1. Annals of Applied Biology (2005), 146:193–201
*Corresponding Author Email: david.powlson@bbsrc.ac.uk
© 2005 Association of Applied Biologists
193
Biofuels and other approaches for decreasing fossil fuel emissions from
agriculture
D S POWLSON1
*, A B RICHE2
and I SHIELD2
1
Agriculture and Environment Division, 2
Plant and Invertebrate Ecology Division, Rothamsted Research,
Harpenden, Herts AL5 2JQ, UK
Summary
Biofuels offer one method for decreasing emissions of carbon dioxide (CO2
) from fossil fuels, thus
helping to meet UK and EU targets for mitigating climate change. They also provide a rational option
for land use within the EU that could be economically viable, provided that an appropriate financial and
policy environment is developed. If 80% of current set-aside land in the UK were used for production
of biomass crops for electricity generation, about 3% of current UK electricity demand could be met
from this source. Considering possibilities for increasing yields and land area devoted to such crops
over the coming decades, this could possibly rise to 12%. These estimates exclude consideration of
developments in electricity generation which should increase the efficiency of conversion. Also, the
use of combined heat and power units at local level (e.g. on farms or in rural communities) gives
additional energy saving. Dedicated biomass crops such as willow, poplar, miscanthus, switchgrass
or reed canary grass are perennials: in comparison with annual arable crops they would be expected
to deliver additional environmental benefits. The elimination of annual cultivation should give a more
stable environment, beneficial for farmland biodiversity. Some increase in soil organic matter content
is likely, leading to some sequestration of carbon in soil and long-term improvements in soil quality.
The impact on water quality may be positive as nitrate losses are small and a similar trend is expected
for phosphate and pesticides. However, these crops may well use more water than arable crops so their
impact on water resources could be negative – an issue for further research. Agricultural land can also
be used to produce liquid fuels for use in transport. At present biodiesel can be produced from oilseed
rape and ethanol from either sucrose in sugar beet or cellulose from virtually any plant material. In
the short-term, liquid biofuels are an easy option as they require little change to either agriculture or
transport infrastructure. However, their benefits for CO2
emissions are much less than for biomass used
for generating electricity. It is therefore necessary to debate the priorities for land use in this context.
Key words: Biofuels, emissions, miscanthus, switchgrass, reed canary grass, willow, poplar
Introduction
The Third Assessment Report of the
Intergovernmental Panel on Climate Change (IPCC)
concluded that there is now a discernable human
influence on climate caused mainly by increasing
emissions of greenhouse gases (IPCC, 2001). The
UK Royal Commission on Environmental Pollution
(RCEP) recommended cuts in greenhouse gas
emissions of 60% of 1990 levels by 2050 (RCEP,
2000) and the UK government has set targets broadly
in line with this. If challenging cuts in emissions,
especially of carbon dioxide (CO2
), are to be achieved
all aspects of human activity, including agriculture,
have to be assessed. Agriculture could contribute to
emissions cuts in several ways including:
1. Decrease the use of fossil fuel or inputs
having a large fossil fuel emission during their
manufacture – includes fertilisers, agrochemicals
and machinery;
2. Use some agricultural land to produce bioenergy
crops to partially replace fossil fuels for
electricity production, heating or transport;
3. Use existing agricultural crops as energy sources
instead of their current use;
4. Utilise agricultural wastes for energy
production;
5. Adopt management practices that cause
sequestration of carbon (C) in soil or long-
lived plants in order to remove it from the
atmosphere;
6. Adopt management practices that decrease net
emissions of trace greenhouse gases (mainly
nitrous oxide and methane).
This article concentrates on the opportunities for
greater production of crops specifically grown as
sources of renewable energy, particularly perennial
grasses and short rotation coppice (SRC) of tree
species. CO2
emissions associated with agricultural
inputs are considered briefly. Possibilities for C
sequestration and associated impacts on trace gases
have been considered previously (Smith et al.,
2001).

2. D S POWLSON ET AL.194
Types of Biofuel Crops
Table 1 summarises information on the main
biomass crops currently being considered as sources
of material for electricity generation. In addition,
oilseed rape can be grown specifically for production
of biodiesel and ethanol can be produced from many
different plant sources; these are considered later.
