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Soil organic carbon changes in the cultivation of energy crops:Implications for GHG balances and soil quality for use in LCA
Miguel Brandao a,b,*, Llorenc Mila i Canals a,c, Roland Clift a
a Centre for Environmental Strategy, University of Surrey, GU2 7XH, UKb Institute for Environment and Sustainability, Joint Research Centre, European Commission T.P. 270,
Via Enrico Fermi, 2749, I-21027 Ispra (VA), Italyc SEAC, Unilever, Colworth Park, Sharnbrook, Bedford, MK44 1LQ , UK
a r t i c l e i n f o
Article history:
Received 19 September 2007
Received in revised form
2 January 2009
Accepted 4 December 2009
Available online xxx
Keywords:
Energy crops
Oilseed rape
Miscanthus
Willow short-rotation coppice (SRC)
Forest residues
Life cycle assessment (LCA)
Land use
Soil quality
Carbon sequestration
Soil organic carbon (SOC)
* Corresponding author. Institute for EnvironFermi, 2749, I-21027 Ispra (VA), Italy. Tel.: þ
E-mail address: [email protected] Expressed in CO2 equivalents, excluding
ments. Had LULUFC been considered, the U
Please cite this article in press as: Brandafor GHG balances and soil quality for use
0961-9534/$ – see front matter ª 2009 Elsevidoi:10.1016/j.biombioe.2009.10.019
a b s t r a c t
The environmental impact of different land-use systems for energy, up to the farm or
forest ‘‘gate’’, has been quantified with Life Cycle Assessment (LCA). Four representative
crops are considered: OilSeed Rape (OSR), Miscanthus, Short-Rotation Coppice (SRC) willow
and forest residues. The focus of the LCA is on changes in Soil Organic Carbon (SOC) but
energy use, emissions of GreenHouse Gases (GHGs), acidification and eutrophication are
also considered. In addition to providing an indicator of soil quality, changes in SOC are
shown to have a dominant effect on total GHG emissions. Miscanthus is the best land-use
option for GHG emissions and soil quality as it sequesters C at a higher rate than the other
crops, but this has to be weighed against other environmental impacts where Miscanthus
performs worse, such as acidification and eutrophication. OSR shows the worst perfor-
mance across all categories. Because forest residues are treated as a by-product, their
environmental impacts are small in all categories. The analysis highlights the need for
detailed site-specific modelling of SOC changes, and for consequential LCAs of the whole
fuel cycle including transport and use.
ª 2009 Elsevier Ltd. All rights reserved.
1. Introduction agricultural sector – estimated at 6.9% [2] – to total national
UK greenhouse gas (GHG) emissions in 2005 were estimated at
15.7% below 1990 levels1 [1] primarily because of the shift from
coal to gas for power generation. Under the Kyoto Protocol, the
UK government agreed to reduce GHG emissions by 12.5%
below 1990 levels on average over the commitment period
2008–2012. Despite the relatively low contribution of the
ment and Sustainability,39 332 785 969; fax: þ39 3ropa.eu (M. Brandao).emissions from Land Us
K emissions would have
o M, et al., Soil organic cain LCA, Biomass and Bi
er Ltd. All rights reserved
emissions, it is the largest emitter of two of the most powerful
GHGs: methane (CH4) and nitrous oxide (N2O), which are 25
and 298 times, respectively, more potent contributors to global
warming than carbon dioxide (CO2) (100-year GWP [3]). In
addition, since agriculture occupies 77% of total UK land area
[2], the potential of agriculture to mitigate GHG emission by
acting as a C sink through its sequestration in soil and by
Joint Research Centre, European Commission, T.P. 270, Via Enrico32 786645.
e, Land-Use Changes and Forestry (LULUCF) and aircraft move-been lower in 2005, but higher in 1990.
rbon changes in the cultivation of energy crops: Implicationsoenergy (2010), doi:10.1016/j.biombioe.2009.10.019
.
b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 0 ) 1 – 1 42
ARTICLE IN PRESS
producing energy crops to displace fossil fuels is promising
[4,5].
As part of the strategy to reduce GHG emissions, govern-
ment policy in the UK and elsewhere is promoting the
production and use of biomass energy (liquid biofuels for
transport and energy crops for heat and/or power). This policy
also contributes to other political and social objectives, such
as reduced reliance on fuel imports (energy security) and
increased income to farmers and rural communities [6–8]. The
UK Biomass Strategy published in 2007 sets out a clear hier-
archy for biomass which shows that the preferred uses are for
heat and combined heat and power (CHP) [5].
Debate over the advantages or otherwise of producing
biofuels or bioenergy is intense in both peer-reviewed journals
and the general media [9–17]. Biofuels and bioenergy can only
supply a small fraction of current energy demand due to the
fact that land is a finite resource. The competition for land
with food crops highlights that net energy yield per hectare is
a major concern between alternative energy crops [18–20].
Rising food prices have been attributed in part to demand for
biofuels [e.g. Ref. [19]. The potential GHG emission savings
from bioenergy and biofuels are not clear and vary widely
according to:
� the crop/feedstock chosen and how it is used (power, heat,
CHP, or ethanol and biodiesel for transport),
� the reference system for land use [21] and non-renewable
fuels, and
� the amount of agrochemical use [22].
Righelato and Spracklen [16] suggest that ‘‘the carbon
sequestered by restoring forests is greater than the emissions
avoided by the use of the liquid biofuels’’ due to the C
sequestered in the soil and in above-ground biomass, and
therefore argue for reforestation and forest maintenance. The
high variability in the impacts of bioenergy has led the WWF
to develop their own standards [23].
