Energy performance and ef

ﬁciency of two sugar crops for the biofuel

supply chain. Perspectives for sustainable

ﬁeld management in

southern Italy

Pasquale Garofalo

, Laura D'Andrea

, A. Vittorio Vonella

, Michele Rinaldi

A. Domenico Palumbo

Consiglio per la Ricerca in Agricoltura e l'Analisi dell'Economia Agraria

e Centro di Ricerca per la Cerealicoltura (CRA-CER), S.S. 673 km 25,200,

Foggia, Italy

Consiglio per la Ricerca in Agricoltura e l'Analisi dell'Economia Agraria

e Unita di Ricerca per i Sistemi Colturali degli Ambienti Caldo-Aridi (CRA-SCA),

via Celso Ulpiani, 5, Bari, Italy

a r t i c l e i n f o

Article history:
Received 3 May 2015
Received in revised form
6 August 2015
Accepted 11 October 2015
Available online 19 November 2015

Keywords:
Bioethanol
Fertilization
Soil tillage
Sweet sorghum
Sugar beet

a b s t r a c t

Improvement of the energy balance and ef

ﬁciency for reduced input of cropping systems is one of the

main goals for the cultivation of energy crops. In this

ﬁeld study, two sugar crops for bioethanol pro-

duction were cultivated under different soil tillage management (conventional; no tillage) and mineral
nitrogen application (0, 75, 150 kg N ha

): sweet sorghum and sugar beet. The energy performance and

ﬁciency along the bioethanol supply chain were analysed and compared. Both of these crops showed

good growth adaptation to the different soil and nitrogen management, and thus the energy return,
resource and energy ef

ﬁciencies were signiﬁcantly improved in the low-input system. Sweet sorghum

provided better responses in terms of water and nitrogen use ef

ﬁciency for biomass accumulation, as

well as its energy yield and net gain, compared to sugar beet, whereas sugar beet showed higher energy
ef

ﬁciency than sorghum. According to these data, both of these crops can be cultivated in a Mediter-

ranean environment with low energy input, which guarantees good crop and energy performances for
biofuel strategy planning.

1. Introduction

European countries are largely dependent on imported fossil

fuels, and the transport sector accounts for more than 30% of the
imported energy

[1]

. Most greenhouse gas emission is due to the

transport industry, and CO

emission into the air has risen by 90%

over the last two decades (Biofuels Advisory Council,

[1]

). To cope

with further increases in greenhouse gas emission, the EU
Renewable Energy Directive

[2]

ﬁned a framework for the pro-

motion of energy from renewable sources. The target of this
Directive is that by 2020 with the use of biofuels, it will be possible
to achieve a 20% share of energy from renewable sources, and a
reduction of 20% in greenhouse gas emissions, with 10% of this

being renewable in the transport sector. Moreover the

“Climate

Action and Renewable Energy Package

”

[3]

has been proposed to

save energy use through an improved energy ef

ﬁciency of 20% by

2020. In view of these policy targets, some investigations have
focused on the role of the agricultural sector for biofuel production
and the energy ef

ﬁciency of different cropping systems

e6]

while others have focused on the reduction of greenhouse gas
emission through the introduction of biofuels and the replacement
of fossil fuel use

[7]

Some studies have reported that crop productivity of several

species (e.g., wheat, corn, soybean) is not compromised when the
energy input is reduced at the

ﬁeld level (mainly through soil tillage

and nitrogen management), to improve the energy balance and
ef

ﬁciency

[8,9]

. Other studies have instead indicated signi

ﬁcant

reductions in crop and energy performance with reduced energy
input applied at the

ﬁeld level

[10]

, as also observed for the pro-

ductivity of winter sorghum

[11]

, where a reduced tillage system

lowered the water-use ef

ﬁciency of the crop, and consequently the

grain yield.

* Corresponding author. Consiglio per la Ricerca e la Sperimentazione in Agri-

coltura

e Centro di Ricerca per la Cerealicoltura (CRA-CER), S.S. 673 km 25,200,

Foggia, Italy. Tel.:

þ39 0881742972; fax: þ39 0881713150.

E-mail address:

pasquale.garofalo@entecra.it

(P. Garofalo).

Contents lists available at

ScienceDirect

Energy

j o u r n a l h o me p a g e :

w w w . e l s e v i e r . c o m/ l o ca t e / e n e r g y

http://dx.doi.org/10.1016/j.energy.2015.10.031

0360-5442/

Energy 93 (2015) 1548

e1557

Currently, the debate on second-generation biofuels remains

open

[12]

, and their energy and environmental performance are

under study, although they are unlikely to have any effective impact
before 2020

[13]

. The conversion tecnologies are well known for

the

ﬁrst generation of biofuels

[14]

, whereas for second generation

biofuels, these are still under development

[15]

. Moreover, the

advantage in terms of energy performance and climate impact of
second generation biofuels is achieved through low-input peren-
nial crops

[16]

. Recently, the CAP reform 2014-2020 (EU Regulation

1307/2013)

[17]

established that two or more crops should be

cultivated on farms to allow the claiming of subsidies (i.e., crop
diversi

ﬁcation as a ‘greening’ measure). Thus, herbaceous crops

with a short growing cycle and high biomass production (e.g.,
sorghum) or high sugar content (e.g., sugar beet) can satisfy both
the greening measure and the biofuel planning strategy.

