Biofuels and other approaches for decreasing fossil fuel emissions


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Biofuels and other approaches for decreasing fossil fuel emissions

  1. 1. Annals of Applied Biology (2005), 146:193–201*Corresponding Author Email:© 2005 Association of Applied Biologists193Biofuels and other approaches for decreasing fossil fuel emissions fromagricultureD S POWLSON1*, A B RICHE2and I SHIELD21Agriculture and Environment Division, 2Plant and Invertebrate Ecology Division, Rothamsted Research,Harpenden, Herts AL5 2JQ, UKSummaryBiofuels offer one method for decreasing emissions of carbon dioxide (CO2) from fossil fuels, thushelping to meet UK and EU targets for mitigating climate change. They also provide a rational optionfor land use within the EU that could be economically viable, provided that an appropriate financial andpolicy environment is developed. If 80% of current set-aside land in the UK were used for productionof biomass crops for electricity generation, about 3% of current UK electricity demand could be metfrom this source. Considering possibilities for increasing yields and land area devoted to such cropsover the coming decades, this could possibly rise to 12%. These estimates exclude consideration ofdevelopments in electricity generation which should increase the efficiency of conversion. Also, theuse of combined heat and power units at local level (e.g. on farms or in rural communities) givesadditional energy saving. Dedicated biomass crops such as willow, poplar, miscanthus, switchgrassor reed canary grass are perennials: in comparison with annual arable crops they would be expectedto deliver additional environmental benefits. The elimination of annual cultivation should give a morestable environment, beneficial for farmland biodiversity. Some increase in soil organic matter contentis 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 expectedfor phosphate and pesticides. However, these crops may well use more water than arable crops so theirimpact on water resources could be negative – an issue for further research. Agricultural land can alsobe used to produce liquid fuels for use in transport. At present biodiesel can be produced from oilseedrape and ethanol from either sucrose in sugar beet or cellulose from virtually any plant material. Inthe short-term, liquid biofuels are an easy option as they require little change to either agriculture ortransport infrastructure. However, their benefits for CO2emissions are much less than for biomass usedfor 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, poplarIntroductionThe Third Assessment Report of theIntergovernmental Panel on Climate Change (IPCC)concluded that there is now a discernable humaninfluence on climate caused mainly by increasingemissions of greenhouse gases (IPCC, 2001). TheUK Royal Commission on Environmental Pollution(RCEP) recommended cuts in greenhouse gasemissions of 60% of 1990 levels by 2050 (RCEP,2000) and the UK government has set targets broadlyin line with this. If challenging cuts in emissions,especially of carbon dioxide (CO2), are to be achievedall aspects of human activity, including agriculture,have to be assessed. Agriculture could contribute toemissions cuts in several ways including:1. Decrease the use of fossil fuel or inputshaving a large fossil fuel emission during theirmanufacture – includes fertilisers, agrochemicalsand machinery;2. Use some agricultural land to produce bioenergycrops to partially replace fossil fuels forelectricity production, heating or transport;3. Use existing agricultural crops as energy sourcesinstead of their current use;4. Utilise agricultural wastes for energyproduction;5. Adopt management practices that causesequestration of carbon (C) in soil or long-lived plants in order to remove it from theatmosphere;6. Adopt management practices that decrease netemissions of trace greenhouse gases (mainlynitrous oxide and methane).This article concentrates on the opportunities forgreater production of crops specifically grown assources of renewable energy, particularly perennialgrasses and short rotation coppice (SRC) of treespecies. CO2emissions associated with agriculturalinputs are considered briefly. Possibilities for Csequestration and associated impacts on trace gaseshave been considered previously (Smith et al.,2001).