Despite recognition that global emissions due to land-use
changes since the Industrial Revolution have contributed to
Global Warming at the same order of magnitude as the
combustion of fossil fuels [24,25], degradation of soil carbon
has not been properly and consistently addressed in the
environmental assessment of agriculture and forestry
systems. Some studies have attempted to quantify the rates of
C sequestration/emissions associated with land use and land-
use changes, mainly using models simulating organic matter
turnover as part of the renewable carbon cycle [26]. In addition,
SOC loss is a major cause of soil quality degradation [27–30],
which is a major concern due to the scarcity of fertile land
[18,31]. Land use is recognised as the main driver of soil
degradation, although impacts on soil quality can also be
beneficial depending on land management practices. In fact, in
contrast to annual crops, perennial cropping systems tend to
accumulate SOC and some energy crops, such as Short-Rota-
tion Coppice (SRC) willow, can also serve for remediation of
contaminated soil [32,33]. It is therefore of paramount impor-
tance to include SOC changes [26] and soil quality effects when
comparing the environmental impacts of different bioenergy
land uses. As recognised by Larson [21], many LCA studies
ignore the changes in SOC associated with growing biomass;
Please cite this article in press as: Brandao M, et al., Soil organic cafor GHG balances and soil quality for use in LCA, Biomass and B
e.g. of the 24 LCA studies published in Biomass & Bioenergy [34–
57], only six consider SOC [34,36,43,44,48,50]. Furthermore,
there is no common methodology in these studies and others
[58–60]. Clearly a systematic and harmonised method for
considering SOC changes in LCA is needed.
In addition to soil organic carbon (SOC) [31,61–64]; erosion
[64], microbial biomass, salinisation [65], and ecosystem ther-
modynamics/exergy [66] have been suggested as possible
indicators for land-use impacts in LCA. However, SOC,
expressed as kg C$year present or absent in soil due to the land
use studied, is selected as a promising stand-alone indicator
because of its close association with most soil functions; oper-
ational methods to include it in LCA have already been sug-
gested [31] and it is of obvious importance for carbon inventory
calculations. Indeed, the SOC indicator provides directly rele-
vant information for the assessment of a system’s net contri-
bution to GWP through the effect on the soil carbon pool.
Besides, the effects on SOC from the same land use in different
regions in the world are potentially very variable; thus, current
development of life cycle inventory techniques to identify the
indirect land-use changes from bioenergy production will be
highly relevant for the application of this indicator [67].
The purpose of this paper is to compare different land uses
for energy and to assess the importance of SOC changes for
the GHG balance and for the soil quality impacts of energy
crops. Four types of land use relevant to bioenergy production
in the UK have been assessed here: an arable crop, two
different ligno-cellulosic energy crops and forest. OilSeed
Rape (OSR) (Brassica napus) is chosen because of its popularity
in Europe as a non-cereal arable crop using some 43% of arable
land in the UK [2]. The ligno-cellulosic crops considered are
willow (Salix spp.) under a short-rotation coppice (SRC)
regime, and elephant grass (Miscanthus x giganteus), as these
are the most relevant for UK conditions. Their current
proportion of agricultural land use is insignificant (<0.01%) but
would grow substantially if recommendations by the Royal
Commission on Environmental Pollution [68] are followed and
the market for energy crops develops. Forest is a major land
use (11.7% of UK total). The popular forestry crop Sitka Spruce
(Picea sitchensis) and its residues are modelled.
Section 2 briefly explains the methodology followed in the
study and describes the systems under analysis, as well as the
assumptions made, goals and boundaries, data sources, etc.
The main environmental impacts are reported in Section 3,
along with the sensitivity analyses, and discussed in Section 4.
Finally Section 5 highlights the main conclusions derived from
this study.
2. Application of life cycle assessment toland use for energy
LCA is a systems analysis tool that provides information on
the full environmental effects of a product, service or system
from its cradle (extraction of raw materials) to its grave (waste
management). It gathers information on all the inputs and
outputs to and from a product system, and assesses the
potential environmental impacts associated with these inputs
and outputs. Further information on this method can be found
in Refs. [69–72]. This LCA study forms part of a larger
rbon changes in the cultivation of energy crops: Implicationsioenergy (2010), doi:10.1016/j.biombioe.2009.10.019
Propagation material
CUTTING, SWATHING and BALING
OR
CULTIVATION
OR
DRYING
STORAGE
TRANSPORT
Bioenergy Feedstocks
Machinery/spares
Agrochemicals
Diesel fuelMotor spirit
Lubricating oil
Fue loil
Electricity
SteelSoftwood
Preservative
Diesel fuel
Fig. 1 – Flow Chart for the Production of Bioenergy
Feedstocks.
b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 0 ) 1 – 1 4 3
ARTICLE IN PRESS
comparison of the environmental impacts associated with
different systems for energy production and use from land in
the UK. The exposition follows the steps in LCA identified in
the relevant international standard [73].
2.1. Goal and scope of the LCA
The focus of this study is the cultivation of crops for energy.
The scope covers the ‘cradle-to-gate’ stage of the life cycle,
from extraction of raw materials through agricultural
Table 1 – Summary of the main inputs in each land use per ref
Life cycle stage O
Seeds (kg) Propagation 5
Cuttings/sets Propagation
Rhizomes (kg) Propagation
N fertiliser (kg N) Fertilisation 19
P fertiliser (kg P2O5) Fertilisation 50
K fertiliser (kg K2O) Fertilisation 48
Lime (kg CaO) Fertilisation 19
Manganese (l MnSO4) Fertilisation
Pesticidesa (kg) Pest control 2.
Softwood (kg) Weed control
Steel (kg) Weed control
Preservative (kg) Weed control
Diesel fuel (GJ) Mechanisation, transport, storage 2.
Motor spirit (GJ) Mechanisation
Fuel oil (GJ) Drying 1
Lubricating oil (MJ) Mechanisation
Electricity (kWh) Storage 33
Machinery/spares (MJ) Mechanisation
a Includes herbicides and fungicides.
Please cite this article in press as: Brandao M, et al., Soil organic cafor GHG balances and soil quality for use in LCA, Biomass and Bi
activities and production to the point where the crop is har-
vested and ready for transport. The specific objectives are to:
1. Determine which life cycle stages of representative
biomass feedstocks contribute the greatest environmental
impacts, including land-use related soil emissions;
2. Compare different land uses in the UK for the production of
biomass/biofuels.
This paper reports on an LCA study of four specific land
uses in the UK. It is intended to be representative of average
UK practices, and care should be taken not to draw conclu-
sions from this study for comparison between different
regions within or outside the UK.