Crops like sugar beet are not suitable for second-generation

biofuels, because the accumulation of large amounts of lignocel-
lulose can delay the harvest time, with the consequent loss of
biomass (i.e., dead leaves and relocation of sugar from roots to new
leaves). For other crops such as sorghum, the harvest time at full
maturity can compromise the

ﬁeld operations and/or the sowing

time of the autumn species that follows. Finally, the cultivation of
sucrose crops for feed has dramatically declined in Italy over past
years (72% decrease from 2001 to 2012; ISTAT 2015,

[18]

), because

of the European policies (EU Regulation 320/2006); however, the
use of these crops can provide diversi

ﬁcation of the farm income

using the expertise and machinery available on the farm.

Assessment of the energy performance of various crops has

been carried out using different parameters and/or at different
stages of the supply chain. Koga (2008)

[19]

reported the energy

balance at the

ﬁeld scale as the difference between the gross energy

output from sugar beet and the energy cost for its cultivation. Other
studies

[20]

have de

ﬁned the EROI (energy return on the energy

investment) as the ratio of the quantity of energy delivered by a
biofuel society to the energy used as an input in the process to
produce the energy, with a comparison of EROI of bioethanol ob-
tained from corn and lignocellulose materials.

Most of the studies that have analysed energy crop perfor-

mances have been limited to a single species

[8,21]

or a single

growing season

[22]

, or have been based on literature data

[23]

rather than applied to a speci

ﬁc environment. Moreover, in Italy,

ﬁeld experiments have been mainly carried out in central or
northern regions

[24,25]

, with pedo-climatic condition that are

relatively dissimilar to the typical climate of the Mediterranean
area of the present study. In addition, the system boundaries are
often limited to the farm, thus excluding the cost of transportation
and the conversion of biomass into biofuel, although the energy for
transport is not negligible and can reach up to 23% of the total
energy cost

[26]

Comparisons of the energy performances of sucrose crops for

bioethanol production under different cropping systems (e.g., with
modulation of soil tillage and nitrogen supply) under speci

ﬁc pedo-

climatic conditions that are prolonged over several years need to be
investigated. Thus, the present study was designed to: (i) determine
the biomass and sugar yield of sweet sorghum and sugar beet in
southern Italy cultivated for

ﬁrst-generation biofuel; and (ii)

compare the energy performances between these two crops under
different management (i.e., conventional and no tillage practices, and
different nitrogen levels), with consideration also of the

ﬂow along

all of the supply chain (i.e., from farm to bioethanol conversion plant).

2. Materials and methods

The

ﬁeld experiment was carried out over a 3-year period from

2009 to 2012 in Foggia (latitude, 41

N; longitude, 15

altitude, 90 m a.s.l.), in the Apulia region of southern Italy. The soil
was a vertisol of alluvial origin, Typic Calcixeret

[27]

classi

ﬁed as

silty-clay, with the following characteristics: organic matter, 2.1%;
total nitrogen, 0.122%; NaHCO

-extractable phosphorus, 41 ppm;

OAc extractable K

, 1598 ppm; pH (water), 8.3;

ﬁeld capacity

water content, 0.396 m

; permanent wilting point water

content, 0.195 m

; and available soil water, 202 mm m

. The

climate was

‘accentuated thermo-Mediterranean’

[28]

, with tem-

peratures below 0

C in winter and above 40

C in summer. The

annual rainfall (mean, 550 mm) was mostly concentrated in the
winter months. The class

‘A pan’ evaporation was 1033 mm over

the year, and 657 mm from May to August. The daily meteorological
data of temperature, humidity, rainfall, wind velocity, and solar
radiation were recorded at a meteorological station located at the
same experimental farm.

2.1. Field experiment

Sweet sorghum (cv

‘Sucro 506’) was sown at the beginning of

May, in rows 0.5 m apart and at a distance of 0.08 m between the
seeds in each row (i.e., 250,000 seeds per hectare). The crop was
harvested before heading (mid-August) to maintain an adequate
plant water content (75%), as necessary for the fermentation pro-
cess. Sugar beet (cv

‘Autave’) was sown between late November and

early December, with row spacing of 0.5 m, and plant-to-plant
spacing of 0.20 m (i.e., 100,000 seeds ha

). The crop was harvest

at the beginning of August, when the plant achieved a good
compromise between water content and root sugar content.

Irrigation of both of these crops was managed according to the

water consumed by the plants, as estimated by the gravimetric
method (at 0

e0.8 m soil depth). Each time the water used by the

sweet sorghum and sugar beet reached 60 mm and 30 mm,
respectively, the irrigation was triggered. To ensure uniform water
distribution, a drip irrigation system was used, with one line for
each plant row, and drippers with a 4 L h

ﬂow. The total amount

of water applied for the irrigation of the sorghum was 120 mm,
176 mm and 300 mm for the

ﬁrst, second and third experimental

years, respectively, with rainfall of 79 mm, 73 mm and 68 mm
during the growing seasons in 2010, 2011 and 2012, respectively.
For the sugar beet, 90 mm, 65 mm and 108 mm of irrigation water
were applied in the

ﬁrst, second and third growing seasons,

respectively, with the total rainfalls of 473 mm, 399 mm and
236 mm during the growing seasons in 2010, 2011 and 2012,
respectively.