  2. 2. D S POWLSON ET AL.194Types of Biofuel CropsTable 1 summarises information on the mainbiomass crops currently being considered as sourcesof material for electricity generation. In addition,oilseed rape can be grown specifically for productionof biodiesel and ethanol can be produced from manydifferent plant sources; these are considered later.Converting agricultural land to the production oftree crops (SRC of willow or poplar) or miscanthusrepresents a long-term commitment by the farmer orlandowner. Establishment costs are high and it willbe several years before the crops yield at their fullpotential. At present SRC is established by plantingstem cuttings and miscanthus is established by theplanting of rhizomes, both of which techniques arelabour intensive and hence costly. However recentresearch has indicated the possibility of establishingmiscanthus from seed (Christian et al., 2005). Ifthis is possible in practice it would greatly decreasethe cost of establishment. By contrast to SRC ormiscanthus, the other perennial grasses have a lowestablishment cost as they are propagated fromseed. With the tree crops, conversion of land back toconventional cropping will require significant effortdue to the presence of stools and roots. Althoughmiscanthus produces large rhizomes, experience inexperimental plots indicates that their destruction byploughing is not particularly difficult.At present yields of 12 t dry matter ha–1ofmiscanthus are routinely achieved in the UK andthere is scope for further improvement as discussedlater. The relatively little research so far conductedwith switchgrass and reed canary grass under UKconditions shows rather smaller yields than formiscanthus (Table 1) but this may be acceptable inview of the relative ease of establishment. Also,these grasses could be suitable for growing forshorter periods (e.g. 5 years) within more traditionalrotations as opposed to the longer-term conversionrequired for the other crops. Yields of switchgrassin the USA have been in the range 0.9 to 34.6 t DMha–1(Pfeifer et al., 1990).It is estimated that willow SRC yields currentlyachieved 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–1are now being recorded infield experiments with the latest varieties. But it alsoreflects the fact that, in general, the poorest soils areused for willows. This is likely to be the trend for allenergy crops under current economic conditions andis a point that should be borne in mind when makingestimates of the quantities likely to be available.With the perennial grasses very few pest ordisease problems have been encountered underUK conditions, though most experience so far hasbeen on relatively small experimental plots. Thepossibility of greater problems if they were grownmore widely cannot be excluded. The only seriouspest encountered was double lobed moth (Apameaophiogramma) on reed canary grass. With willowtwo serious problems have been identified: rust(Melampsora epitea) and willow beetle (Phratoravulgatissima and P. vitellinae). Much currentresearch is directed towards identifying and utilisingwider genetic sources of resistance to rust. Recentwork at Rothamsted has led to the breeding of thefirst varieties with potential resistance to willowbeetles and other invertebrates. The latest rustresistant varieties are frequently established instands comprising a mixture of varieties so that thespread of the different pathotypes of the rust fungusis limited and, potentially, the breakdown in rustresistance will be delayed (McCracken & Dawson2001). This is important as the stands are plantedwith the intention of maintaining production for 20years or more.In general pesticide use is low with all biomasscrops, except for herbicides to control weeds inthe establishment years. Knowledge of the nutrientTable 1. Summary of information currently available on biomass crops in use or being researched in the UKCropSRC Poplar(Populus spp.)SRC Willow(Salix spp.)Miscanthus(Miscanthusgiganteus)Switchgrass(Panicum virgatum)Reed canarygrass (Phalarisarundinacea)Current typical yield,t dry matter ha–1 7a7a12 10 8Establishment time 3 years + 3 years + 3 years + 2–3 years + 1–2 yearsPesticide requirements Low Low Low Low Very lowFertiliser requirements Low/medium Low/medium Low Very low MediumAgronomic knowledge Good Good Reasonable Low LowEstablishment costs High High Very high Very low Very lowPest/disease problems ? Beetle Rust None serious None seriousPossible insect pestproblemsPlantation longevity 20 years + 20 years + 20 years + ? 20 years + ?? 10 years + ??Other issues Resistant to lodging Resistant to lodging Resistant to lodgingaSRC Poplar and willow yields are estimates from farmers’ willow fields and represent the fact that many crops are grown on lowgrade land. Plot yields are greater (c. 10 t DM ha–1)