The reference unit, i.e. the reference measure for which the
environmental burdens are expressed, is taken as 1 ha of land
for one year (1 ha� yr). To show the significance of the results
for LCA of energy systems, they are converted into indicative
values for the whole fuel cycle, expressed in terms of energy-
based units (1 GJ of energy in this case), using representative
values of net energy yield and avoided energy use. However, any
detailed study will require specific figures for these parameters.
UK-specific data have been used. Input data were collected
from various studies [74–76]. Data for production of ancillary
materials and machinery has been obtained from existing
databases, as described in the relevant sections. The main
LCA database used is ecoinvent 2000 version 1.2 (http://www.
ecoinvent.ch) [77–80] which is sufficiently comprehensive to
cover the operations in the bioenergy supply chain.
2.2. Systems description
2.2.1. Land use I: oilseed rapeOilSeed Rape (OSR) is an annual arable crop, farmed primarily
for its vegetable oils, used in human food. However, OSR is
becoming increasingly popular for energy purposes: refined
rapeseed oil and biodiesel from OSR can both be used as fuels.
erence unit (haL1 yrL1) throughout, after Elsayed et al. [74].
SR Miscanthus Willow SRC Forest residues
6250/313
53
6.0 5.3
.0 4.8
.0 5.1
158
4
80 0.51 2.25
8.6
12.3
2.8
39 2.48 1.14 0.40
0.59
1 0
158 107
rbon changes in the cultivation of energy crops: Implicationsoenergy (2010), doi:10.1016/j.biombioe.2009.10.019
Table 2 – Emissions of ammonia (NH3-N as % loss of Ncontent) from mineral fertilisers (adapted from [101] in[100]).
Inputs (mineral fertilisers) Ammonia (NH3-N) emissionsto air (% loss of N content)
Ammonia, direct application 1.0
Ammonium nitrate 2.0
Ammonium phosphate 4.0
Ammonium sulphate 8.0
Calcium ammonium nitrate 2.0
Compound N 4.0
Nitrogen solutions 2.5
NK N 2.0
NPK Na 4.0
Other NP N 3.0
Other straight nitrogen 2.5
Total straight nitrogenb 4.0
Urea 15.0
a Assumed to be half nitrate, half ammonium.
b This should only be used if no information is available on
fertiliser consumption of the individual categories.
Table 3 – Emissions of nitrous oxide (N2O-N as % loss of Ncontent) from mineral fertilisers (adapted from [101] in[100]).
Inputs (mineral fertilisers) Nitrous oxide (N2O-N)emissions to air
(% loss of N content)
Ammonium (soil temperatures 0–10 �C) 0.40
Ammonium (soil temperatures 10–20 �C) 0.50
Nitrate (soil temperatures 0–10 �C) 1.70
Nitrate (soil temperatures 10–20 �C) 1.10
NPK Na (soil temperatures 0–10 �C) 1.05
NPK Na (soil temperatures 10–20 �C) 0.80
Urea (soil temperatures 0–10 �C) 0.80
Urea (soil temperatures 10–20 �C) 3.00
a Assumed to be half nitrate, half ammonium.
b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 0 ) 1 – 1 44
ARTICLE IN PRESS
Rape straw is assumed here to be ploughed back into the land.
In a different scenario where straw would have been exported
from the system, the soil carbon emissions would be around
0.40 t C ha�1 yr�1 [81]; incorporating the straw reduces the loss
by typically 0.16 t C ha�1 [81] so that the total SOC loss is around
0.24 t C ha�1. If the straw were removed as a co-product, then
allocation of the total environmental burdens between the
rapeseed and the rape straw would be needed. The subsequent
stages of solvent extraction, refining and esterification, in
which dried rapeseed is processed into crude rapeseed oil and
rape meal; and the oil is further processed into refined rape-
seed oil which is then converted into biodiesel and crude
glycerine, are not part of the scope and, thus, not considered.
2.2.2. Land use II: MiscanthusMiscanthus (M. x giganteus), also known as elephant grass, is a C4
perennial energy and fibre crop. Alternative uses include
animal bedding, paper making, biopolymer manufacture, and
biodegradable products production (e.g. flowerpots). Miscanthus
is indigenous to Africa and Asia but is now grown commercially
in the UK [68,76,82–84]. There are, currently, around
12,600 ha of Miscanthus cultivation in England and Wales [85].
Miscanthus is propagated vegetatively from rhizomes or by
micro-propagation, from commercially available materials
which can be planted using existing machinery for more
common crops. Weed control and fertiliser inputs are essen-
tial at establishment but not subsequently. Miscanthus is
harvested annually in winter by cutting and baling into 5–
600 kg Heston bales, which are stored outside prior to being
transported to the end user.
Miscanthus typically has a useful cropping cycle of 20 years,
although it takes one year for establishment [86]; this study
assumes one year for establishment followed by 19 years of
production. The yield of a mature crop can be up to 20 oven-
dried tonnes (odt2) per hectare and year (odt ha�1 yr�1) [87,88].
Miscanthus takes three years to mature, during which period
2 At 0% moisture content, for practical purposes.
Please cite this article in press as: Brandao M, et al., Soil organic cafor GHG balances and soil quality for use in LCA, Biomass and B
the yield is lower. Data on yield, inputs, outputs, and C
sequestration were obtained from the literature [74,89–92]. An
effective annual yield of 19.2 t ha�1 yr�1 (25% moisture content
– 14.4 odt ha�1 yr�1) after 20% losses during harvest and
storage is assumed3 [74]. However, improved cultivation
practices and cultivars have been shown to give higher yield
or, alternatively, to enable poorer land to be used [86,93].
Miscanthus is a simple product so no allocation of envi-
ronmental impacts is necessary.
2.2.3. Land use III: willow SRCWillow SRC is a fast-growing perennial woody crop that, when
harvested, chipped and dried, can be used as a fuel for heat
and power generation [84]. There are, currently, around
2600 ha of Willow SRC in England and Wales [85].
Like Miscanthus, willow is propagated vegetatively using
commercially available cuttings; pest and weed control are
essential in the first two years. SRC takes usually one year to
get established, followed by a productive period of 15–30 years.