For both of these crops, the soil management was carried out

according to CT (conventional tillage) and NT (no tillage). For CT,
shallow ploughing (soil depth, 25 cm) was performed with a

ﬁve-

furrow plow, followed by disc harrowing, power harrowing, and
seeding with a precision driller. For NT, no soil practice was
scheduled, with only direct seeding with a Gaspardo No-Till 1040,
which ensured light and shallow tillage in the strip area affected by
the furrowers. For NT, before seeding, 5 L ha

glyphosate was

applied for weed control.

Mineral nitrogen fertilisation was managed with 75 kg ha

(N75) and 150 (N150) kg ha

nitrogen in the form of ammonium

nitrate (34%), as compared with no nitrogen fertilisation (N0). The
nitrogen fertilizer was split into two doses, one as basal dressing
before sowing, and the second as top dressing in the middle of May
for sugar beet, and between the end of June and the beginning of
July for sorghum. Phosphate fertilizer was applied before sowing
(100 kg ha

The main treatment of these crops was related to soil tillage,

while the secondary treatment was for the different nitrogen
supplies. The experimental design was a split-plot design with
three replications. The area of each subplot was 84 m

. In the 2-year

P. Garofalo et al. / Energy 93 (2015) 1548

e1557

1549

rotation, the farm was split into two halves, with one crop for each
half. In the following year, the crop placings were inverted.

Diesel consumption was measured directly at the farm, as the

difference between the amount of fuel necessary to re

ﬁll the tractor

tank after one soil operation and the following one. The time to
fully complete each

ﬁeld operation was also recorded.

At harvest, a range of parameters were determined: the weight

of the total fresh and dry biomass (sweet sorghum, leaves and
stems; sugar beet, roots and leaves); the soluble solids content in

Brix (PR model 32 ATAGO Palette digital refractometer), with the

Brix multiplied by the fresh biomass/roots for the TSS (total soluble

solid); the potential sugar yield from the biomass and roots, as
reported by Wortmann et al. (2010)

[29]

(see also

Table 1

); and the

WUE (crop water) and nitrogen (NUE) use ef

ﬁciency, calculated as

the ratio between the water used by the crop (i.e., irrigation plus
rainfall), the nitrogen applied, and the parameters analysed.

2.2. Energy balance and ef

ﬁciency

To determine the energy

ﬂuxes for the input and output of the

product systems analysed, the system boundaries were established
(

Fig. 1

). The energy input at the

ﬁeld level was calculated separately

for the tractor, implements, fuel, seed, and chemicals used during
crop management, taking into account the direct and indirect in-
puts. The indirect input included the materials to build the tractor,
and the machinery and implements, and their maintenance. To
follow all the energy cost along the bioethanol supply chain, the
estimations were performed as reported in

Table 1

For a full overview of the energy performance and ef

ﬁciency,

several indicators were used. The EY (energy yield) provided the
gross energy output for the potential bioethanol production, minus
the cost to convert the sugar into biofuel, but not including the
transport, and hence indicating the effective energy deliverable by
the power plant. In the NEG (net energy gain), the direct and in-
direct costs for crop cultivation, transport of materials, and biomass
and sugar-bioethanol conversion were subtracted from the EY to
indicate the energy that was effectively gained from the biofuel
supply chian. Finally, to determine how much of the production
process of the biofuel was energy advantageous, and to quantify the
positive returns on investment obtained, the EROI was calculated.
All of the equations necessary for these parameter calculations are
reported in

Table 1

, and the parameters are reported fully in

Table 2

The ratio between NEG and the sum of the rainfall plus the

irrigation, and between NEG and the N applied, provided the water

energy use ef

ﬁciency (EWUE; MJ m

) and the nitrogen energy use

ﬁciency (ENUE; MJ kg

)

2.3. Statistical analysis

The analysis was performed using the statistical analysis soft-

ware SAS/STAT

(SAS Institute Inc., Cary, NC, USA). The data un-

derwent ANOVA (analysis of variance), considering the Year as the
random effect and the appropriate error test for the soil manage-
ment (in the main plot), nitrogen (in the sub-plot) and their in-
teractions. Differences among treatments were assessed using
Tukey

eKramer tests and signiﬁcance was accepted at P ¼ 0.05.

3. Results

3.1. Crop productivity

The crop performances for sweet sorghum and sugar beet are

reported in

Tables 3 and 4

. For the soil management, there were no

differences across all of the analysed parameters for either of the
crops. On average over the 3-year experimental period, the fresh
biomass of sweet sorghum was 99.9 t ha

, and the water content at

harvest was 25%. The

Brix was a little higher for NT (

þ4%)

compared to CT, although compared to NT, CT promoted a better
response in terms of TSS (

þ6%). The water and nitrogen use efﬁ-

ciencies were slightly improved for CT over NT, but globally, these
parameters were not statistically different between CT and NT.

Sweet sorghum saw little bene

ﬁt of the increased nitrogen

supply, with an increase in fresh biomass from 98.1 t ha

with N0

to 102.4 t ha

with N150, and for dry biomass from 23.4 t ha

25.4 t ha

. These productivity levels are comparable with those

reported by Habyarimana et al. (2004)

[33]

for sorghum biomass,

where they showed dry biomass accumulation (20

e29 t ha

) close

to the experimental values in the present study, under similar
environmental and rainfall conditions.

