The significance of nitrous oxide emission from biofuel crops on arable land : a Swedish perspective

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Introduction
In June 2009 the European Union Directive "Promotion of the use of energy from renewable sources" (EC directive 98/70/EG) came into force, setting goals for 20 % use of renewable energy in 2020.Under the Directive, the transport sector will be required to use at least 10 % renewable fuel.The motives are local energy security and reduced greenhouse gas (GHG) emissions, compared with those emanating from fossil fuels.However, one problem is that cropping of the feedstocks for the first-generation liquid biofuels -mainly cereal grains and oilseeds in the temperate zone, and sugar cane and palm oil in the tropics -inevitably involves emissions of greenhouse gases, nitrous oxide in particular, and some of these fuels may actually cause more emissions than Figures the gasoline and diesel fuels they replace (Crutzen et al., 2008;Mosier et al., 2009).
Thus the Directive contains sustainability rules for biofuels; for example, the savings of greenhouse gases expressed in g CO 2 -equivalents per MJ when biofuel is used must be at least 35 %, compared to fossil fuel use, and this differential will be increased to 50 % in 2017.Recent modelling suggests that these targets are not being achieved in many circumstances (Ogle et al., 2008;Mosier et al., 2009;Smeets et al., 2009) and this raises the question of how to estimate nitrous oxide emissions caused by biofuel cropping in different regions, to reduce the uncertainties.Nitrous oxide (N 2 O) is a naturally occurring and chemically stable greenhouse gas, with a global warming potential about 300 times greater than that of carbon dioxide, and a lifetime of more than 100 yr in the atmosphere (Forster et al., 2007).It is produced mainly by microbial activity in soils, and expansion of agriculture and increasing use of nitrogen (N) in synthetic fertilisers and manures has resulted in agricultural soils becoming globally the main source of N 2 O -65 % of anthropogenic emissions according to IPCC (2006) and as much as 80 % according to Crutzen et al. (2008).In recent years these changes have caused the atmospheric N 2 O concentration to increase by 0.25 % yr −1 (Prather et al., 2001).Nitrous oxide emissions from soil have increased by 50 % during the last 150 yr due to increased nitrogen use (Crutzen et al., 2008).The discussion on the importance of N 2 O emission in agricultural production in relation to the achievement of lower emissions with biofuels compared with fossil fuels has also thrown light on GHG emissions in relation to agricultural production in general.Globally, the efficiency with which N fertiliser is used by crops is only of the order of 40 %, as measured by the recovery of N in the harvested crop (Cassman et al., 2002); the figure is slightly higher for crops in northern Europe, e.g.47 % for Sweden (Oenema et al., 2009).This inefficiency is important in agro-ecosystems with newly added nitrogen amendments and high soil fertility, increasing the likelihood of surplus nitrogen and N 2 O production (Wang and Bakken 1997;Korsaeth et al., 2001).
A key question is: how much N 2 O will be emitted by the cropping for biofuel production?Detailed earlier life cycle analyses (LCAs) on three different biofuel production Introduction

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Full systems showed both higher and lower GHG emissions than those from the use of fossil fuels and the estimated emission of N 2 O was a decisive factor for the overall GHG emission level (Mosier et al., 2009).In each EU country, biofuel producers and regulatory authorities need to know whether or not their products will achieve the threshold set by the EU directive; our purpose here was to examine ways to estimate the size of N 2 O emissions from crop-based biofuels grown under Swedish conditions.

Lifecycle assessment for Swedish conditions
The RES directive [EC directive 98/70/EG] states a need for at least 35 % savings compared to fossil fuels like petrol, emitting 83.8 g CO 2eq MJ −1 .Thus 54.5 g CO 2eq