Converting agricultural land to the production of
tree crops (SRC of willow or poplar) or miscanthus
represents a long-term commitment by the farmer or
landowner. Establishment costs are high and it will
be several years before the crops yield at their full
potential. At present SRC is established by planting
stem cuttings and miscanthus is established by the
planting of rhizomes, both of which techniques are
labour intensive and hence costly. However recent
research has indicated the possibility of establishing
miscanthus from seed (Christian et al., 2005). If
this is possible in practice it would greatly decrease
the cost of establishment. By contrast to SRC or
miscanthus, the other perennial grasses have a low
establishment cost as they are propagated from
seed. With the tree crops, conversion of land back to
conventional cropping will require significant effort
due to the presence of stools and roots. Although
miscanthus produces large rhizomes, experience in
experimental plots indicates that their destruction by
ploughing is not particularly difficult.
At present yields of 12 t dry matter ha–1
of
miscanthus are routinely achieved in the UK and
there is scope for further improvement as discussed
later. The relatively little research so far conducted
with switchgrass and reed canary grass under UK
conditions shows rather smaller yields than for
miscanthus (Table 1) but this may be acceptable in
view of the relative ease of establishment. Also,
these grasses could be suitable for growing for
shorter periods (e.g. 5 years) within more traditional
rotations as opposed to the longer-term conversion
required for the other crops. Yields of switchgrass
in the USA have been in the range 0.9 to 34.6 t DM
ha–1
(Pfeifer et al., 1990).
It is estimated that willow SRC yields currently
achieved in practice average about 7 t dry matter ha–1
.
In part this is a result of the varieties being grown
– yields of 12–14 t ha–1
are now being recorded in
field experiments with the latest varieties. But it also
reflects the fact that, in general, the poorest soils are
used for willows. This is likely to be the trend for all
energy crops under current economic conditions and
is a point that should be borne in mind when making
estimates of the quantities likely to be available.
With the perennial grasses very few pest or
disease problems have been encountered under
UK conditions, though most experience so far has
been on relatively small experimental plots. The
possibility of greater problems if they were grown
more widely cannot be excluded. The only serious
pest encountered was double lobed moth (Apamea
ophiogramma) on reed canary grass. With willow
two serious problems have been identified: rust
(Melampsora epitea) and willow beetle (Phratora
vulgatissima and P. vitellinae). Much current
research is directed towards identifying and utilising
wider genetic sources of resistance to rust. Recent
work at Rothamsted has led to the breeding of the
first varieties with potential resistance to willow
beetles and other invertebrates. The latest rust
resistant varieties are frequently established in
stands comprising a mixture of varieties so that the
spread of the different pathotypes of the rust fungus
is limited and, potentially, the breakdown in rust
resistance will be delayed (McCracken & Dawson
2001). This is important as the stands are planted
with the intention of maintaining production for 20
years or more.
In general pesticide use is low with all biomass
crops, except for herbicides to control weeds in
the establishment years. Knowledge of the nutrient
Table 1. Summary of information currently available on biomass crops in use or being researched in the UK
Crop
SRC Poplar
(Populus spp.)
SRC Willow
(Salix spp.)
Miscanthus
(Miscanthus
giganteus)
Switchgrass
(Panicum virgatum)
Reed canary
grass (Phalaris
arundinacea)
Current typical yield,
t dry matter ha–1 7a
7a
12 10 8
Establishment time 3 years + 3 years + 3 years + 2–3 years + 1–2 years
Pesticide requirements Low Low Low Low Very low
Fertiliser requirements Low/medium Low/medium Low Very low Medium
Agronomic knowledge Good Good Reasonable Low Low
Establishment costs High High Very high Very low Very low
Pest/disease problems ? Beetle Rust None serious None serious
Possible insect pest
problems
Plantation longevity 20 years + 20 years + 20 years + ? 20 years + ?? 10 years + ??
Other issues Resistant to lodging Resistant to lodging Resistant to lodging
a
SRC Poplar and willow yields are estimates from farmers’ willow fields and represent the fact that many crops are grown on low
grade land. Plot yields are greater (c. 10 t DM ha–1
)

3. 195Approaches for decreasing fossil fuel emissions from agriculture
requirements of all biomass crops is still limited but
current research suggests that the requirement for
fertilisers is low compared to most arable crops.
