP GWP ODP HP FEP MEP TEP LC POFP CED

Maize class 300 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 300-CAN �9.8% �59.9% �52.1% �4.2% �13.9% þ1.1% �1.2% þ0.3% þ1.2% �7.2% �6.8% �9.0%Maize class 400 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 400- CAN �9.8% �59.9% �52.1% �3.7% �13.9% þ1.1% �1.2% þ0.3% þ1.2% �7.2% �6.8% �9.0%Maize class 500 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 500- CAN �9.8% �59.9% �52.1% �3.2% �13.9% þ1.1% �1.2% þ0.3% þ1.2% �7.2% �6.8% �9.0%Maize class 600 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 600- CAN �8.5% �41.6% �34.6% �0.4% �10.4% �4.3% �5.1% �4.4% �3.7% �7.3% �7.1% �8.1%Maize class 700 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 700- CAN �8.5% �41.6% �34.6% �0.4% �10.4% �4.3% �5.1% �4.4% �3.7% �7.3% �7.1% �8.1%Triticale 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Triticale-CAN �9.5% �47.6% �40.5% �1.2% �10.7% �0.8% þ0.3% þ0.1% �5.1% �7.4% �6.8% �9.3%Wheat 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Wheat- CAN �9.5% �47.6% �40.5% �1.4% �10.7% �0.8% þ0.3% þ0.1% �5.1% �7.4% �6.8% �9.3%

Table 6Comparative environmental results of the assessed energy crops (current scenarios) and the hypothetical alternative based on the substitution of urea by urea ammoniumnitrate (UAN) e Scenario C. The comparative results are displayed in terms of the variation (%) in relation to the corresponding current scenario.

ADP AP EP GWP ODP HP FEP MEP TEP LC POFP CED

Maize class 300 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 300-UAN þ0.3% �30.3% �25.5% �3.4% �3.3% þ7.7% þ6.5% þ7.2% þ7.6% þ2.9% þ2.9% þ1.1%Maize class 400 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 400- UAN þ0.3% �30.3% �25.5% �3.0% �3.3% þ7.7% þ6.5% þ7.2% þ7.6% þ2.9% þ2.9% þ1.1%Maize class 500 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 500-UAN þ0.3% �30.3% �25.5% �2.6% �3.3% þ7.7% þ6.5% þ7.2% þ7.6% þ2.9% þ2.9% þ1.1%Maize class 600 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 600-UAN þ1.6% �18.8% �14.4% �0.5% �0.0% þ4.5% þ4.1% þ4.5% þ4.8% þ2.8% þ2.8% þ2.1%Maize class 700 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Maize class 700-UAN þ1.6% �18.8% �14.4% �0.6% �0.0% þ4.5% þ4.1% þ4.5% þ4.8% þ2.8% þ2.8% þ2.1%Triticale 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Triticale-UAN þ0.6% �0.2% þ0.6% �1.3% �0.3% þ6.6% þ7.4% þ7.1% þ4.1% þ2.8% þ2.9% þ0.8%Wheat 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 100%Wheat-UAN þ0.6% �0.2% þ0.6% �1.5% �0.3% þ6.6% þ7.4% þ7.1% þ4.1% þ2.8% þ2.9% þ0.8%

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wheat þ maize class 300, wheat þmaize class 400, wheat þmaizeclass 500, triticale þ maize class 300, triticale þ maize class 400,triticaleþmaize class 500, andmaize classes 600 and 700. The totalyields in terms of dry basis biomass per hectare are: 27 t ha�1,29 t ha�1, 31 t ha�1, 29 t ha�1, 32 t ha�1, 33 t ha�1, 29.3 t ha�1 and26 t ha�1 respectively. Since the triticale and wheat, as well as themaize classes 300, 400 and 500, were cultivated following the samemanagement practices but with different biomass yields, there areno differences between these six scenarios except in terms of GWPsince this category is affected by the CO2 uptake during the biomassgrowth and, as previously mentioned, the yields are differentbetween the scenarios. Thus, between all these six scenarios(Fig. 6), the triticale þ maize class 500 should be the best option interms of derived GHG emissions and dry basis biomass yield. Thesame situation occurs for the maize 600 and 700, obtaining thesame environmental performance in all the categories except in theGWP where maize class 600 shows the best result due to thehighest dry basis biomass yield. Fig. 6 shows the comparativeperformances. As a result, maize 600 and maize 700 present thebest environmental results compared to the scenarios based onrotations, with reductions ranging from 34% to 67% depending onthe category. On the contrary, the best scenario from the GWPperspective is the rotation based on triticale and maize 500 since itpresents the highest yield of biomass.

The consideration of an area-oriented functional unit leads tointeresting results when there are limitations on arable landavailable for biomass productionwith energy purposes. However, itis also important to guarantee the supply and availability ofbiomass to the biogas plant. In this case, a functional unit based ontonne of biomass ready to be used in the plant could also beinteresting. Fig. 7 shows the comparative results for all thesescenarios based on the production of one tonne of biomass. Theresults are considerably different as the other shown in Fig. 6.Therefore, the production of biomass in a rotation system between

triticale and maize class 400 should be the worst option except interms of GWP. In the remaining impact categories, the maize class700 should be the best biomass production alternative.

4. Conclusions

In this study, the identification of the best source of biomass tobe supplied to a biogas plant located in Italy, from an environ-mental point of view was assessed in detail. Three different energycrops well-known by Italian farmers were evaluated andcompared: triticale, wheat and maize. Within the maize, fiveclasses were also considered. The interest on these crops lies on thefact that energy crops are expected to result in remarkable socialand environmental benefits, on a short-medium term. According tothe results, the selection of the best biomass source depends onseveral factors such as the functional unit and biomass yield.Therefore, a sensitivity assessment was performed in order toidentify these differences. Moreover, the most critical processesthroughout the life cycle of the cropping systems were identifiedand improvement alternatives were proposed specifically for themineral fertilization (one of the most important hot spots). Thus,different scenarios based on alternative nitrogen based fertilizerswere assessed in detail and discussed, identifying that the use ofCAN instead of urea should improve the environmental profileregardless of the energy crop. Finally, the combination of triticale orwheat with maize classes 300, 400 or 500 in rotation systems inorder to obtain biomass yields per ha similar to that of maizeclasses 600 and 700 were also evaluated. The best results wereobtained for maize classes 600 and 700 regardless of the functionalunit considered in all the categories assessed except in GWP, wheretriticale with maize 400 or with maize 500 were the best options.Thus, the choice of the best source of biomass based on GHGemissions originates different results compared to the resultsderived from other impact categories.

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ADP AP EP GWP ODP HP FEP MEP TEP LC POFP CED

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Fig. 7. Comparative environmental results between the scenarios considering, or not, rotations based on 1 ton of biomass as a functional unit ready to be processed in the biogasplant.

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Acknowledgements

Dr. S. González-García would like to express her gratitude to theSpanish Ministry of Education for the financial support (Grantreference: EX2009-0740) granted for a Postdoctoral ResearchFellowship performed at the University of Aveiro (Portugal) duringwhich this paper was prepared.

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