MJ
−1 is the maximum allowed emission.We began by asking: how much N 2 O emission can be allowed in order to meet this criterion, in production of ethanol from wheat using current farming techniques in Sweden?The calculations were made for two southern regions for which standard yields were obtained from the Swedish Agricultural Statistics (Statistics Sweden, 2011) (Fig. 1).We used information showing that only 60.8 % of the harvested energy can be converted into ethanol energy, equal to 7.9 MJ kg −1 grain (Bernesson et al., 2006, cited by Ahlgren et al., 2009); the other part goes into co-products such as distillers' grain.Also, in the refinery the conversion of the grains into ethanol needs energy, which according to B örjesson ( 2008) is equal to half the amount contained in the ethanol produced.We then assumed the same energy mix as in the overall Swedish energy system, with 43 % from renewable energy sources (Swedish Energy Agency, 2010), having low CO 2 emissions with only 1 g CO 2eq MJ −1 (a number given by the RES directive [directive 98/70/EG]).For fossil fuels the corresponding number is 83.8 gCO 2eq MJ −1 .The refinery emissions were then estimated to be 24 g CO 2eq MJ −1 ethanol-derived energy.Energy used for transportation and crop management, field operations and drying result in 3 g CO 2eq MJ −1 ethanol-derived energy (Ahlgren et al., 2009).The manufacture of N fertiliser (and to a lesser extent biocides) needs much energy and also produces N 2 O.The fertiliser Introduction

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Full  Summing the emissions, we concluded that for the two Swedish regions N 2 O emission from agricultural fields cannot exceed in total 2 kg N 2 O-N ha −1 yr −1 (1.5 for direct emission and 0.5 for indirect), in order to achieve the required saving of 35 % (Fig. 1).Measurements on unfertilised organic soils in Sweden have shown much larger emissions from barley production (a direct emission of 10 kg N 2 O-N ha −1 yr −1 ) (Kasimir Klemedtsson et al., 2009); use of such land would not be possible if the "sustainability goal" were to be achieved.Therefore in the remainder of our investigation we only include mineral soils, and determine the magnitude of the emissions we may expect on the basis of flux measurements and estimation models.Introduction

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Data from measurements
To estimate the size of agricultural N 2 O emissions as a consequence of cropping of the biofuel feedstock, we need measurements.The simplest and most common measurement method is by use of flux chambers: frames inserted permanently (apart from having to be removed briefly during seeding and soil management operations) in the soil surface.The rate of accumulation of N 2 O in the chambers is measured by gas chromatography.This gives data on direct emissions, i.e. those from the surface of the agricultural field.Chamber measurements, in spite of their limitations, are still the staple method for obtaining emission data, and are the only technique readily available to most researchers.In principle, better measurements can be made by micrometeorological techniques, which make it possible to measure the emission while disturbing neither the soil nor the crop (Wagner-Riddle et al., 2007).The emissions are often characterised by large temporal variations; thus it is important to detect sudden increases that commonly occur after fertiliser N additions or rainfall/irrigation events.This makes field measurements complicated and prolonged -they should run for at least a 12month cycle -and consequently costly.However, since the 1980s, data have been collected in this way in many different countries and locations.Swedish data used here include measurements during winter periods when soils were frozen, since emissions can then be as great as during other seasons, with large emission peaks often being found during thawing events (Jungkunst et al., 2006).Emission measurements in Sweden on crops appropriate for producing biofuels such as ethanol or biodiesel include wheat, rye, barley and oilseed rape.Here we report data on emissions from two experimental farms, both on mineral soils: Mellby g ård in Halland, south west Sweden, and Log ården in V ästra G ötaland, west Sweden.Both farms have conventional plant production systems with no livestock, and in both cases the crop was spring wheat.The first farm has coarse sandy soil, with 5 % organic matter and 5-10 % clay.Calcium ammonium nitrate (CAN), 120 kg N ha −1 yr −1 , was added Introduction

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Full to two plots by broadcasting and drilling and one was maintained without fertiliser addition.The second farm has a light clay soil with 30-40 % clay in the topsoil and more than 50 % in the subsoil, and an organic matter content of >4 %: a soil type identified as one providing a risk of high emissions, due to small soil pores having high waterholding capacity, increasing the risk of lack of oxygen.Here the fertiliser (NH 4 NO 3 , Axan) additions were 117 and 128 kg N ha −1 yr −1 .
Published data for similar systems elsewhere were taken from the large data set compiled by Stehfest and Bouwman (2006) (which also is the basis for the new IPCC ( 2006) direct emission factor -see below), comprising 1008 measurements of N 2 O from agricultural systems, of which 223 measurements were made on the crops wheat, rye, barley and rape.We selected only data measured in northern Europe (>46 • N) and Canada, with the quality criterion of at least a one-year measurement period.Moreover, data from organic soils were omitted.This selection provided 28 values, and we have added three data points from Kavdir et al. (2008) and our five values obtained from the measurements at Mellby and Log ården, Thus the total number of measurements comes to 36, and the annual emissions are plotted against N application rates in Fig. 2. The full dataset can be obtained in Supplementary data A.