Sustainability of Biomass Crop Yields
Particularly for SRC or miscanthus, where
establishment is costly and yields may be sub-
optimal for the early years, it is essential that high
yields of biomass continue for a prolonged period.
At Rothamsted, miscanthus and switchgrass have
both been grown continuously for at least 10 years
with no indication of yields declining (Fig. 1).Yields
of switchgrass varied considerably from year to
year probably due to weather conditions, especially
rainfall.
The yields shown in Fig. 1 are for crops receiving
no nitrogen (N) fertiliser. Other treatments did
receive N (60 kg ha–1
for switchgrass, 60 and 120
kg ha–1
for miscanthus) but there was no yield
response (Christian et al., 2002). These crops were
established on land that had a long history of grass
which was ploughed up 5 years earlier, so the soil
was presumably mineralising more N than would be
the case in old arable soil. It seems likely that at some
point in the future N fertiliser will be required. In the
USAabout 112 kg N ha–1
is normally recommended
for switchgrass (http://www.extension.iastate.edu/
Publications/PM1866.pdf). For switchgrass and
reed canary grass offtakes of P and K were about 5
and 20 kg ha–1
year–1
, respectively, in experiments at
Rothamsted. These values are lower than for most
arable crops; typical annual offtakes in winter wheat
grain are 20–30 kg P ha–1
and 30–40 kg K ha–1
.
Very little data has been published on the fertiliser
requirements of SRC. Johnson (1999) reviewed the
available data and drew up the guidelines adopted
in the Defra Growers Guide (Defra, 2002). This
recommends a minimum of 40 kg N ha–1
in the
first year after cutback rising to 100 kg N ha–1
in
the third year. However it is virtually impossible to
apply fertiliser in the pre harvest year because the
tree canopy fills the space between rows, hampering
the entry of machinery. It is common practice to
apply sewage sludge to the land following cutback
or harvest. It is beneficial to apply sewage sludge
to non-food crops grown in long rotations because,
as opposed to crops entering the food chain, it is
consistent with UK sludge disposal legislation
and water company guidelines. Sewage sludge
is a relatively slow release fertiliser that supplies
N and also P and K to the crop over much of the
harvest cycle. Hilton (2001) recommended applying
sludge shortly after harvest at the maximum rate
permitted for Good Agricultural Practice at the site
(Defra, 1998; usually based on N inputs in view of
nitrate leaching risk). This is because it is generally
possible to apply sludge only once during a rotation,
immediately after harvesting, when the land is clear
of the tree canopy.
How Much of UK Energy Requirement could
be Met from Biofuel Crops?
A recent estimate by the UK Department of
Trade and Industry (DTI) was that a combination
of willow and miscanthus could potentially supply
11.8% of UK electricity by 2020 (http://www.
supergen-bioenergy.net/docs/03%20Shanahan%20-
%20supergen%20conference%20010404.pdf). This
assumed that 1 Mha of land would be devoted to
these crops. If other biomass sources such as straw
and wood wastes were included the figure rose to
16.6%. However, DTI recognised that constraints on
the rate of construction of generating capacity could
decrease this to 9% and if a smaller area of land was
available (350 000 ha) to 6%. Table 2 shows our
estimates of the amount of electricity that could be
generated in the UK using biomass crops, based on
a range of scenarios for crop availability.
As about 600 000 ha of arable land in the UK is
currently in set-aside annually, this area is potentially
available for energy production and is the starting
point for our scenarios. However, as other non-food
crops may increasingly be grown, we have assumed
that only 80% of the set-aside area is actually
available. Table 2 shows that if this area alone were
used for biomass crops it would provide sufficient
plant material to produce electricity equivalent to
2.7% of current demand. Clearly this is not large
but, if taken together with other renewables such as
wind (likely to be the major source in the next few
decades), wave and photovoltaics, biomass crops
could make a significant contribution to partial
replacement of fossil fuels. There are three main
ways in which the contribution from biomass crops
could be increased:
1. Increase yields of dry matter per ha;
0.0
0.2
0.4
0.6
0.8
0.01
0.21
0.41
0.61
0.81
2002100200029991899179916991599149913991
raeY
Yield(tha-1
DM)
Fig. 1. The yield of switchgrass (cv. Cave in rock) and
Miscanthus giganteus over 10 years at Rothamsted. Both
experiments received no nitrogen fertiliser. (Christian et al.,
2002 with later data added). N, switchgrass; I, miscanthus.