The sweet sorghum

Brix was more stable among the nitrogen

treatments, whereas TSS was improved with N150 when compared
to the lower nitrogen levels (

þ12%). However, the differences in

productivity and the WUE indicators did not reach statistical sig-
ni

ﬁcant. On the other hand, with the doubling of the nitrogen dose,

NUE was halved for the fresh and dry biomass and TSS parameters.

In comparison to sweet sorghum, the sugar beet growth was

more dependent on the different nitrogen supplies. Indeed, the
differences in the fresh and dry roots as well as TSS indicated a

Table 1
Estimation of direct and indirect energy input for crop cultivation, transport, sugar-bioethanol conversion, energy performance and ef

ﬁciency indicators.

Equation

Unit

Description

CSY

¼ ðFMY DMYÞ*

Brix

*0:75

(1)

t ha

Conservative sugar yield

; 80%extracted ¼ ½FMY ðDMY CSYÞ*0:8

(2)

t ha

Juice yield

¼ JY*

Brix

*0:75

(3)

t ha

Sugar yield

(4)

MJ ha

Energy coef

ﬁcient for each implement

¼ T

(5)

MJ ha

Energy coef

ﬁcient for human labour

¼ E

(6)

MJ ha

Energy coef

ﬁcient for seed

;P;G

¼ EC

;P;G

(7)

MJ ha

Energy coef

ﬁcient for materials

d*g*H*Q

*10

(8)

MJ ha

Energy input for the irrigation

ðI;IIÞ

(9)

MJ ha

Energy for transportation

¼ ðBE*BYÞ ðCE*BYÞ

(10)

MJ ha

Energy yield

NEG

¼ EY ðEFI þ ET

þ ET

(11)

MJ ha

Net energy gain

EFI

¼ ðEt

ðI;IIÞ

þ D

þ E

;

þ I

(12)

MJ ha

Total energy input

EUE

EFI

(13)

kg ha

Energy use ef

ﬁciency

EROI

NEG

ðEFIþE

ðI;IIÞ

(14)

Energy return on energy invested

Sugar concentration of juice is 75% of Brix expressed in g kg

sugar juice and 95% of the extracted sugar is converted to ethanol.

P. Garofalo et al. / Energy 93 (2015) 1548

e1557

1550

different pattern of plant responses according to the nitrogen dose.
Here, N150 showed the best response for fresh (38.1 t ha

) and dry

(10.5 t ha

) roots, and TSS production (7.7 t ha

), with these data

signi

ﬁcantly worst for N0 (decreases, 23%, 31%, 25%, respectively),

with intermediate data for the N75 treatment. A similar response
was observed for water use ef

ﬁciency, with the same trend re-

ported under the same environment conditions by Rinaldi and
Vonella (2006)

[34]

, although they reported higher water

Fig. 1. Flow diagram (input and output) and system boundaries of the bioethanol production in sweet sorghum and sugar beet.

Table 2
Variables and parameters, and their values as used in the equations given in

Table 1

Symbol

Description

Unit

Value

Reference

FMY

Fresh matter yield

t ha

See text

DMY

Dry matter yield

t ha

See text

Bioethanol yield

t ha

See text

Energy cost for building, maintenance, transport for
the machineries and implements

MJ kg

See

Table 3

[46]

Mass

See

Table 3

[46]

Lifespan

See

Table 3

[46]

Human labour

See text

Human energy consumption

MJ h

1.08

[30]

Energy value for the seed production,

MJ kg

36.98 for sugar beet; 54 MJ kg

for sweet sorghum

[2,22]

Seed rate

1 for sugar beet; 14 MJ kg

for sweet sorghum

N, P, G

Nitrogen, phosphorous, glyphosate

Energy coef

ﬁcient of materials

MJ kg

48.89 for N, 15.23 for P, 268.4 for G

[2]

Application rate of materials

See text

Water density

kg m

1000

Total dynamic head

Water applied

See text

Pump ef

ﬁciency

0.65

[31]

ﬁciency of the electric motor

0.22

[31]

t(I)

Transport of materials from the factory to the farm

MJ ha

See text

t(II)

Transport of biomass from the farm to the conversion plant

MJ ha

See text

Diesel consumption of the truck

l km

or kg km

0.34 l or 0.27

[32]

Payload

(I)

Distance from the factory (materials) to the farm

100

(II)

Distance from the farm to the conversion plant

Weight of the mass transported

See text

Energy density of diesel

MJ kg

43.1

[2]

Energy density of bioethanol

MJ kg

26.8

[23]

Energy to convert biomass in anhydrous ethanol

MJ kg

18.33

[22]

Differences between the level of water suction in the well (10 m) and the level of the water surface in the collecting basin (5 m).

70 km as maximum distance in the short supply chain for the Apulia region; Reg. No 42/2012.