  3. 3. 195Approaches for decreasing fossil fuel emissions from agriculturerequirements of all biomass crops is still limited butcurrent research suggests that the requirement forfertilisers is low compared to most arable crops.Sustainability of Biomass Crop YieldsParticularly for SRC or miscanthus, whereestablishment is costly and yields may be sub-optimal for the early years, it is essential that highyields of biomass continue for a prolonged period.At Rothamsted, miscanthus and switchgrass haveboth been grown continuously for at least 10 yearswith no indication of yields declining (Fig. 1).Yieldsof switchgrass varied considerably from year toyear probably due to weather conditions, especiallyrainfall.The yields shown in Fig. 1 are for crops receivingno nitrogen (N) fertiliser. Other treatments didreceive N (60 kg ha–1for switchgrass, 60 and 120kg ha–1for miscanthus) but there was no yieldresponse (Christian et al., 2002). These crops wereestablished on land that had a long history of grasswhich was ploughed up 5 years earlier, so the soilwas presumably mineralising more N than would bethe case in old arable soil. It seems likely that at somepoint in the future N fertiliser will be required. In theUSAabout 112 kg N ha–1is normally recommendedfor switchgrass ( For switchgrass andreed canary grass offtakes of P and K were about 5and 20 kg ha–1year–1, respectively, in experiments atRothamsted. These values are lower than for mostarable crops; typical annual offtakes in winter wheatgrain are 20–30 kg P ha–1and 30–40 kg K ha–1.Very little data has been published on the fertiliserrequirements of SRC. Johnson (1999) reviewed theavailable data and drew up the guidelines adoptedin the Defra Growers Guide (Defra, 2002). Thisrecommends a minimum of 40 kg N ha–1in thefirst year after cutback rising to 100 kg N ha–1inthe third year. However it is virtually impossible toapply fertiliser in the pre harvest year because thetree canopy fills the space between rows, hamperingthe entry of machinery. It is common practice toapply sewage sludge to the land following cutbackor harvest. It is beneficial to apply sewage sludgeto non-food crops grown in long rotations because,as opposed to crops entering the food chain, it isconsistent with UK sludge disposal legislationand water company guidelines. Sewage sludgeis a relatively slow release fertiliser that suppliesN and also P and K to the crop over much of theharvest cycle. Hilton (2001) recommended applyingsludge shortly after harvest at the maximum ratepermitted for Good Agricultural Practice at the site(Defra, 1998; usually based on N inputs in view ofnitrate leaching risk). This is because it is generallypossible to apply sludge only once during a rotation,immediately after harvesting, when the land is clearof the tree canopy.How Much of UK Energy Requirement couldbe Met from Biofuel Crops?A recent estimate by the UK Department ofTrade and Industry (DTI) was that a combinationof willow and miscanthus could potentially supply11.8% of UK electricity by 2020 ( Thisassumed that 1 Mha of land would be devoted tothese crops. If other biomass sources such as strawand wood wastes were included the figure rose to16.6%. However, DTI recognised that constraints onthe rate of construction of generating capacity coulddecrease this to 9% and if a smaller area of land wasavailable (350 000 ha) to 6%. Table 2 shows ourestimates of the amount of electricity that could begenerated in the UK using biomass crops, based ona range of scenarios for crop availability.As about 600 000 ha of arable land in the UK iscurrently in set-aside annually, this area is potentiallyavailable for energy production and is the startingpoint for our scenarios. However, as other non-foodcrops may increasingly be grown, we have assumedthat only 80% of the set-aside area is actuallyavailable. Table 2 shows that if this area alone wereused for biomass crops it would provide sufficientplant material to produce electricity equivalent to2.7% of current demand. Clearly this is not largebut, if taken together with other renewables such aswind (likely to be the major source in the next fewdecades), wave and photovoltaics, biomass cropscould make a significant contribution to partialreplacement of fossil fuels. There are three mainways in which the contribution from biomass cropscould be increased:1. Increase yields of dry matter per ha; 1. The yield of switchgrass (cv. Cave in rock) andMiscanthus giganteus over 10 years at Rothamsted. Bothexperiments received no nitrogen fertiliser. (Christian et al.,2002 with later data added). N, switchgrass; I, miscanthus.