4. D S POWLSON ET AL.196
2. Increase the area used for energy crops;
3. Increase efficiency of conversion of biomass
into energy.
These are considered in turn.
Increase yields of dry matter per ha
For the calculations in Table 1 we assumed a dry
matter yield of 12 t ha–1
for all biomass crops. For
miscanthus this is conservative as yields of up to
20 t ha–1
and above have been achieved in various
European experiments (Lewandowski et al., 2000).
It is likely that there is scope for significant increase
as so little research has yet been conducted on the
grasses. Experience from willows is promising as
yields have increased greatly as a result of research
on genetics and breeding. For example, willow
varieties yielding up to 14 t dry matter ha–1
are now
available compared to only 5–7 t ha–1
before this
research was conducted. We consider that it is not
unreasonable to expect miscanthus yields of about 25
t ha–1
during the next 20 years. Similar increases may
be possible with switchgrass and reed canary grass.
If so, the contribution to UK electricity generation
could increase to about 5%, even with no increase
in the area devoted to biomass crops.
Increase the area used for energy crops
In Table 2 we indicate some possibilities. For
example,inviewofCAPreformthereissomeconcern
that sugar beet may no longer be economically viable
within the EU. In this situation the area currently
used for this crop could potentially be available for
biofuels (scenario 2 in Table 2). Another possibility
would be to replace cereals currently grown in
environments that are sub-optimal for yield or those
grown on soils where erosion is a major risk such
as the South Downs. Conversion from an annual
arable crop to a perennial would greatly decrease
erosion risk.
In the UK a large area of land is currently used
for grass (over 6 Mha; Table 2). Some of this is
becoming uneconomic and some conversion to
biomass crops could be economically beneficial.
Climate change during the 21st
century is likely
to result in hotter drier summers in many parts of
the UK, probably decreasing the area well suited
for grass production. C4 crops such as miscanthus
and switchgrass are well suited to drier conditions
and could therefore replace some current grassland
(scenario 5; Table 2). However this change would
need to be made with great care; if old grassland
were ploughed this would cause a large release of
CO2
to the atmosphere and nitrate to water (Smith
et al., 1996). It would be necessary to establish the
biomass crops with minimum physical disturbance
to the soil, for example by direct drilling.
Other areas of land not suitable for normal
agriculture such as landfill sites or former industrial
sites could be available for biomass crops. The
total area is likely to be small so the contribution
to national electricity generation will be small, but
there could an economic benefit locally and could
help to stabilise and restore land through the input
of root material.
Increase efficiency of conversion of biomass into
energy
Adiscussion of the technology is beyond the scope
of this paper but there appear to be considerable
possibilities through furnace design or use of
biomass for gasification. Large efficiency benefits
are possible through the use of combined heat and
power (CHP) systems such that heat produced during
burning for electricity generation is utilised in district
heating systems. To maximise the use of CHPsmall
scale generating plants are required as heat must be
used close to its source. Small scale generation is the
preferred use for biomass crops as this decreases the
need for transporting bulky materials, thus cutting
costs and energy use.