P. Garofalo et al. / Energy 93 (2015) 1548

e1557

1551

availability (mean irrigation, 249 mm; mean rainfall, 353 mm) and
crop productivity (fresh roots, 38.2

e60.0 t ha

Finally, the halving of the nitrogen dosing from N150 to N75

resulted in large difference in nitrogen use ef

ﬁciencies (i.e., almost a

doubling;

Table 5

3.2. Energy input

Table 6

shows the amount of energy required for crop man-

agement of the sorghum and sugar beet. The highest impact of the
energy needed was due to the diesel used for the movement of
tractors and equipment, as well as the nitrogen fertilizer applica-
tions. The diesel input for cultivation accounted for 37% and 49% for
sorghum and sugar beet, respectively for the CT_N0 treatment,
although these values dropped to around 15% when compared to
the full nitrogen supply management in the NT treatment. Indeed,
the nitrogen fertilizer required an energy cost of 3647 MJ ha

and

7349 MJ ha

, when passing from the N75 to the N150 treatments,

with a minimum effect of 25% (sweet sorghum, CT_N75) to a
maximum effect of 50% (sugar beet, NT_N150) of the total energy
use.

The indirect input due to the tractors and implements used was

between 1% for sorghum (NT_N150) to 7% for sugar beet (NT_N0), of
the total energy use. In sweet sorghum the cost for seeding and
irrigation accounted for 750 MJ ha

and 933 MJ ha

respectively,

with the maximum percentage of 9% for the irrigation cost at the
lowest energy use. In sugar beet, the input due to the seed and
irrigation supply was negligible, as it was constantly below 6% of
the total energy budget. Generally, the estimated energy input at
ﬁeld scale is higher in northern European countries than in
southern European countries. Indeed, in the UK, the total energy
cost for sugar beet was reported to be up to 26.8 GJ ha

[35]

, and in

Germany, 29.7 GJ ha

[36]

. On the other hand, in Greece, the en-

ergy input for sun

ﬂower cultivation was reported as 10.5 GJ ha

Table 3
Energy coef

ﬁcients for the indirect energy input used for the machinery for the sorghum and sugar beet

[47]

Item

Mass (kg)

Life (h)

Energy cost

Energy coef

ﬁcient

(MJ h

)

Materials (MJ kg

)

Assembly (MJ kg

)

Transport (MJ kg

)

Maintenance (MJ kg

)

Tractor (132 kW)

6300

12,000

35.2

18.7

1.9

29.6

44.8

Tractor (51 kW)

4000

12,000

35.2

18.7

1.9

29.6

28.5

Plough

1000

3000

34.5

12.3

1.3

25.7

24.6

Disk harrow

1200

3000

34.5

12.3

1.3

25.7

29.5

Fertilizer distributor

100

3000

34.5

12.3

1.3

25.7

2.5

Sprayer

200

2000

34.5

12.3

1.3

25.7

7.4

Power harrow

500

3000

34.5

12.3

1.3

25.7

12.3

Seeder (sorghum)

800

3000

34.5

12.3

1.3

25.7

19.7

(Sugar beet)

1000

3000

34.5

12.3

1.3

25.7

24.6

Harvester (sorghum)

8000

3000

41.3

14.8

1.5

30.9

354.0

Harvester (sugar beet)

8500

3000

41.3

14.8

1.5

30.9

250.8

Trailer

1000

3500

34.5

12.6

1.3

25.9

22.3

Mass and life based on local survey.

For fertilizing, spraying and for the trailer.

Table 4
Biomass performance, and water and nitrogen use ef

ﬁciency of sweet sorghum, as affected by soil tillage and nitrogen fertilization during the experimental trial (2009e2012).

Treatment Parameter

FB (t ha

) DB (t ha

)

Brix TSS (t ha

) WUEfb (kg m

) WUEdm (kg m

) WUEtss (kg m

) NUEfb (kg kg

) NUEdb (kg kg

) NUEtss (kg kg

)

101.9

24.8

8.9

8.7

39.9

9.8

3.6

1007.1

244.5

85.4

97.7

24.6

9.3

8.2

37.8

9.5

3.3

995.4

262.2

81.7

98.1

23.4

9.1

8.3

38.2

9.2

3.4

N75

99.0

25.3

9.1

8.2

38.3

9.8

3.4

1319.6 a

337.5 a

108.8 a

N150

102.4

25.4

9.2

8.8

40.1

9.9

3.7

682.8 b

169.1 b

58.3 b

Mean

99.8

24.7

9.1

8.4

38.9

9.6

3.5

1001.3

253.3

83.6

FB, fresh biomass; DB, dry biomass; TSS, total soluble solid; WUE, water use ef

ﬁciency for fresh biomass (fb), dry biomass (db) and total soluble solid (tss); NUE, nitrogen use

ﬁciency for biomass (fb), dry biomass (db) and total soluble solid (tss); CT, reduced tillage; NT, no tillage; N0, N75, N150, nitrogen application (0, 75, 150 kg ha

Different letters indicate different means at P

¼ 0.05 (TukeyeKramer tests).

Table 5
Biomass performance, water and nitrogen use ef

ﬁciency of sugar beet as affected by soil tillage and nitrogen fertilization during the experimental trial (2009e2012).