The crop is harvested every three years. There are different
harvesting techniques: combined harvesting and baling, and
stick harvesting and baling. The harvested crop is commonly
stored on-farm as billets. It can then be chipped or processed
into granules [94].
The average yield of a mature crop of current cultivars is
typically 9.5 odt ha�1 yr�1 [74], but yields are rising with
the introduction of improved strains and can be in excess
of 20 odt ha�1 yr�1 [95]. A 16-year rotation averaging
9.5 odt ha�1 yr�1 is assumed here, although rotations can last for
30 years. Biomass yields for the whole rotation are therefore
assumed to be 152 odt ha�1. Stick harvesting and baling was also
assumed because it is the most dominant. No allocation is
necessary as there is no co-production. A sensitivity analysis is
presented, showing the variability of results according to
different yields, rotation periods and carbon sequestration rates.
2.2.4. Land use IV: forest residuesForest residues refer to all forest material which may be of too
poor a quality for traditional timber markets, including resi-
dues arising from forestry operations, such as tops of stems,
3 Losses are due to crop trampling by machinery, to excessivemoisture content and to fallen leaves and tops.
rbon changes in the cultivation of energy crops: Implicationsioenergy (2010), doi:10.1016/j.biombioe.2009.10.019
Table 4 – Carbon sequestration in soils under differentland uses in UK per reference unit (haL1 yrL1).
Land use C sequestereda (t C) Reference
OSR �0.24� 0.08 [81]
Miscanthus 0.62 [103]
Willow SRC 0.09 – 0.18 [103]
Forest 0.32 [103]
a Negative value indicates C-emission to atmosphere.
Quality
SOCpot
t ini tfin trelax,pot
SOCfin
SOCini
trelax
relaxation
time
Time
(tC·ha-1)
R, Relaxation rate
Fig. 2 – Calculation of impacts on soil quality measured by
SOC.
b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 0 ) 1 – 1 4 5
ARTICLE IN PRESS
side branches (which may include foliage), diseased wood and
deadwood [96], as well as those derived from chunks from
sawn timber milling waste [74]. These materials are normally
treated as waste, or at best considered a by-product. The
Biomass Energy Centre [97] estimates that almost 6.5 million
odt per year can be made available from UK forest and
woodlands, considering technical and environmental
constraints, but not economic and market constraints.
Insensitive harvesting of residues from forests can have
detrimental impacts on biodiversity.
Like the other land uses, the system is based on typical
practice in the UK [74]. Since the residues will be produced
whether they are used or not, allocation of the environmental
load between forest residues and the other co-products
(sawlogs and small roundwood) was not necessary as forest
residues are essentially waste. As a result, only the environ-
mental load related to the collection, extraction, transport,
drying and storage and chipping is included. Part of the resi-
dues originates from chunks from the sawlogs route. The
environmental load of transport and milling of sawlogs were
allocated to timber as the primary product. The chunks were
considered to be a waste product and their transformation into
chips regarded as a means of valorisation; therefore, all inputs
related to chipping of chunks were allocated to the chips.
2.3. Inventory analysis
A life cycle inventory analysis has been carried out for each land
use. The stages adopted include: cultivation, regeneration,
harvesting, baling, transport, chipping, drying, and storage. The
inputs considered include agrochemicals (fertiliser, pesticides,
herbicide, manganese), seeds, tree seedlings, cutting/sets,
rhizomes, liquid fuels, lubricating oil, electricity, machinery/
spares, softwood, steel, preservative; the latter three used for
fencing (Fig. 1). Post-farm processing is not considered.
Generic LCA data used for farm operations include fuel,
farm machinery and steel production for machinery spares.
All come from the ecoinvent 2000 v 1.2 database. Fertiliser
production data have been obtained from an existing study
Table 5 – Effects on soil quality at the cropping stage (typical v
SOCini
(t C ha�1)SOCini� SOCfin
(t C ha�1)SO
(t C
OSR 80 [102] �0.24 [81] 150
Miscanthus 80 [102] þ0.62 [103] 150
Willow SRC 80 [102] þ0.14 [103] 150
Forest residues 130 [102] 0 150
Please cite this article in press as: Brandao M, et al., Soil organic cafor GHG balances and soil quality for use in LCA, Biomass and Bi
[98] (used within the ecoinvent database), as well as common
practice in the field in the UK [74]. Generic LCA data used for
harvest and post-harvest operations include fuel and elec-
tricity (UK generating mix). Table 1 summarises the main
inputs in each land use.
Nutrient-related emissions from soil (NH3; N2O; NOx; NO3�;
PO43�; CH4) have been obtained from literature values for crops
in general:
- NH3-N emission factors (expressed as % loss of N content)
from [99] have been used following the recommendation of
[100]; see Table 2.
- For N2O emissions, the emission factors for mineral fertil-
isers [101] have been used; see Table 3. For organic fertil-
isers, the content of nitrate and ammonium N has been
used with the factors in Table 2 for nitrate and ammonium.
- NOx-N has been considered as 10% of N2O-N [100], [p. 49].
- NO3� and PO4
3� have been obtained from literature values as
15 kg N-NO3�ha�1 yr�1 and 1 kg P-PO4
3�ha�1 yr�1 [62].
- An emission of 1 kg of CH4 to the air per each 150 kg of N
applied as ammonium fertiliser has been included [100],
[p. 58].
Calculation of changes in soil quality and GHG emissions
requires estimates of the effects of the production system on
soil organic carbon (SOC). Data on these are based on Refs.
[81,102,104]. Table 4 includes values of soil carbon emissions or
sequestration for all land uses and Table 5 includes initial stocks
of carbon for all land uses. It is assumed that all C captured as
SOC comes from atmospheric CO2 through photosynthesis, and
that all SOC degraded is emitted as CO2 to the atmosphere.
alues).