Table 2. Potential for biomass crops to supply UK electricity requirements (in 2002) based on various scenarios
for biomass availability. See footnotes and text for assumptions
Scenario no. Scenario details Percentage of UK electricity provided (%)g
1 80% of current set-aside areaa
converted to biomass cropsb
2.7
2 50% of current sugar beetc
area converted to biomass crops 0.5
3 50% of wood and forestry wastesd
1.6
4 50% of wheat strawe
3.7
5 10% of current grassland areaf
converted to biomass crops 3.7
Total all scenarios 12.2
a
611 000 ha in 2002 (http://statistics.defra.gov.uk/esg/)
b
Yield assumed to be 12 t ha–1
c
169 000 ha in 2002 (http://statistics.defra.gov.uk/esg/)
d
Includes forestry wastes, sawdust from sawmills and prunings from municipal authorities: http://www.woodfuelresource.org.uk/
e
Assuming 16 Mt produced annually (8 t straw ha–1
from 2 Mha)
f
6.65 Mha in 2002 of which 1.23 Mha under 5 years old and 5.42 Mha over 5 years old (http://statistics.defra.gov.uk/esg/)
g
Assumes 1.6 MWhe
t–1
biomass. UK electricity demand (2002) 343.8 TWh (http://www.dti.gov.uk/energy/inform/energy_in_
brief/)

5. 197Approaches for decreasing fossil fuel emissions from agriculture
Renewable Transport Fuels
For transport purposes liquid fuels are required;
the two usually considered from plant sources
are ethanol and rapeseed methyl ester (RME or
biodiesel). RME is produced from oilseed rape
and ethanol can be produced from virtually any
plant material containing cellulose or from sucrose
obtained from sugar beet or sugar cane. There is an
EU target for renewable transport fuels of 2% by
2007 and 5.75% by 2010. Currently all car engines
are designed to run on at least 5% ethanol or RME
blends. Table 3 shows some possible scenarios for
the production of liquid fuels. These indicate that it
should be possible to meet the EU 2007 target for
renewable transport fuels by using current set-aside
land. However, it should be noted that this would
be at the expense of using that area for biomass for
electricity generation.
Relative Efficiency of Different Bioenergy
Options
Table 4 presents a selection of energy ratios (energy
available : energy expended) from recent literature
for comparison with the authors own calculations.
Table 3. Potential for wheat and oilseed rape to supply UK demand for road transport fuels
Scenarioa Percentage of UK petrol requirement
replaced by ethanolb
(%)
Percentage of UK diesel requirement
replaced by RMEc
(%)
Convert 80% of current set-aside area to liquid biofuel
production (wheat grain for ethanol or oilseed rape
seeds for RME)
5.5 3.4
Convert sugar beet area to ethanol or RME
production
1.8 1.2
a
Areas as in Table 2
b
Assumes 276 kg ethanol t–1
wheat grain, wheat yield of 8.0 t ha–1
. UK petrol demand (2002) 19.8 million t (http://www.dti.gov.
uk/energy/inform/energy_in_brief/)
c
Assumes a rapeseed yield of 3.33 t ha–1
with a 37% oil yield. UK diesel demand (2002) 17.7 million t (www.dti.gov.uk/energy/
inform/energy_in_brief/)
Table 4. Energy ratios for a selection of energy crop options. Energy ratio is defined as the ratio (energy available
from product: energy expended in producing it). This is expressed as x:1, where a large value of x is desirable and
a value less than 1 is undesirable, meaning that more energy is expended in producing the fuel than is obtainable
from it. Table shows values of x
Harvested crop Distributed fuel
Generation of
electricity2
Caliceti et
al. (2001)3
Metcalfe
& Bullard
(2001)
Authors’
own data
Woods
& Bauen
(2003)
Elsayed et
al. (2003)
Saynor et
al. (2003)
Elsayed et al.
(2003)
Solid fuels
SRC Willow combustion 22.2 2.6
Miscanthus combustion 35.9 37.5 3.7
Wheat straw combustion 4
60.5 4
1.7
Switchgrass combustion 29.0 35.4
Reed canary grass combustion 20.4 18.0
Liquid fuels
Compressed Natural Gas 0.86
Petrol 0.91
Diesel 0.94
Bio-ethanol from sugar beet 12.6 1.4 to 2.1 2.0
Bio-ethanol from wheat grain 7.2 0.7 to 2.7 2.2
Bio-ethanol from wheat straw 4
60.5 0.8 to 2.4 4
50.0
Biodiesel, oilseed rape 4.3 4.4 0.7 to 4.4 2.3 0.7 to 3.2
Biodiesel, sunflower 6.1
Fischer Tropsch reaction from SRC Willow1
22.2 18.1 to 62.4 13.9 to 44.6
Gas fuels
Hydrogen from SRC Willow 22.2 18.0 to 54.0
1
50% kerosene, 25% diesel and 25% naptha
2
Electricity generation, no utilisation of heat. 20 MWe generating capacity, 25% efficient conversion process
3
Under Mediterranean conditions
4
Assumes all growing costs allocated to the grain crop, only baling, carting and stacking allocated to the straw crop. Takes no account
of the value of incorporating the straw into the soil

6. D S POWLSON ET AL.198
Energy ratio is a means of comparing different fuel
options based on partial life cycle analysis. It is
defined as the ratio energy available from product:
energy expended in producing it.