Treatment

Parameter

FR (t ha

)

DR (t ha

)

Brix

TSS (t ha

)

WUEfr (kg m

)

WUEdr (kg m

)

WUEtss (kg m

)

NUEfr (kg kg

)

NUEdr (kg kg

)

NUEtss (kg kg

)

35.8

8.8

20.1

7.2

7.5

1.8

1.5

378.1

93.5

76.7

33.4

9.0

20.9

6.8

6.9

1.9

1.4

360.9

97.2

74.1

29.3 b

7.2 b

20.2

5.8 c

6.0 b

1.5 b

1.2 b

N75

36.4 ab

9.0 ab

20.8

7.5 a

7.6 a

1.9 ab

1.6 a

485.2 a

120.6 a

99.4 a

N150

38.1 a

10.5 a

20.5

7.7 a

8.0 a

2.3 a

1.6 a

253.8 b

70.1 b

51.4 b

Mean

34.6

8.9

20.5

7.0

7.2

1.9

1.5

369.5

95.3

75.4

FR, fresh biomass; DR, dry root biomass; TSS, total soluble solid; WUE, water use ef

ﬁciency for fresh root (fr), dry root (dr) and total soluble solid; NUE, nitrogen use efﬁciency for

fresh root (fr), dry root (dr) and total soluble solid (tss); CT, reduced tillage; NT, no tillage; N0, N75, N150, nitrogen application (0, 75, 150 kg ha

Different letters indicate different means at P

¼ 0.05 (TukeyeKramer test).

P. Garofalo et al. / Energy 93 (2015) 1548

e1557

1552

[37]

, and in Italy, for giant reed cultivation, this value has oscillated

from 4 GJ ha

to 18 GJ ha

[38]

. Despite this large variability

across these studies, they all agreed that diesel consumption and
nitrogen supply are the components that have the greatest effects
on the energy

ﬂux.

For the transport of feedstock from the farm to the plant con-

version platform, the mean energy costs were 3123 MJ ha

and

1082 MJ ha

, for the biomass of sorghum and the roots of sugar

beet, respectively. This was mainly due to the fresh biomass weight
(three-fold greater in sorghum than beet). Finally, passing from the
highest to the lowest energy intensity treatment, the energy saving
was 6720 MJ ha

for sorghum (reduction, 38%) and 7011 MJ ha

for sugar beet (reduction, 48%), when compared to CT_150.

3.3. Energy performance

The energy yield, or EY, for sweet sorghum oscillated between

35.2 GJ ha

(CT_N150) and 30.0 GJ ha

(NT_N75), as shown in

Fig. 2

, although no statistically signi

ﬁcant differences were

observed among the treatments. For the sweet sorghum NEG,
although the unfertilized treatments (N0) provided the highest
values (mean, 21.9 GJ ha

;

þ35%) with respect to the other treat-

ments, there were no statistical differences in the responses across
all of the treatments for NEG. Consequently, sweet sorghum
showed a relatively stable response for NEG (mean, 17.7 GJ ha

)

across the different management practices, which is in line with
data reported in other studies

[39,40]

For sugar beet, compared to the unfertilized treatment, EY was

improved by the N75 to N150 change, with an increase of 28% (24.3
vs. 31.0 GJ ha

). However, these differences did not reach statistical

signi

ﬁcance among the treatments, and unlike for sweet sorghum,

the performances among the years were more stable. The sugar
beet NEG was 17.3 GJ ha

, 20.1 GJ ha

, and 15.6 GJ ha

for the N0,

N75, and N150 treatments, respectively (

Fig. 3

), and CT provided a

small advantage over the NT treatment of 8%, although globally the
net energy returns were comparable for all of the soil and nitrogen
management treatments.

3.4. Resource ef

ﬁciency

In sorghum, the nitrogen application rather than the soil man-

agement (

Fig. 2

) improved WEUE. Indeed, for N0, there was 9.2 MJ

of energy gain per m

water used by the crop, followed by the N_75

and N_150 treatments (mean, 7.1 MJ m

), with small changes due

to soil management (

þ6% for CT treatment) that did not reach

statistical signi

ﬁcance. The halving of the nitrogen that was applied

to the sweet sorghum provided strongly improved bene

ﬁts in terms

of energy gain, as when the nitrogen supply was reduced from
150 kg ha

to 75 kg ha

, the ENUE increased from 0.11 GJ kg

0.23 GJ kg

Compared to sweet sorghum, for sugar beet, the best energy

return per m

water used was for the N75 treatment (mean,

4.2 MJ m

), with the lowest for N150 (3.3 MJ m

) (

Fig. 3

). As for

sorghum, CT slightly improved WEUE, enhancing the value from
3.7 GJ ha

for NT to 3.8 GJ ha

for CT, as the means of all of the

nitrogen fertilizer treatments. Finally, ENUE more than doubled
when the nitrogen application was halved (0.11 MJ kg

for N150 vs.

0.27 MJ kg

for N75).

3.5. Energy ef

ﬁciency

The reduction in energy input intensity together with no

meaningful biomass yield reduction led to a signi

ﬁcant increase in

energy ef

ﬁciency for sweet sorghum. Indeed, as shown in

Fig. 2

NT_N0 and CT_N0 provided the best EUE, with the mean production
of 0.37 kg ethanol per MJ energy applied in the

ﬁeld, followed by

Table 6
Energy input at the

ﬁeld level for the different soil and crop management for the sweet sorghum and sugar beet.