IPCC emission factor
The Intergovernmental Panel on Climate Change (IPCC) has agreed on a relatively simple estimation method which can be used by all nations to estimate their national emissions of nitrous oxide.Originally the emission factor of 1.25 % for the direct nitrous oxide emission from soil and fertiliser additions (IPCC, 1997) was based on 20 data points (Bouwman 1996), measured during one year mainly from mineral soils in USA and UK, which had different crops and types of N additions.A linear increase in nitrous oxide emission with the amount of nitrogen added to the field was obtained.More data have been obtained since then and the correlation has become weaker, Introduction

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Full especially where <100 kg N ha −1 has been added to the field (Stehfest and Bouwman, 2006).This is also evident in Fig. 2. In the 2006 updating of the IPCC "Guidelines" the emission factor for nitrous oxide from arable fields has been modified to 1 % (IPCC, 2006) based on Bouwman et al. (2002), Stehfest and Bouwman (2006) and Novoa and Tejeda (2006).The IPCC report points out that reporting countries should use the most detailed and appropriate estimation method available, preferably their own equations and emission factors (Tier 2) and, where possible, process modelling (Tier 3).If no such methods are available the reporting country has to use Tier 1, based on the 1996 Guidelines, as the 2006 revised version has not yet come into force.Sweden uses a modified emission factor for agricultural land receiving only inorganic fertilisers, 0.8 % of added N (Kasimir Klemedtsson, 2001).
The original purpose of the IPCC emission factors was not to estimate nitrous oxide emission for every field, crop or year, but only to give a method to estimate the possible scale of emissions from a country to include in reports to UNFCCC, and to indicate trends from year to year.Inevitably, as long as the Tier 1 methodology is employed, there will be countries or regions with greater-than-average emissions, and others with lower-than-average emissions, depending on environmental and local agricultural management factors.The IPCC emission factors have led to a general acceptance of a clear connection between N-addition and N 2 O emission, despite increasing data collection showing a weak connection to inorganic N-additions up to 400 kg N ha −1 yr −1 (Novoa and Tejeda 2006).It is important to recognise that even if no fertiliser nitrogen is added the nitrous oxide emission will still continue since agricultural soils contain much labile nitrogen (Freibauer et al., 2004).Moreover, factors other than nitrogen have an influence on the emission, in particular soil wetness, temperature and carbon availability.This leads into more complex models: Introduction

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Statistical models
Statistical models that try to include other influencing factors in addition to the nitrogen input have been made by Freibauer (2003), Freibauer and Kaltschmitt (2003) and Stehfest and Bouwman (2006).The first two aimed for a regionalized estimation procedure and compiled data on soil emissions from European measurement and analyzed it by a stepwise multivariate method.The very south of Sweden was included into their region with a "mild westerly climate", but most of Sweden was categorized as being in the sub-boreal region.In the south, determining factors for direct N 2 O emission were, besides nitrogen fertiliser addition, the topsoil carbon and sand contents.

Estimation by use of process-based models
Estimation by use of process-based models has been suggested since the production of nitrous oxide is complex and depends on many soil physical, chemical and biological factors.Also, interactions with the crop or other plants where the nitrogen uptake efficiency can be important.This is why process-based models have been and are being developed, aiming at estimating nitrous oxide emission accurately.One example is the DAYCENT model which is currently in use for estimation of nitrous oxide in the USA's national reporting of greenhouse gas emissions (Del Grosso et al., 2008).Another widely used process model is the Denitrification-Decomposition model, DNDC, to which several sub-models have been attached (Li et al., 1992(Li et al., , 2000)), and which can use geographic databases connected to modelling of soil processes to simulate N 2 O emission from different ecosystems (Kesik et al., 2005;Beheydt et al., 2008;Giltrap et al., 2010;Smith et al., 2010b).Yet another model platform is COUP (Coupled heat and mass transfer model for soil-plant-atmosphere systems), which originally was a soil physical model based on Swedish soil physical data for calculating processes in the soil-plant system, and has been further developed to be applicable to whole crop rotations (Jansson and Karlberg 2004).A module for estimating nitrification/denitrification processes via the PnET-N-DNDC model has been connected to the Coup model (Norman et al., 2008).This combined model has so far been used for research purposes to understand soil processes.Introduction