Cpot
ha�1)tini� tfin
(years)trelax
(years)DC CF
(t C yr ha�1 yr�1)
[102] 1 100.8 122.7
[102] 1 98.1 �65.3
[102] 1 99.6 40.3
[102] 1 100.0 20.0
rbon changes in the cultivation of energy crops: Implicationsoenergy (2010), doi:10.1016/j.biombioe.2009.10.019
-3,000
-2,000
-1,000
0
1,000
2,000
3,000
4,000
OSR Miscanthus Willow SRC ForestResidues
Land Use
OC
gk
(s
noi
ssi
me
GH
Gl
at
oT
2)
qe Soil
Storage
Drying
Transport
Mechanisation
Pesticides
Fertilisers
Propagation
Fig. 3 – Global Warming Potential of different land uses per
reference unit (haL1 yrL1) and their relative contribution
from different sources.
b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 0 ) 1 – 1 46
ARTICLE IN PRESS
2.4. Impact assessment
The impact assessment phase has been performed using
mainly the CML 2001 method [104] due to its comprehen-
DCht C yr ha�1 yr�1
i¼�SOCpot � SOCini
�� ðtrelax � tiniÞ þ 1=2ðtrelax � tiniÞ �
�SOCini � SOCfin
��tfin � tini
� (1)
siveness in terms of environmental issues covered and its
scientific soundness. The following impact categories and
indicators have been considered because of their relevance to
agricultural and forestry systems:
� Primary energy use (measured in MJ);
� Climate change (measured as GWP of the GHG emitted [104],
including emissions from SOC degradation);
� Acidification Potential [104];
� Eutrophication Potential [104];
� Soil quality (through changes in SOC [31]),
The most novel aspects in terms of life cycle impact
assessment methodology are the inclusion of SOC degrada-
tion both for GWP and soil quality assessment. In the case of
GWP, one additional kg C stored in soil (a positive value in
Table 4) is equivalent to avoided GHG emissions of �3.67 kg
DCht C yr ha�1 yr�1
i¼
�150 t C ha�1 � 80 t C ha�1
�� ð100:8 yr� 99 yrÞ þ 1=2ð100:8 yr� 99 yrÞ �
�80 t C ha�1 � 79:76 t C ha�1
�
ð100 yr� 99 yrÞ ¼ 122:7
CO2-eq. (44 kg CO2/12 kg C), whereas 1 kg C released to the
atmosphere from SOC degradation has a GWP of 3.67.
It may be argued that carbon sequestered as SOC may be
re-released to the atmosphere in a short period of time, and
therefore the GWP attributed to such sequestration should be
closer to zero. The values we have used for the sequestration
rates are calculated as mid-term trends and thus incorporate
Please cite this article in press as: Brandao M, et al., Soil organic cafor GHG balances and soil quality for use in LCA, Biomass and B
this issue at least partially. More sophisticated modelling is
required to include the temporal aspect in the GWP values, in
a similar way as suggested for wood products [105].
Soil quality refers to the ability of soil to sustain life support
functions [31,63]: biotic production; substance cycling and
buffer capacity; climate regulation. The impacts of production
systems on soil quality have not traditionally been included in
LCA, and the recommendations of Mila i Canals et al. [33] have
been followed here. Particularly, Mila i Canals et al. [31,33] and
others argue that soil organic matter (SOM) can be used as an
indicator for soil quality within LCA of agricultural systems:
an increase in soil organic matter due to the soil management
practices implies a benefit, whereas any decrease in SOM is
accounted as damage to the system. The impact is measured
as a carbon deficit (or credit, expressed by negative values)
with the unit ‘kg C$year’, referring to the amount of extra
carbon temporarily added to or removed from the soil in the
system studied compared to a reference system [31].
The method developed for land-use LCIA by Mila i Canals
et al. [31] has been slightly modified to follow the consider-
ations in [33]. The general formula used to calculate charac-
terisation factors (CF) for land-use flows is shown in Eq. (1); see
Fig. 2 for an explanation of the formula’s parameters.
where SOCpot is the potential level of SOC if land is left
undisturbed; SOCini the SOC level at the start of the land use
studied; SOCfin is the SOC level at the end of the cultivation
period; the studied land use starts at time tini and ends at time
tfin; and at time trelax soil quality has reverted to the level prior
to land use. trelax may be calculated from the relaxation rate R
(see third assumption below). The equation assumes very
simplified shapes of the evolution of soil quality, as suggested
in [33]. The first component of the numerator refers to the
impacts due to the postponed relaxation of the system (light-
coloured area in Fig. 2), whereas the second component is the
dark-coloured ‘‘triangle’’, referring to the impacts due to the
change in quality during the occupation. The denominator
serves to express the characterisation factors per ha yr, with
all SOC values expressed as t C per ha.
For example, in calculating the soil quality impacts of OSR,
we have:
The following assumptions have been made:
– All transformation impacts are allocated to the subsequent
100 years of cropping, as opposed to the suggestion by
Ref. [106], who allocated all transformation impacts to the
first year of cropping. All land transformations took place
more than 100 years ago. For more information on the
rbon changes in the cultivation of energy crops: Implicationsioenergy (2010), doi:10.1016/j.biombioe.2009.10.019
Table 6 – Summary of yields in each land use perreference unit (haL1 yrL1) (NB: excludes gate-to-grave lifecycle stages).
OSRa Miscanthusb WillowSRCc
Forestresiduesc
Biomass yield (t) 2.9 19.2 9.5 0.5
Energy yieldd (GJ) 52.0 345.6 169.5 8.6
Energy
requirement (GJ)
14.1 6.9 6.4 0.1
Net energy
yield (GJ)
37.9 338.7 163.2 8.5
a Dried rapeseed.
b Miscanthus fuel feed (25% moisture content).
c Dried wood chips (25% moisture content).
d Net calorific value of the crop (indicative values only).
Table 7 – Summary of GHG emissions (kg CO2-eq.) in eachland use per reference unit (haL1 yrL1).
OSRa Miscanthusb WillowSRCc
Forestresiduesc
(A) Cradle-to-gate
excl. SOC
1833 707 332 10
(B) SOC 880 �2273 �497 0
(C) Cradle-to-gate
(Aþ B)
2763 �1567 �165 10
(D) Gate-to-graved 344 980 432 28
(E) Totale (CþD) 3107 �587 267 38
(F) Avoidede �3509 �10,509 �10,638 �540
(G) Total incl.
avoidedd (Eþ F)
�402 �11,096 �10,371 �502
a Dried rapeseed.
b Miscanthus fuel feed (25% moisture content).
c Dried wood chips (25% moisture content).
d Indicative values only.
e Biodiesel from OSR displaces diesel, whereas power from the
combustion of Miscanthus displaces electricity from UK grid. Heat
from the combustion of wood chips from Willow SRC and Forest
Residues displaces heat from an oil-fired boiler.
b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 0 ) 1 – 1 4 7
ARTICLE IN PRESS
calculation of the characterisation factors for all land-use
flows, refer to Ref. [107].