  4. 4. D S POWLSON ET AL.1962. Increase the area used for energy crops;3. Increase efficiency of conversion of biomassinto energy.These are considered in turn.Increase yields of dry matter per haFor the calculations in Table 1 we assumed a drymatter yield of 12 t ha–1for all biomass crops. Formiscanthus this is conservative as yields of up to20 t ha–1and above have been achieved in variousEuropean experiments (Lewandowski et al., 2000).It is likely that there is scope for significant increaseas so little research has yet been conducted on thegrasses. Experience from willows is promising asyields have increased greatly as a result of researchon genetics and breeding. For example, willowvarieties yielding up to 14 t dry matter ha–1are nowavailable compared to only 5–7 t ha–1before thisresearch was conducted. We consider that it is notunreasonable to expect miscanthus yields of about 25t ha–1during the next 20 years. Similar increases maybe possible with switchgrass and reed canary grass.If so, the contribution to UK electricity generationcould increase to about 5%, even with no increasein the area devoted to biomass crops.Increase the area used for energy cropsIn Table 2 we indicate some possibilities. Forexample,inviewofCAPreformthereissomeconcernthat sugar beet may no longer be economically viablewithin the EU. In this situation the area currentlyused for this crop could potentially be available forbiofuels (scenario 2 in Table 2). Another possibilitywould be to replace cereals currently grown inenvironments that are sub-optimal for yield or thosegrown on soils where erosion is a major risk suchas the South Downs. Conversion from an annualarable crop to a perennial would greatly decreaseerosion risk.In the UK a large area of land is currently usedfor grass (over 6 Mha; Table 2). Some of this isbecoming uneconomic and some conversion tobiomass crops could be economically beneficial.Climate change during the 21stcentury is likelyto result in hotter drier summers in many parts ofthe UK, probably decreasing the area well suitedfor grass production. C4 crops such as miscanthusand switchgrass are well suited to drier conditionsand could therefore replace some current grassland(scenario 5; Table 2). However this change wouldneed to be made with great care; if old grasslandwere ploughed this would cause a large release ofCO2to the atmosphere and nitrate to water (Smithet al., 1996). It would be necessary to establish thebiomass crops with minimum physical disturbanceto the soil, for example by direct drilling.Other areas of land not suitable for normalagriculture such as landfill sites or former industrialsites could be available for biomass crops. Thetotal area is likely to be small so the contributionto national electricity generation will be small, butthere could an economic benefit locally and couldhelp to stabilise and restore land through the inputof root material.Increase efficiency of conversion of biomass intoenergyAdiscussion of the technology is beyond the scopeof this paper but there appear to be considerablepossibilities through furnace design or use ofbiomass for gasification. Large efficiency benefitsare possible through the use of combined heat andpower (CHP) systems such that heat produced duringburning for electricity generation is utilised in districtheating systems. To maximise the use of CHPsmallscale generating plants are required as heat must beused close to its source. Small scale generation is thepreferred use for biomass crops as this decreases theneed for transporting bulky materials, thus cuttingcosts and energy use.Table 2. Potential for biomass crops to supply UK electricity requirements (in 2002) based on various scenariosfor biomass availability. See footnotes and text for assumptionsScenario no. Scenario details Percentage of UK electricity provided (%)g1 80% of current set-aside areaaconverted to biomass cropsb2.72 50% of current sugar beetcarea converted to biomass crops 0.53 50% of wood and forestry wastesd1.64 50% of wheat strawe3.75 10% of current grassland areafconverted to biomass crops 3.7Total all scenarios 12.2a611 000 ha in 2002 ( assumed to be 12 t ha–1c169 000 ha in 2002 ( forestry wastes, sawdust from sawmills and prunings from municipal authorities: 16 Mt produced annually (8 t straw ha–1from 2 Mha)f6.65 Mha in 2002 of which 1.23 Mha under 5 years old and 5.42 Mha over 5 years old ( 1.6 MWhet–1biomass. UK electricity demand (2002) 343.8 TWh (