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Top-down estimation
A top-down approach based on calculations by Crutzen et al. (2008) can be used to give a global average figure, if no local or regional possibilities exist.The method is based on the fact that atmospheric nitrous oxide concentration before industrialization was fairly stable (as evidenced by ice-core data), with the rate of addition to the atmosphere balanced by the rate of loss by stratospheric decomposition (Prather et al., 2001).At present, however, the N 2 O concentration in the atmosphere is increasing at about 0.7 ppb yr −1 and the annual addition is 50 % higher than in the pre-industrial period (Crutzen et al., 2008).They argued an overall connection between the N 2 O addition to the atmosphere and the global creation of reactive nitrogen by fixation, which also has increased by 50 %.Nitrogen fixation in this context means biological fixation and fixation by the Haber-Bosch process.Thus 3 to 5 % of the newly fixed nitrogen is emitted as nitrous oxide -similar to the proportion of the N that Galloway et al. (2004) estimated to have been newly fixed in natural ecosystems in the pre-industrial era.

Swedish measurements
Measured emission from the Mellby farm was on average 2-3 kg N 2 O-N ha −1 yr −1 where the fertilised plots did not differ from the control.The emission peaks in spring and early summer (June) (Fig. 3) made the difference between the median emission and the average.The control plot had a lower yield, which can be important when expressing emission per unit yield, as in this case of ethanol production.For the Log ården farm, in spite of the clay soil and fertiliser addition, the emission was low -but had a few emission peaks, in early spring and after harvest (Fig. 4).This resulted in the average emission being two times higher than the median (Table 1).The lower emission in the second year may have been due to lack of measurements during soil thawing in early Introduction

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Full spring when we frequently have observed higher emissions.The emission from Mellby farm was double that from Log ården, in spite of Mellby having a sandy soil, which was unexpected, as was the fact that the fertiliser addition at the Mellby farm did not increase the emission compared with the plot without fertilisation.At the Log ården farm the design did not include a zero addition plot but a parallel organic rotation had even lower emissions (half the size) than the conventional wheat.Our interpretation is that preceding management actions and crops also have influence on the emissions.But the important issue here is the overall size of the emissions, affecting the sustainability of the conversion of agricultural produce into liquid fuels.

Collected data
It is common practice to present emissions in relation to fertiliser addition.Figure 2 shows the compiled literature data together with the Swedish data presented above.Average emission for the whole dataset is 2.8 kg N 2 O-N ha −1 yr −1 , with a median of 1.9 kg N 2 O-N ha −1 yr −1 .Sites receiving fertiliser have higher emissions compared to those with no fertiliser addition, averaging 3.0 ± 0.6 compared to 1.2 ± 0.6 kg N 2 O-N ha −1 yr −1 .It is also possible to see that the Swedish data are in the same range as other data from the northern European region.The nitrogen addition explains less than 50 % of the emissions shown in Fig. 2, estimated from linear regression.Thus, other environmental factors have an important influence on the emissions.Like natural ecosystems, fertile arable fields contain tons of nitrogen per hectare, which has accumulated after many years of N fixation and/or addition in manure or fertiliser.Varying proportions of this organic N can be mineralized, and contribute to the N 2 O production, depending on environmental influences and management actions.Effective cropping systems that assimilate most of the available nitrogen may give low nitrous oxide emissions, at moderate fertiliser nitrogen additions (Snyder et al., 2009).This can be seen in Fig. 2, with a more pronounced tendency for high emissions to follow after high N addition but a less clear connection at low N addition.Since the flux could not exceed Introduction

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Full for studies performed during similar conditions as prevailing in Sweden, the risk of exceeding this limit is of the order of >50 %.