Energy ratios are usually expressed in the form x:1,
so a large value of x (well in excess of 1) is desirable
meaning that the fuel option offers considerable
efficiency. By contrast a value of x less than 1 means
that more energy is expended in producing the fuel
than is available from it when used – clearly an
inefficient situation. The values in Table 4 in the
column headed “authors’own data” were calculated
using a combination of our assessment of typical
crop yields and agronomic operations associated
with each crop and the data of West & Marland
(2002) on energy consumption in US agriculture,
modified to UK conditions.
The data are separated into three classifications
dependent upon the extent of the life cycle analysis
involved in the calculation. It can be seen that
for each “harvested crop” assessed ‘on farm’,
immediately after harvesting and without further
transport or processing, the energy ratio values are
all much greater than 1 (range: 4–60; Table 4), i.e.
a much greater quantity of energy is potentially
available from the material than was expended in
growing and harvesting the crop. The dedicated
energy crops (willow, miscanthus, switchgrass and
reed canary grass) have large values (18–38; Table
4) due to their perennial and low input characters,
i.e. amounts of fuel used for agricultural operation
are small relative to annual arable crops and inputs
of agrochemicals are small. There is good agreement
between values calculated by the present authors and
the values of Metcalfe & Bullard (2001). The annual
crops (sugar beet, wheat, oilseed rape and sunflower)
have lower values as they require annual cultivations
and often greater agrochemical inputs (especially
nitrogen fertiliser). Thus biodiesel and bioethanol,
assessed at the “harvested crop stage” mostly have
energy ratios in the range 4–12.
Cereal straw is a special case as its energy ratio can
be assessed in different ways. If straw is regarded
as a by-product of grain, and effectively a waste
product, it is reasonable to allocate the energy (and
CO2
emissions) required for its growth entirely to
grain. Doing this gives energy ratios for straw as high
as 60 at the “harvested crop” stage (Table 4). Also,
following this logic, Elsayed et al. (2003) obtained a
value of 50 at the “distributed fuel” stage for wheat
straw used as a feedstock for bioethanol production.
The much lower value (0.8–2.4) calculated by
Woods & Bauen (2003) reflects the case where
energy inputs to the growing crop were allocated to
both grain and straw.
Transporting the crop to a site of utilisation ‘off
farm’ (“Distributed fuel” in Table 4) can incur a
significant energy cost, reducing the value of the
energy ratio. Avoiding this energy expenditure is a
primary reason behind the requirement for power
stations etc. to source the fuel locally (often defined
as a 40 km or occasionally 100 km radius).All three
fossil fuels listed by Woods & Bauen (2003) show
an energy ratio of less than 1.
This selection of published work shows broad
agreement in the calculation of the energy ratios
between different authors where comparisons are
possible. However it should be noted that many
authors rely on the same, relatively small, set
of baseline data. Expanding that baseline with
independent research should be a priority. One
anomaly in Table 4 is the value for processed willow
by Saynor et al. (2003). Their upper value is far in
excess of our calculated ‘on farm’value; the reason
for this discrepancy is not known.
The final classification in Table 4 highlights the
energy losses when the crop is utilised to generate
electricity. The energy ratios fall again, but remain
greater than 1 so are still an improvement on fossil
fuels. During the generation of electricity large
quantities of energy are lost as heat. The same
is true of the use of transport fuels but it is less
well quantified. Table 5 shows how technology
can improve on these inefficiencies. Utilising the
heat through combined heat and power (CHP)
installations greatly increases the energy ratio
(Elsayed et al., 2003). This was also pointed out
by the UK Royal Commission on Environmental
Pollution (RCEP, 2004) who recommended the
widespread adoption of CHP. Gasification and
pyrolysis increase the efficiency of the electricity
generating process, partly by utilising some of the
heat given off to begin the process and partly because
the end products are commonly utilised in an internal
combustion engine rather than a steam turbine.