Crop

Parameter

Energy input according to soil tillage management and nitrogen application in the

ﬁeld (MJ ha

)

Conventional tillage

No tillage

N75

N150

N75

N150

Sorghum

Seed

750

Pesticide

1342

Nitrogen

3674

7349

3674

7349

1523

Transport (I)

Machinery

242

255

163

171

172

Tractor

408

476

114

115

Labour

Diesel

4079

4271

4508

2260

2560

2526

Irrigation

933

Transport (II)

3198

3088

3276

2939

3105

3133

Total

11151

15000

19107

10032

14194

17871

Sugar beet

Seed

Pesticide

1342

Nitrogen

3674

7349

3674

7349

1523

Transport (I)

Machinery

517

518

427

Tractor

468

483

114

122

123

Labour

Diesel

3662

3902

4122

1663

1978

2287

Irrigation

369

312

Transport (II)

965

1154

1240

867

1123

1142

Total

7559

11,690

15,678

6356

10,616

14,570

Data are means over the three experimental years (2009

e2012).

N0, N75, N150, nitrogen application in the

ﬁeld (0, 75, 150 kg ha

); Transport (I), energy input for transport of seed, fertilizers and herbicides at the farm (100 km); Transport

(II), energy input for transport of sorghum biomass or sugar beet root to the conversion plant (70 km).

P. Garofalo et al. / Energy 93 (2015) 1548

e1557

1553

the N75 and N150 treatments (0.26 and 0.22 kg MJ

, respectively).

N0 provided the highest EROI for the bioethanol supply chain, at
3.2, which was 44% and 65% greater than for N75 and N150,
respectively.

For sugar beet, the analysis of EUE showed different ef

ﬁciencies

as a consequence of the nitrogen doses applied. Indeed, lowering
the energy input (essentially due to lowering the nitrogen levels),
the ef

ﬁciency for potential bioethanol production per energy

applied in the

ﬁeld increased (

Fig. 3

), with the lowest for N150

(0.24 MJ kg

), followed by N75 (

þ37%), and the highest for N0

(

þ69%). These differences were conﬁrmed also in the EROI analysis,

which indicated that the highest energy return in terms of bio-
ethanol energy output per unit energy used was for N0 (3.5), then
N75 (2.8), and

ﬁnally N150 (2.0), with NT slightly better than CT

(

þ4%).

4. Discussion

The soil management had a small impact on the crop produc-

tivity response for both of these crops for this mid-term period,
whereas the nitrogen management indicated greater growth of
sugar beet with increasing nitrogen doses. The nitrogen also
in

ﬂuenced the water use efﬁciency differently between these two

crops: the water use ef

ﬁciency remained unchanged for sorghum,

irrespective of the nitrogen management, while it increased for
sugar beet as the nitrogen availability increased. Our

ﬁndings are in

line with those reported by Ceotto et al. (2014)

[41]

, who indicated

that the productivity of sorghum under N0 treatment was com-
parable to that for partial and fully fertilized sorghum even after 5
years. For sugar beet, a signi

ﬁcant growth reduction was observed

as a consequence of reduced nitrogen supply

[5]

, even if in the

present study these differences emerged between N0 and N150,
rather than between N0 and N75. Thus, these data indicated that
soil with good nutrient availability provides good and stable re-
sponses for sweet sorghum in terms of nitrogen use ef

ﬁciency and

plant growth (with reduced or even no nitrogen supply) for several
years, while for sugar beet, the productivity is more susceptible to
the nitrogen doses.

However, in sugar beet, the increase in energy intensity at the

ﬁeld scale did not lead to dramatic increases in terms of the root
and potential bioethanol production, as indicated above. This will
be due to the very deep root system of both of these crops, which
will allow greater soil exploration when compared to other crops
(i.e., wheat), in which the different nitrogen doses can have an
appreciable effect on plant growth

[42]

. As a consequence, the best

energy performances were achieved by reducing the energy

Fig. 2. Energy balance and ef

ﬁciencies for the sweet sorghum (as indicated). Data are means ± standard errors over the three experimental years (2009e2012).

P. Garofalo et al. / Energy 93 (2015) 1548

e1557

1554

intensity at the

ﬁeld scale, for both of these crops. This was espe-

cially the case when accounting for the nitrogen supply, high-
lighting nitrogen as the key factor for improvement in the energy
gain. Also, the coupling of wise fertilization management with
conservative tillage practices has been shown to reduce the fossil
energy requirements and greenhouse gas emission in previous
investigations

[43,44]

. These bene

ﬁts can be further emphasized

when the reduced input to the cropping system does not dramat-
ically affect the crop production, as con

ﬁrmed in the mid-term in

the present study. These data are, however, in contrast to those of
Ceccon et al. (2003)

[45]

, who indicated that the gross energy

output and ef

ﬁciency that was achievable in their crops was

improved, as the input increased due to better productivity in
comparison with the low-input treatment at the

ﬁeld scale.

For the biomass production, TSS and potential sucrose yield,

these data indicate that sorghum provides greater improvements
for the resource ef

ﬁciency than sugar beet. This is true at the farm

scale, where different energy patterns were seen when analysing
the whole supply chain, and thus with the introduction of the
transport costs. This direct comparison between sweet sorghum
and sugar beet gave a mixed response in terms of the most suitable
species for energy purposes according to the bioethanol supply
chain, as also illustrated by the spider plots in

Fig. 4

. For the energy

achievable from the potential bioethanol production (i.e., EY),
sweet sorghum and sugar beet gave comparable values at the
highest energy intensity, although a better response emerged with
sorghum compared to sugar beet with the reduction in the nitrogen
and soil-tillage input. With the subtraction of the cost for cultiva-
tion and transport from EY, to obtain NEG, the best energy return
was for sweet sorghum with the N0 management and for both soil
treatments.