– SOCpot for UK and all background uses is 150 t C ha�1
(temperate warm forest, [102])
– Changes in soil quality due to land use have been assessed
relative to a situation where this activity is not undertaken.
Thus, natural relaxation has been used as the reference
system4 [33]. The relaxation rate, R, during natural relaxation
has been estimated as 0.32 t C ha�1 yr�1 [103].
3. Results
For the reasons outlined in Section 2.1, the reference unit for
the study is production of the specified crop on 1 ha of land for
one year; i.e. 1 ha yr. The results in Sections 3.1–3.5 are
expressed on this basis.
3.1. Global warming potential
Fig. 3 shows the GHG emissions resulting from the different
land uses, showing the contributions of the different life cycle
stages. These emissions are clearly dominated by changes in
SOC (see Section 2.2). Table 6 gives the numerical values for
the energy balances and Table 7 shows the GHG balances.
Fertiliser use causes much of the impact for OSR, primarily
due to field emissions of greenhouse gases, mainly N2O from
soil and CO2 from oxidation of soil organic carbon, with
additional emissions of CO2 and N2O from fertiliser produc-
tion. For Miscanthus and willow SRC, SOC sequestration more
than compensates for the emissions. The impacts allocated to
recovering forest residues are low.
3.2. Soil quality (soil organic carbon)
The effect of the different land uses on soil quality is shown
diagrammatically in Fig. 4, while Fig. 5 and Tables 4 and 5 give
the quantitative estimates for SOC changes. Oilseed rape has
4 An alternative more consequential approach would assumethat, instead of natural relaxation, UK cropland would most likelybe used for food or feed production. The reference would beadopted accordingly.
Please cite this article in press as: Brandao M, et al., Soil organic cafor GHG balances and soil quality for use in LCA, Biomass and Bi
the highest detrimental impact on soil quality because of the
differences between SOCpot and SOCini, and between SOCini
and SOCfin. In this way, not only does OSR delay relaxation,
but it also decreases levels of SOC during occupation. Most of
the impact is due to the delay of relaxation rather than the
occupation itself. Miscanthus has a beneficial impact as it
increases SOC levels at rates higher than the relaxation rate.
Willow SRC has a detrimental effect because, even though this
land use shows a net increase in SOC, this increase is smaller
than the reference system considered (0.136 t C ha�1 yr�1
compared to 0.320 t C ha�1 yr�1 during natural relaxation, see
Section 2.4).
In the case of forest residues, zero accumulation of SOC has
been assumed, even though forests tend to accumulate SOC.
SOC is assumed to increase at 0.32 t C ha�1 yr�1 [102] if the
forest residues are not removed, but removal of residues is
assumed to eliminate this increase. This is a conservative
assumption, because it assumes that all the SOC accumula-
tion results from the residues and none from leaves, twigs and
roots. The effect of removing forest residues on soil quality is
slightly lower than that of Willow SRC, but it is still
detrimental.
Most of the total impact is due to the change in relaxation
in all land uses, and occupation impacts are negligible.
3.3. Acidification potential
Fig. 6 shows the acidification potential of emissions from the
different land uses. OSR is the land use generating the highest
acidification potential. Fertiliser use contributes most to
acidification for both Miscanthus and OSR, mainly due to fer-
tiliser-related ammonia emissions. On the other hand,
nitrogen oxide emissions from diesel use dominate for willow
SRC.
rbon changes in the cultivation of energy crops: Implicationsoenergy (2010), doi:10.1016/j.biombioe.2009.10.019
Fig. 4 – Representation of the effect of OSR (top left) Miscanthus (top right), Willow SRC (bottom left) and Forest Residues on
Soil Quality (not to scale).
)
30
b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 0 ) 1 – 1 48
ARTICLE IN PRESS
3.4. Eutrophication potential
Fig. 7 shows the eutrophication potential arising from the
different land uses. Again, OSR shows the highest impact,
dominated by the estimated nutrient emissions from fertiliser
use: ammonia (NH3) and nitrate (NO3�) emissions to water.
3.5. Primary non-renewable energy use
Fig. 8 shows the primary non-renewable energy requirements
(expressed in GJ) to produce bioenergy from 1 ha of different
-100
-50
0
50
100
150
Oilseed rape Miscanthus Willow SRC Forest Residues
Land Use
)r
yC
t(
ytil
au
qli
os
no
tc
ap
mI
Fig. 5 – Impact of the different land uses on Soil Quality per
reference unit (haL1 yrL1).
Please cite this article in press as: Brandao M, et al., Soil organic cafor GHG balances and soil quality for use in LCA, Biomass and B
land uses. It is noteworthy that oilseed rape uses more than
twice the energy per ha of any of the other land uses, even
without considering post-farm processing. This is mainly due
to the nitrogen fertiliser used. Energy use in the Miscanthus life
cycle arises mainly from herbicide production and diesel use.
Fencing and diesel use in mechanical operations in the culti-
vation, harvesting and chipping stages account for most
PA
(l
ai
tn
et
oP
no
it
ac
if
id
ic
A
OS
gk
[2
]q
e-
0
5
10
15
20
25
Oilseed rape Miscanthus Willow SRC Forest Residues
Fig. 6 – Acidification Potential per reference unit
(haL1 yrL1).
rbon changes in the cultivation of energy crops: Implicationsioenergy (2010), doi:10.1016/j.biombioe.2009.10.019
)P
E(
l
ai
tn
et
oP
n
oi
ta
ci
hp
or
tu
E
]q
e-
et
ah
ps
oh
P
gk
[
0
5
10
15
20
25
30
Oilseed rape Miscanthus Willow SRC Forest Residues
Fig. 7 – Eutrophication Potential per reference unit
(haL1 yrL1).