  5. 5. 197Approaches for decreasing fossil fuel emissions from agricultureRenewable Transport FuelsFor transport purposes liquid fuels are required;the two usually considered from plant sourcesare ethanol and rapeseed methyl ester (RME orbiodiesel). RME is produced from oilseed rapeand ethanol can be produced from virtually anyplant material containing cellulose or from sucroseobtained from sugar beet or sugar cane. There is anEU target for renewable transport fuels of 2% by2007 and 5.75% by 2010. Currently all car enginesare designed to run on at least 5% ethanol or RMEblends. Table 3 shows some possible scenarios forthe production of liquid fuels. These indicate that itshould be possible to meet the EU 2007 target forrenewable transport fuels by using current set-asideland. However, it should be noted that this wouldbe at the expense of using that area for biomass forelectricity generation.Relative Efficiency of Different BioenergyOptionsTable 4 presents a selection of energy ratios (energyavailable : energy expended) from recent literaturefor comparison with the authors own calculations.Table 3. Potential for wheat and oilseed rape to supply UK demand for road transport fuelsScenarioa Percentage of UK petrol requirementreplaced by ethanolb(%)Percentage of UK diesel requirementreplaced by RMEc(%)Convert 80% of current set-aside area to liquid biofuelproduction (wheat grain for ethanol or oilseed rapeseeds for RME)5.5 3.4Convert sugar beet area to ethanol or RMEproduction1.8 1.2aAreas as in Table 2bAssumes 276 kg ethanol t–1wheat grain, wheat yield of 8.0 t ha–1. UK petrol demand (2002) 19.8 million t ( a rapeseed yield of 3.33 t ha–1with a 37% oil yield. UK diesel demand (2002) 17.7 million t ( 4. Energy ratios for a selection of energy crop options. Energy ratio is defined as the ratio (energy availablefrom product: energy expended in producing it). This is expressed as x:1, where a large value of x is desirable anda value less than 1 is undesirable, meaning that more energy is expended in producing the fuel than is obtainablefrom it. Table shows values of xHarvested crop Distributed fuelGeneration ofelectricity2Caliceti etal. (2001)3Metcalfe& Bullard(2001)Authors’own dataWoods& Bauen(2003)Elsayed etal. (2003)Saynor etal. (2003)Elsayed et al.(2003)Solid fuelsSRC Willow combustion 22.2 2.6Miscanthus combustion 35.9 37.5 3.7Wheat straw combustion 460.5 41.7Switchgrass combustion 29.0 35.4Reed canary grass combustion 20.4 18.0Liquid fuelsCompressed Natural Gas 0.86Petrol 0.91Diesel 0.94Bio-ethanol from sugar beet 12.6 1.4 to 2.1 2.0Bio-ethanol from wheat grain 7.2 0.7 to 2.7 2.2Bio-ethanol from wheat straw 460.5 0.8 to 2.4 450.0Biodiesel, oilseed rape 4.3 4.4 0.7 to 4.4 2.3 0.7 to 3.2Biodiesel, sunflower 6.1Fischer Tropsch reaction from SRC Willow122.2 18.1 to 62.4 13.9 to 44.6Gas fuelsHydrogen from SRC Willow 22.2 18.0 to 54.0150% kerosene, 25% diesel and 25% naptha2Electricity generation, no utilisation of heat. 20 MWe generating capacity, 25% efficient conversion process3Under Mediterranean conditions4Assumes all growing costs allocated to the grain crop, only baling, carting and stacking allocated to the straw crop. Takes no accountof the value of incorporating the straw into the soil
  6. 6. D S POWLSON ET AL.198Energy ratio is a means of comparing different fueloptions based on partial life cycle analysis. It isdefined 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 desirablemeaning that the fuel option offers considerableefficiency. By contrast a value of x less than 1 meansthat more energy is expended in producing the fuelthan is available from it when used – clearly aninefficient situation. The values in Table 4 in thecolumn headed “authors’own data” were calculatedusing a combination of our assessment of typicalcrop yields and agronomic operations associatedwith 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 classificationsdependent upon the extent of the life cycle analysisinvolved in the calculation. It can be seen thatfor each “harvested crop” assessed ‘on farm’,immediately after harvesting and without furthertransport or processing, the energy ratio values areall much greater than 1 (range: 4–60; Table 4), i.e.a much greater quantity of energy is potentiallyavailable from the material than was expended ingrowing and harvesting the crop. The dedicatedenergy crops (willow, miscanthus, switchgrass andreed canary grass) have large values (18–38; Table4) due to their perennial and low input characters,i.e. amounts of fuel used for agricultural operationare small relative to annual arable crops and inputsof agrochemicals are small. There is good agreementbetween values calculated by the present authors andthe values of Metcalfe & Bullard (2001). The annualcrops (sugar