Some studies have indicated that the minimun threshold of the

net energy achievable by a crop suitable for energy purposes is
20 GJ ha

[23]

, a value that was reached only by sweet sorghum at

the lowest energy management. However, this estimation was
based on the low heating value of the biomass and only considered
the cost at the

ﬁeld scale. This means that in the present study, the

NEG would have far exceeded 20 GJ ha

if it had relied on the low

heating value and included only the energy needed at the

ﬁeld

scale, which is also the case for the sugar beet for the mid-input
management.

With sorghum, the high amounts of biomass that need to be

moved from the farm to the plant added from 17% to 29% onto the
total energy costs, which thus negatively affected the EUE and EROI
of sorghum with respect to sugar beet (i.e., with lower biomass
production but higher sugar content than sorghum); the greater

Fig. 3. Energy balance and ef

ﬁciencies for the sugar beet (as indicated). Data are means ± standard errors over the three experimental years (2009e2012).

P. Garofalo et al. / Energy 93 (2015) 1548

e1557

1555

the energy saving at the

ﬁeld scale, the greater the advantage of the

sugar beet over the sorghum (see

Fig. 4

). This was based on a dis-

tance from the farm to the conversion plant of 70 km (i.e., the
maximum distance for the short supply chain in the Apulia region),
although with a smaller distance (e.g.,

<25 km) the efﬁciency pa-

rameters become more comparable between these crops (data not
shown).

These

ﬁndings are in line with those reported by de Vries et al.

(2010)

[4]

, who indicated that crops with high energy yields (e.g.,

sweet sorghum) do not necessarily have the highest energy ratios.
However, both sorghum and sugar beet gave higher energy yields
(i.e., the energy content of the bioethanol minus the cost to convert
the sugar into bioethanol) from 2.2-fold (i.e., the mid-energy in-
tensity for sweet sorghum) to 3.5-fold (i.e., the lowest energy
management for sugar beet) the total energy costs, with the values
over the minimum threshold of 2.0 indicated by Venturi and
Venturi (2003)

[23]

Moreover, although ENUE was similar or slightly better for sugar

beet (i.e., for the mid-input treatment), WEUE underlined that

sweet sorghum (a C4 species) can produce higher amounts of
biomass per water used by the crop

[24]

, with respect to sugar beet

(see

Fig. 4

), with this also coupled to a shorter and faster growing

cycle. This is a crucial point for the selection of energy crops in an
environment with reduced water availability and/or with the need
to introduce summer-energy crops in rotation with, rather than
substitution of, winter-food crops.

5. Conclusions

In the present study, both sweet sorghum and sugar beet

showed good potential as energy crops in southern Italy, as they can
maximize the energy balance (sweet sorghum) or the energy ef

ﬁ-

ciency (sugar beet) with lowered energy input.

Due to its deep roots and drought resistance, sweet sorghum

showed favourable exploitation of the water resource and nitrogen
availability, which allows signi

ﬁcant energy saving with stable

productivity under reduced energy intensity. However, when tak-
ing into account the energy costs due to transport of the sorghum

Fig. 4. Relative sustainabilities (as indicated) of the assessed cropping systems for the sweet sorghum (continuous lines) and the sugar beet (dotted lines). Data are percentages
relative to the highest value for the energy balance and ef

ﬁciencies.

P. Garofalo et al. / Energy 93 (2015) 1548

e1557

1556

biomass to the biofuel conversion plant, the results are reversed,
due to the better energy use ef

ﬁciency for sugar beet than sweet

sorghum.

When the land that can be dedicated to energy crops is limited,

then the goal is to achieve the greatest energy return from a
cropped unit to ful

ﬁl energy demands, and hence sweet sorghum

will be the most suitable crop. However, vice versa (i.e., no limits in
the land available), the crop with the higher energy ef

ﬁciency

should be preferred (i.e., sugar beet), to achieve improved energy
output with reduced fossil energy use during the crop cycle.

The overall result here, which highlights that conservative

agriculture allows energy saving and improvements in energy ef-
ﬁciency, should be considered valid in the short-term to mid-term,
as carried out in the present study. A long-term approach is needed
to determine for how long the soil fertility can satisfy the crop ni-
trogen requirements, and to validate the effects of conservative
tillage on the water and nutrient dynamics.

Finally, other factors other than the energy performance and

ﬁciency might lead to a different approach in the selection of one

crop over another, such as the available technologies on the farm
(i.e., a beet harvester rather than a forage harvester), the expertise
of the farmer relative to a speci

ﬁc crop, or the different income

levels (i.e., due to the selling price of the raw materials, and to the
costs of the starting materials and the transport).

References

[1]

European Commission. Biofuels in the European Union: a vision for 2030 and
beyond. Final Report of the biofuels advisory Counsil, DG Res. Sust. Energy
Systems EU 22066. 2006

[2] RED;

2009/28/EC;

http://eur-lex.europa.eu/legal-content/EN/ALL/?

uri

¼CELEX:32009L0028

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[3]