Table 8 – Summary of energy and carbon-equivalentbalances per GJ in each land use (NB: includes all life cyclestages – indicative values only).
OSR Miscanthus WillowSRC
Forestresidues
Land area (ha yr) 0.025 0.015 0.010 0.194
Energy requirement (GJ) 0.44 0.27 0.11 0.10
GHG emissions excl.
SOC (kg CO2-eq.)
40.7 26.0 7.5 7.4
GHG emissions from
SOC (kg CO2-eq.)
36.4 �35.0 �4.9 0.0
Total GHG emissions
(kg CO2-eq.)
77.0 �9.0 2.6 7.4
Total GHG emissions
incl. avoided
emissions (kg CO2-eq.)
�10.0 �171.0 �102.4 �97.7
b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 0 ) 1 – 1 4 9
ARTICLE IN PRESS
energy use in willow SRC, and forest residues use energy
mostly through mechanisation (diesel fuel use in the regen-
eration and harvesting stages, and chipping). The cultivation/
regeneration stages therefore represent the hotspots in
energy use, mainly due to the use of agrochemicals and fossil
fuels.
3.6. Net energy yield
Although the focus here has been on land use, it is of interest
to give broad comparisons in terms of net energy yield. Table 6
gives indicative values for the net calorific value of the
biomass leaving the ‘‘gate’’ (Energy Yield), not allowing for
energy used in subsequent processing or transport. Table 8
gives the resulting figures for the impacts and land use per GJ.
In order to show the significance of the results, they are con-
verted into indicative values for the whole fuel cycle using
representative values of net energy yield and avoided energy
use (see Tables 6–8).
The energy crops present the highest energy yield per ha,
with the yield of Miscanthus being more than twice that of
willow SRC (see Table 6). The relatively low energy yield of OSR
Fig. 8 – Primary energy use per reference unit (haL1 yrL1).
Please cite this article in press as: Brandao M, et al., Soil organic cafor GHG balances and soil quality for use in LCA, Biomass and Bi
combined with the relatively high energy requirements
results in a low net energy yield.
3.7. Sensitivity analyses
The results obtained will vary according to the adopted
assumptions of yield, productive period, and carbon seques-
tration rates. These varying assumptions found for Willow
SRC are tested in this section and presented in Tables 9–11.
The other land uses were not subject to sensitivity analyses
since there were no data variability found for their
parameters.
The yield of wood chips from SRC can be as much as
20 odt ha�1 yr�1 (see Section 2.2) and the production period
can be up to 30 years. The crop energy yield increases
accordingly (�356 GJ ha�1 yr�1). The energy requirement of
SRC now varies between 6.4 and 7.9 GJ ha�1 yr�1, resulting in
a net energy yield of 163–348 GJ ha�1 yr�1 (see Table 9).
If we further assume that the carbon sequestration rate
varies from 0.09 to 0.18 t C ha�1 yr�1, total GHG emissions are
between �328 and 98 kg CO2-eq. ha�1 yr�1, implying that,
without considering the rest of the life cycle, SRC on its own
may not be carbon-negative. Table 10 presents indicative
values for the whole fuel cycle. These are in the range of 104–
1091 kg CO2-eq. ha�1 yr�1. Assuming that heat production
from the combustion of wood chips displaces heat provided
by a small-scale oil-fired boiler, between 10,638 and 22,340 kg
CO2-eq. ha�1 yr�1 are avoided, depending on the yield
assumptions. The total GHG emissions, therefore, vary
between �21,576 and �10,208 kg CO2-eq. ha�1 yr�1 when the
whole fuel cycle is considered. All variances are explained by
Table 9 – Analysis of sensitivity to Willow SRC yields andproductive period per reference unit (haL1 yrL1) (NB:excludes gate-to-grave life cycle stages).
Biomass yield (t) 9.5–20.0
Energy yielda (GJ) 169.5–356.0
Energy requirement (GJ) 6.4–7.9
Net energy yield (GJ) 163.2–348.1
a Net calorific value (indicative values only).
rbon changes in the cultivation of energy crops: Implicationsoenergy (2010), doi:10.1016/j.biombioe.2009.10.019
Table 10 – Sensitivity analysis of GHG emissions (kg CO2-eq.) to Willow SRC yields, productive period and carbonsequestration rates per reference unit (haL1 yrL1).
(A) Cradle-to-gate excl. SOC 332–431
(B) SOCa �660 to �334
(C) Cradle-to-gate (Aþ B) �328 to 98
(D) Gate-to-graveb 432–993
(E) Totalb (CþD) 104–1091
(F) Avoided c 10,638–22,340
(G) Total incl. avoidedb (Eþ F) �21,576 to �10,208
a Negative value indicates C sequestration from the atmosphere.
b Indicative values only.
c Heat generated from the combustion of wood chips from Willow
SRC displaces heat from small-scale oil-fired boiler.
b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 0 ) 1 – 1 410
ARTICLE IN PRESS
the extreme assumptions for the values of SOC sequestration,
yield and productive period (see Table 10).
Varying rates of SOC sequestration will also determine the
magnitude of the effects on soil quality. Willow SRC is asso-
ciated with a rate of SOC sequestration of 0.09–0.18 t
C ha�1 yr�1, which results in a change in SOC of 30.6–50.1 t
C yr ha�1 yr�1.
Table 11, finally, shows the above results per GJ of heat
produced by the small-scale combustion of wood chips from
Willow SRC. The land area required per GJ is between 0.005 and
0.010 ha. The energy requirement varies between 0.092 and
0.113 GJ GJ�1(energy input per energy output). Fossil-based
GHG emissions vary between 6.7 and 7.5 kg CO2-eq. ha�1 yr�1
which are counterbalanced by their biogenic counterparts
from soil and result in 1.0–5.1 kg CO2-eq. ha�1 yr�1. These low
figures are fully compensated by the avoided emissions from
heat generated from an oil-fired boiler, to result in a total GHG
emission that is negative (�99.9 to �104.0).
Overall, the sensitivity analyses show that the GHG
balances for Willow SRC may vary substantially, but that this
land use always yields net negative emissions.