beet, wheat, oilseed rape and sunflower)have lower values as they require annual cultivationsand often greater agrochemical inputs (especiallynitrogen fertiliser). Thus biodiesel and bioethanol,assessed at the “harvested crop stage” mostly haveenergy ratios in the range 4–12.Cereal straw is a special case as its energy ratio canbe assessed in different ways. If straw is regardedas a by-product of grain, and effectively a wasteproduct, it is reasonable to allocate the energy (andCO2emissions) required for its growth entirely tograin. Doing this gives energy ratios for straw as highas 60 at the “harvested crop” stage (Table 4). Also,following this logic, Elsayed et al. (2003) obtained avalue of 50 at the “distributed fuel” stage for wheatstraw used as a feedstock for bioethanol production.The much lower value (0.8–2.4) calculated byWoods & Bauen (2003) reflects the case whereenergy inputs to the growing crop were allocated toboth grain and straw.Transporting the crop to a site of utilisation ‘offfarm’ (“Distributed fuel” in Table 4) can incur asignificant energy cost, reducing the value of theenergy ratio. Avoiding this energy expenditure is aprimary reason behind the requirement for powerstations etc. to source the fuel locally (often definedas a 40 km or occasionally 100 km radius).All threefossil fuels listed by Woods & Bauen (2003) showan energy ratio of less than 1.This selection of published work shows broadagreement in the calculation of the energy ratiosbetween different authors where comparisons arepossible. However it should be noted that manyauthors rely on the same, relatively small, setof baseline data. Expanding that baseline withindependent research should be a priority. Oneanomaly in Table 4 is the value for processed willowby Saynor et al. (2003). Their upper value is far inexcess of our calculated ‘on farm’value; the reasonfor this discrepancy is not known.The final classification in Table 4 highlights theenergy losses when the crop is utilised to generateelectricity. The energy ratios fall again, but remaingreater than 1 so are still an improvement on fossilfuels. During the generation of electricity largequantities of energy are lost as heat. The sameis true of the use of transport fuels but it is lesswell quantified. Table 5 shows how technologycan improve on these inefficiencies. Utilising theheat through combined heat and power (CHP)installations greatly increases the energy ratio(Elsayed et al., 2003). This was also pointed outby the UK Royal Commission on EnvironmentalPollution (RCEP, 2004) who recommended thewidespread adoption of CHP. Gasification andpyrolysis increase the efficiency of the electricitygenerating process, partly by utilising some of theheat given off to begin the process and partly becausethe end products are commonly utilised in an internalcombustion engine rather than a steam turbine.A general conclusion from the data in Table 4 isthat, at the point of use, electricity generation fromdedicated biofuels gives a slight benefit in terms ofenergy ratios compared to liquid biofuels (bioethanol,biodiesel). However the difference is small and couldeasily be altered through changes in technology, asillustrated in Table 5. Although beyond the scopeof this paper, it is appropriate to have a discussionon whether to prioritise land use for liquid transportTable 5. Effect of conversion process on energyratios for electricity generation using SRC willowwood chips. Data from Elsayed et al (2003). Tableshows values of x, expressing energy ratios as x:1 asdescribed in Table 4Electrical energyonlyElectrical plus heatenergy (CHP)Heat onlyDirectcombustion2.62 15.10 10.90Gasification 5.92 16.90Pyrolysis 3.02 8.17
  7. 7. 199Approaches for decreasing fossil fuel emissions from agriculturefuels or for biomass for electricity production. Inthe short term it is relatively easy to start liquid fuelproduction, as ethanol or RME can be derived fromtraditional agricultural crops and can be utilised invehicles immediately. Electricity generation frombiomass requires more changes to both agricultureand the generating industry but may give the greatestsaving of CO2emissions at a national scale.Anotheroption in the future will be the use of pyrolysis toconvert solid biomass material to a liquid fuel, butthe product would require significant changes totransport infrastructure.Environmental Impacts of Biomass CropsWaterThe overall impact of any of the biomass crops onwater quality is likely to be positive. Water drainingfrom soil growing these crops is likely have lowerconcentrations of nitrate, phosphate or pesticidesthan