A general conclusion from the data in Table 4 is
that, at the point of use, electricity generation from
dedicated biofuels gives a slight benefit in terms of
energy ratios compared to liquid biofuels (bioethanol,
biodiesel). However the difference is small and could
easily be altered through changes in technology, as
illustrated in Table 5. Although beyond the scope
of this paper, it is appropriate to have a discussion
on whether to prioritise land use for liquid transport
Table 5. Effect of conversion process on energy
ratios for electricity generation using SRC willow
wood chips. Data from Elsayed et al (2003). Table
shows values of x, expressing energy ratios as x:1 as
described in Table 4
Electrical energy
only
Electrical plus heat
energy (CHP)
Heat only
Direct
combustion
2.62 15.10 10.90
Gasification 5.92 16.90
Pyrolysis 3.02 8.17

7. 199Approaches for decreasing fossil fuel emissions from agriculture
fuels or for biomass for electricity production. In
the short term it is relatively easy to start liquid fuel
production, as ethanol or RME can be derived from
traditional agricultural crops and can be utilised in
vehicles immediately. Electricity generation from
biomass requires more changes to both agriculture
and the generating industry but may give the greatest
saving of CO2
emissions at a national scale.Another
option in the future will be the use of pyrolysis to
convert solid biomass material to a liquid fuel, but
the product would require significant changes to
transport infrastructure.
Environmental Impacts of Biomass Crops
Water
The overall impact of any of the biomass crops on
water quality is likely to be positive. Water draining
from soil growing these crops is likely have lower
concentrations of nitrate, phosphate or pesticides
than water from traditional arable crops or high-
productivity grass in view of the lower inputs used.
Christian & Riche (1998) measured nitrate leaching
during the first three winters after establishment
of miscanthus on a silty clay loam soil. During
the first year, when plants were very small and N
uptake negligible, nitrate loss was high: 150 kg N
ha–1
. But in subsequent years it was less than 10 kg
N ha–1
, much less than losses from arable crops that
are commonly in the range 20–60 kg N ha–1
and
sometimes higher (Goulding, 2000;Allingham et al.,
2002).As the biomass crops are perennials, they take
up water over a longer growing period than annual
crops and their canopy can intercept rainfall during
a large part of the year and thus decrease infiltration.
It is possible that this could have a significant impact
on water storage in drier regions if the crops were
grown over large areas. This is an issue requiring
further study.
Soil organic matter content
Two factors make it is very likely that, in the long
term, growth of any biomass crop will increase soil
organic C content compared to that in soil used
continuously for annual arable crops. First, perennial
crops in general tend to deposit more organic C in
soil as roots and root exudates than annual crops; for
example, this is well known for grasses. Observations
on miscanthus suggest large inputs from roots and
rhizomes (Riche & Christian, 2001). Second, the
absence of tillage for perennial crops is likely to
slow the decomposition of soil organic matter to
some extent. A combination of these reasons leads
to soils under pasture for long periods containing
more organic C than an equivalent soil under arable
crops. However, any changes caused by conversion
to perennial biomass crops are likely to be slow
as shown by Garten & Wullschleger (1999) who
could not detect significant increases in soil C under
switchgrass after 5 years in the USA. Similarly,
Hansen et al. (2004) found only a slow increase in
soil C under miscanthus in Denmark.After 16 years
of miscanthus they found an increase in total soil C
content of 15%, but no increase was detected after 9
years. However, changes in C isotope composition
showed that 31% of the organic C in the topsoil
was derived from miscanthus inputs after 16 years.
Later studies at the same site (Foereid et al., 2004)
indicated that the organic C derived from miscanthus
was at least as stable as that derived from grass and
that the turnover time of soil C increased with time
under miscanthus.
Assuming that there is a long-term increase in soil
C under perennial biomass crops, they may be useful
in restoring degraded land (e.g. landfills, brownfield
sites). It is possible that switchgrass or reed canary
grass could be used as part of a long crop rotation
to increase the organic C content of soil that has
been under arable crops for long periods. This could
become an economically viable alternative to the
traditional ley-arable system.