In this paper, impacts form land transformations are allo-
cated over the subsequent 100 years of cropping. Because we
assume that, at the cropping stage, land transformations from
an undisturbed state happened more than 100 years ago, no
impacts from transformation are considered for GWP or for
soil quality. These impacts refer to the change in SOC levels
from 150 t C ha�1 (SOCpot) to 80 t C ha�1 (SOCini). The magni-
tude of these impacts is as follows:
- for GWP, soil emissions of 70 t C ha�1 (257 t CO2 ha�1)
- for soil quality, transformation is responsible for 7656 t
C yr ha�1 yr�1
Table 11 – Sensitivity analysis of energy and carbon-equivalent balances per GJ in Willow SRC (NB: includes alllife cycle stages – indicative values only).
Land area (ha yr) 0.005–0.010
Energy requirement (GJ) 0.092–0.113
GHG emissions excl. soil (kg CO2-eq.) 6.7–7.5
GHG emissions from soil (kg CO2-eq.) �6.5 to �1.6
Total GHG emissions (kg CO2-eq.) 1.0–5.1
Total GHG emissions incl. avoided emissions
(kg CO2-eq.)
�99.9 to �104.0
Please cite this article in press as: Brandao M, et al., Soil organic cafor GHG balances and soil quality for use in LCA, Biomass and B
There is no standard method for allocating the impacts
from transformation along subsequent land uses (see Section
2.4). If we consider that land transformation happened less
than 100 years prior to the land use under study, each annual
land use is responsible for 1% (1 year out of 100 years) of the
impacts from transformation. Consequently, 2.57 t CO2 ha�1
would be added to the GWP, which is more than the emissions
from the rest of the life cycles of these crops. Similarly, 77 t
C yr ha�1 yr�1 would be a significant portion of the total
impacts on soil quality. The results of the impacts of land
transformation are therefore highly sensitive to the time
period adopted between land transformation and land use
under study, as well as the period within which the impact
from land transformation is allocated to subsequent crops.
4. Discussion
Eutrophication emissions are derived from literature values,
and it should be noted that while NH3 emissions are propor-
tional to the amount of fertilisers used, a fixed rate has been
assumed for nitrate emissions (see Section 2.3). Whilst the
authors believe this value is representative, it is presented as
an indication only.
In terms of the energy balance, although it may be relevant
to have information on the energy use of a system, primary
energy use is not per se an environmental impact category.
However, Huijbregts et al. [108] found that Cumulative Energy
Demand (or the total non-renewable energy used along the
life cycle) correlates well with most environmental life cycle
impact categories and can, therefore, be considered an
appropriate proxy indicator for environmental performance.
Indeed, comparison of Figs. 3 and 5–8 shows that primary non-
renewable energy use gives the same ranking of different
crops as acidification and eutrophication. However, when SOC
is included in the analysis, primary energy use does not
correlate with GHG emissions or soil quality, so that it cannot
be used as a proxy.
Estimates of changes in SOC are highly dependent on the
input data for the initial soil quality and on the reference
system used for comparison; the latter point has also been
noted by others [109]. Furthermore, SOC evolution depends
strongly on management practices and location, and so any
decision to use one value or another should be properly
justified. In this work, the estimates are all derived from
literature values. Therefore, while the results obtained here
are plausible, they should also be interpreted as broad
comparisons only. However, the differences found between
different land uses are so large that they may be considered
significant.
5. Conclusions
Changes in SOC dominate the GHG emissions from cultivation
of energy crops, followed by diesel and fertiliser use. It is thus
important to consider changes in SOC in LCA studies of energy
crops, and challenge the results of those studies not including
them. Of the four energy crops studied, OSR shows the biggest
impacts on soil quality. This is due to OSR causing the largest
rbon changes in the cultivation of energy crops: Implicationsioenergy (2010), doi:10.1016/j.biombioe.2009.10.019
b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 0 ) 1 – 1 4 11
ARTICLE IN PRESS
degradation of SOC during the land use (i.e. the dark-coloured
area in Fig. 2); but also, and primarily, because of the change in
relaxation (SOCpot� SOCini), which adds to the magnitude of
the impact for OSR (i.e. light-coloured area in Fig. 2). It should
also be noted that land-use impacts are multi-faceted (e.g.
including effects on biodiversity and water quantity/quality
[93,110]): SOC does not indicate all possible impacts on soil
quality, so that alternative/complementary indicators may be
required in specific cases (e.g. when erosion or salinisation
dominates soil degradation).
Apart from CO2 emissions derived from SOC degradation,
other field emissions – primarily nitrogen fertiliser-related
emissions – dominate many of the impacts considered: N2O
contributing to GHGs, NH3 to acidification, and NH3 and NO3�
to eutrophication.
The focus of this study has been on the GHG balance and it is
clear that land under Miscanthus has the lowest impact, mainly
due to C sequestration. However, Miscanthus is not the best
land-use option for other impacts – acidification and eutro-
phication. The LCA approach is, therefore, a very informative
tool that, among other things, makes clear the existence of
trade-offs but does not necessarily give a simple identification
of the preferred alternative. Decision processes therefore need
to recognise trade-offs between different impacts, for example
using some form of Multi-Criteria Decision Analysis.
This work has confirmed that ligno-cellulosic energy crops
and forest residues give much higher yields and lower GHG
emissions than the arable crop OilSeed Rape. However, anal-
ysis of competition for land between food and energy crops
requires further assessment, using a consequential LCA
approach which allows for displaced land use as well as full
life cycles. This will require data on the most likely effects of
land-use change and the net energy yield associated with the
land uses. This net energy yield will help to estimate the GHG
emissions and other impacts avoided by replacing other
energy sources and other land uses. However, if food
consumption is constant, food production displaced by energy
crops will be replaced by imports so that any net environ-
mental gain will be lowered. Such displacement effects need
to be identified and their environmental consequences
included in any truly holistic assessment of energy crops.
Acknowledgements
Miguel Brandao was funded by a bursary awarded by EPSRC.
Dr. Mila i Canals was funded by the RELU programme (http://
www.relu.ac.uk), and benefited from logistic support from
GIRO CT (http://www.giroct.net) during the preparation of this
paper.
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