water from traditional arable crops or high-productivity grass in view of the lower inputs used.Christian & Riche (1998) measured nitrate leachingduring the first three winters after establishmentof miscanthus on a silty clay loam soil. Duringthe first year, when plants were very small and Nuptake negligible, nitrate loss was high: 150 kg Nha–1. But in subsequent years it was less than 10 kgN ha–1, much less than losses from arable crops thatare commonly in the range 20–60 kg N ha–1andsometimes higher (Goulding, 2000;Allingham et al.,2002).As the biomass crops are perennials, they takeup water over a longer growing period than annualcrops and their canopy can intercept rainfall duringa large part of the year and thus decrease infiltration.It is possible that this could have a significant impacton water storage in drier regions if the crops weregrown over large areas. This is an issue requiringfurther study.Soil organic matter contentTwo factors make it is very likely that, in the longterm, growth of any biomass crop will increase soilorganic C content compared to that in soil usedcontinuously for annual arable crops. First, perennialcrops in general tend to deposit more organic C insoil as roots and root exudates than annual crops; forexample, this is well known for grasses. Observationson miscanthus suggest large inputs from roots andrhizomes (Riche & Christian, 2001). Second, theabsence of tillage for perennial crops is likely toslow the decomposition of soil organic matter tosome extent. A combination of these reasons leadsto soils under pasture for long periods containingmore organic C than an equivalent soil under arablecrops. However, any changes caused by conversionto perennial biomass crops are likely to be slowas shown by Garten & Wullschleger (1999) whocould not detect significant increases in soil C underswitchgrass after 5 years in the USA. Similarly,Hansen et al. (2004) found only a slow increase insoil C under miscanthus in Denmark.After 16 yearsof miscanthus they found an increase in total soil Ccontent of 15%, but no increase was detected after 9years. However, changes in C isotope compositionshowed that 31% of the organic C in the topsoilwas derived from miscanthus inputs after 16 years.Later studies at the same site (Foereid et al., 2004)indicated that the organic C derived from miscanthuswas at least as stable as that derived from grass andthat the turnover time of soil C increased with timeunder miscanthus.Assuming that there is a long-term increase in soilC under perennial biomass crops, they may be usefulin restoring degraded land (e.g. landfills, brownfieldsites). It is possible that switchgrass or reed canarygrass could be used as part of a long crop rotationto increase the organic C content of soil that hasbeen under arable crops for long periods. This couldbecome an economically viable alternative to thetraditional ley-arable system.BiodiversityIt is currently difficult to assess the likely impactof biomass crops on the population sizes or diversityof insects, soil fauna, small mammals or birds as fewlarge areas of the crops exist. Benefits compared toannual arable crops might be expected due to theelimination of annual cultivation and the more stableenvironment provided by perennial species. Thelimited studies conducted to date are consistent withthis (Bullard et al., 1996; Santos, 2001). Managingbiomass crops to enhance farmland biodiversity isan important topic for research. It is likely that thesize of the areas planted, their spatial relationship toother crops or landscape features and the range ofcrop species planted will be important factors.Other Approaches to Decreasing CO2Emissions from AgricultureCO2emissions associated with arable cropproductionWest & Marland (2002) calculated the CO2emissions from the manufacture of agriculturalinputs (fertilisers, agro-chemicals) and the fuelused for farm operations under US conditions. Asignificant conclusion from such studies is that themanufacture of nitrogen fertiliser often representsthe largest emission of CO2due to its high energyrequirement. An emission of 0.86 kg CO2-C kg–1Nis calculated. For winter wheat, even when grownunder US conditions with smaller N applicationsthan in Europe, N fertiliser represents about 60% oftotal CO2emissions. This emphasises the importanceof using N fertiliser efficiently and minimising