Biodiversity
It is currently difficult to assess the likely impact
of biomass crops on the population sizes or diversity
of insects, soil fauna, small mammals or birds as few
large areas of the crops exist. Benefits compared to
annual arable crops might be expected due to the
elimination of annual cultivation and the more stable
environment provided by perennial species. The
limited studies conducted to date are consistent with
this (Bullard et al., 1996; Santos, 2001). Managing
biomass crops to enhance farmland biodiversity is
an important topic for research. It is likely that the
size of the areas planted, their spatial relationship to
other crops or landscape features and the range of
crop species planted will be important factors.
Other Approaches to Decreasing CO2
Emissions from Agriculture
CO2
emissions associated with arable crop
production
West & Marland (2002) calculated the CO2
emissions from the manufacture of agricultural
inputs (fertilisers, agro-chemicals) and the fuel
used for farm operations under US conditions. A
significant conclusion from such studies is that the
manufacture of nitrogen fertiliser often represents
the largest emission of CO2
due to its high energy
requirement. An emission of 0.86 kg CO2
-C kg–1
N
is calculated. For winter wheat, even when grown
under US conditions with smaller N applications
than in Europe, N fertiliser represents about 60% of
total CO2
emissions. This emphasises the importance
of using N fertiliser efficiently and minimising

8. D S POWLSON ET AL.200
losses. Also, emissions are decreased if N can be
supplied from legumes or from the recycling of
organic manures. Although organic production
systems eliminate fertiliser N,it does not necessarily
follow that they are preferable from the emissions
viewpoint. Crop yield is generally only 60–80% of
the “conventional” yield and cereal crops cannot be
grown every year as such systems usually rely on a
rotation that includes several years of a grass/clover
pasture, so the effective yield over a period of years
is even lower. Thus the average C emission per unit
of grain can be similar to that for a system using N
fertiliser.
In view of the high CO2
emission associated with
N fertiliser, its use in the production of energy crops
must be minimised otherwise the environmental
benefit from these crops is severely decreased. This
is one of the factors that decreases the energy ratio
of bioethanol and RME compared to the low-input
biomass crops. A possibility to be investigated is
whether legumes can be intercropped with perennial
biomass crops as a method of meeting their modest
N requirements.
An interesting proposal tested in an EU project is
the establishment of belts of SRC willow between
areas of traditional agricultural crops with the aim of
producing energy equal to that used in agricultural
production within the farm (see: http://www.agsci.
kvl.dk/~bek/cfehtml.htm#energy).
C sequestration in soil or vegetation
Any C that can be transferred from atmospheric
CO2
to a storage pool in the biosphere is beneficial
in mitigating human-induced climate change. On
a global basis soil organic matter contains more
than twice the amount of C in atmospheric CO2
and additional C is stored in vegetation. Because
these pools are so large, relatively small increases
or decreases can be of global significance. There
has been much discussion about the possible role
of carbon sinks in mitigating climate change, both
the opportunities they provide and their limitations;
see for example Smith et al. (2001).ARoyal Society
report (Royal Society, 2001) concluded that the
reform of the EU Common Agricultural Policy
provides opportunities for sequestration of C in
agricultural soils. The organic C content of arable
soils tends to be low so there is often scope for
increase through changes in management practice or
conversion to woodland or pasture. But it was also
noted that any sequestration is finite – as the quantity
of C held in soil organic matter or in the trees of a
new forest accumulates, it tends towards a maximum
and then increases no more.Also, C sequestration is
reversible: if agricultural management or land use
subsequently reverts to the original (e.g. pasture or
woodland converted back to arable), the accumulated
C will be released. Thus sequestration offers a useful
means of CO2
mitigation in the medium term but is
not an alternative to cuts in emissions. Smith et al.
(2001) estimated that sequestration measures applied
to European agricultural land had the potential to
provide mitigation in the range of 1–4% of European
CO2
emissions; mitigation was larger (5% or more)
if the biofuels were grown on former arable land
because of the continuing replacement of some fossil
fuel. In addition, management practices leading to
C sequestration in soil or trees almost invariably
provide additional benefits to the wider environment
or to soil quality. One option is to develop wider field
margins that include trees; these increase habitats for
a range of insects and fauna on farmland in addition
to sequestering carbon (Falloon et al., 2004).
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