  8. 8. D S POWLSON ET AL.200losses. Also, emissions are decreased if N can besupplied from legumes or from the recycling oforganic manures. Although organic productionsystems eliminate fertiliser N,it does not necessarilyfollow that they are preferable from the emissionsviewpoint. Crop yield is generally only 60–80% ofthe “conventional” yield and cereal crops cannot begrown every year as such systems usually rely on arotation that includes several years of a grass/cloverpasture, so the effective yield over a period of yearsis even lower. Thus the average C emission per unitof grain can be similar to that for a system using Nfertiliser.In view of the high CO2emission associated withN fertiliser, its use in the production of energy cropsmust be minimised otherwise the environmentalbenefit from these crops is severely decreased. Thisis one of the factors that decreases the energy ratioof bioethanol and RME compared to the low-inputbiomass crops. A possibility to be investigated iswhether legumes can be intercropped with perennialbiomass crops as a method of meeting their modestN requirements.An interesting proposal tested in an EU project isthe establishment of belts of SRC willow betweenareas of traditional agricultural crops with the aim ofproducing energy equal to that used in agriculturalproduction within the farm (see: sequestration in soil or vegetationAny C that can be transferred from atmosphericCO2to a storage pool in the biosphere is beneficialin mitigating human-induced climate change. Ona global basis soil organic matter contains morethan twice the amount of C in atmospheric CO2and additional C is stored in vegetation. Becausethese pools are so large, relatively small increasesor decreases can be of global significance. Therehas been much discussion about the possible roleof carbon sinks in mitigating climate change, boththe opportunities they provide and their limitations;see for example Smith et al. (2001).ARoyal Societyreport (Royal Society, 2001) concluded that thereform of the EU Common Agricultural Policyprovides opportunities for sequestration of C inagricultural soils. The organic C content of arablesoils tends to be low so there is often scope forincrease through changes in management practice orconversion to woodland or pasture. But it was alsonoted that any sequestration is finite – as the quantityof C held in soil organic matter or in the trees of anew forest accumulates, it tends towards a maximumand then increases no more.Also, C sequestration isreversible: if agricultural management or land usesubsequently reverts to the original (e.g. pasture orwoodland converted back to arable), the accumulatedC will be released. Thus sequestration offers a usefulmeans of CO2mitigation in the medium term but isnot an alternative to cuts in emissions. Smith et al.(2001) estimated that sequestration measures appliedto European agricultural land had the potential toprovide mitigation in the range of 1–4% of EuropeanCO2emissions; mitigation was larger (5% or more)if the biofuels were grown on former arable landbecause of the continuing replacement of some fossilfuel. In addition, management practices leading toC sequestration in soil or trees almost invariablyprovide additional benefits to the wider environmentor to soil quality. One option is to develop wider fieldmargins that include trees; these increase habitats fora range of insects and fauna on farmland in additionto sequestering carbon (Falloon et al., 2004).ReferencesAllingham K D, Cartwright R, Donaghy D, Conway J S,Goulding K W T, Jarvis S C. 2002. Nitrate leaching lossesand their control in a mixed farm system in the Cotswolds,England. Soil Use and Management 18:421–427.Bullard M J, Christian D G, Wilkins C. 1996. Quantifyingbiomass production in crops grown for energy. 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Carbon andenergy balances from a range of biofuels options. Report No.21/3 for the Energy Technology Support Unit, DTI.Falloon P, Powlson D S, Smith P. 2004. Managing field marginsfor biodiversity and carbon sequestration – a UK case study.Soil Use and Management 20:240–247.Foereid B, de Neergaard A, Høgh-Jensen H. 2004. Turnoverof organic matter in a Miscanthus field: effect of time inMiscanthus cultivation and inorganic nitrogen supply. SoilBiology and Biochemistry 36:1075–1085.Garten C T, Wullschleger S D. 1999. Soil carbon inventoriesunder a bioenergy crop (switchgrass): measurementlimitations. Journal of Environmental Quality 28:1359–1365.Goulding K W T. 2000. Nitrate leaching from arable andhorticultural land. Soil Use and Management 16:145–151.Hansen E M, Christensen B T, Jensen L S, KristensenK. 2004. Carbon sequestration in soil beneath long-termMiscanthus plantations as determined by 13C abundance.Biomass and Energy 26:97–105.
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