Comparing measured data with model estimates
The need for estimation models that can match actual emissions is why both IPCC's emissions factors and the more complex models were developed.By using the data from measurements on Mellby and Log ården where auxiliary data are available we were able to validate the different methods.Table 2 shows the evident inability of the IPCC 2006 method to estimate emission for cases where no N-additions were made.
Comparing the use of IPCC emission factors with data from fertilised plots the Swedish results shows an underestimation for Mellby and overestimation for Log ården.The statistical method by Freibauer and Kaltschmitt estimates an emission 3-14 times as large as what was measured, and also the uncertainty range is above measured emissions.We found that the most important factors in the equations deciding the emission are the soil carbon content for Mellby and the soil nitrogen content for Log ården, and the nitrogen addition is of less importance.In contrast we found the Stehfest and Bouwman estimation method to agree, but only for the control plot at Mellby that had no N-addition; however, for all cases receiving fertilisers the emissions was predicted to be 2-8 times higher compared with measured values.In this method, the nitrogen addition and duration of field measurements were the most decisive factors for the estimation.The Stehfest and Bouwman method shows the largest uncertainty and it is somewhat surprising that the length of measurement period was such an important factor for the result.
Both measured and estimated emissions presented in Full Fig. 2, was found to be 2.8 ± 0.5 kg N 2 O-N ha −1 yr −1 , which is smaller than that estimated by both the Freibauer and Kaltschmitt and the Stehfest and Bouwman calculation methods but is in line with our measurements in Sweden.The Crutzen et al. (2008) method would, for the fertilised plots, result in a similar emission to the two statistical ones, with a total emission around 3-6 kg N 2 O-N ha −1 yr −1 .However, with this global method, all subsequent emissions caused by introducing new reactive N into the ecosystem are included, such as indirect emissions following N loss to air and water.

Possibility of achieving required GHG savings in Swedish ethanol production
The refinery in our Swedish LCA releases only 24 g CO 2 equiv MJ −1 , which is low compared with the Mosier et al. ( 2009) study where 43-64 g CO 2 equiv MJ −1 ethanolderived energy was reported for refinery emissions in UK and USA, but this is due to a greater potential to use renewable energy in Sweden, which helps to reduce GHG emissions in this part of the production chain.Despite use of low N 2 O-emitting fertiliser production and high proportion of renewable energy used in the refinery, the LCA shows that a biofuel based on the Mellby data does not reach the sustainability goal of 35 % savings (Table 3), since yields are low and field emissions high, and this ethanol production would give more GHG emission than fossil fuels.But the Log ården case shows a low emission, <0.5 kg N 2 O-N ha −1 yr −1 and despite a low yield it is possible to achieve the 35 % GHG savings (Table 3).With a higher yield and still fairly low emission, 1.1 kg N 2 O-N ha −1 yr −1 , 40 % savings can be obtained.Even if refinery operation uses solely renewable energy, the results still show too high an emission for Mellby, with only 25 % GHG savings.But the same test for Log ården shows it may be possible to gain 70 % savings if both a good yield, normal for the region, and fairly low emissions are achieved.The problem is that these low emissions are often not the case since there is more than a 50 % risk for emissions higher than 1.5 kg N 2 O-N ha −1 yr −1 (Fig. 2), when ethanol production will not reach the sustainability rules.Introduction

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Full Three main factors have been shown to be important for the possibility of reaching the threshold of 35 % savings of GHG compared with fossil fuels; these are harvest size, energy use in the refinery, and nitrous oxide emissions (direct and indirect).These indirect emissions are most often calculated by use of the IPCC emission factors.The 2006 IPCC default EFs for these indirect emissions are 1 % (uncertainty range 0.2-5 %) for volatilised N and 0.75 % (0.05-2.5 %) for leached N. At default volatilization fractions of 10 % (mineral fertiliser) or 20 % (animal manure), and the default leaching fraction of 30 %, indirect emissions can be negligibly small, 0.04 % of the added N using the numbers at the lower end of these ranges, but up to more than the direct emission (1.2 %), at the upper end.In most calculations the default EFs are used, resulting in an estimated emission of 0.3-0.4% of the N applied to the land.More detailed discussion of indirect emissions and the likely EFs can be found in IPCC ( 2006) and Well and Butterbach-Bahl (2010).Here we have assumed the indirect emissions to be 1/3 of the direct emissions, based on the IPCC emission factors.Experiments for similar systems in Sweden to Log ården and Mellby have shown N leaching to be lower from clay soil than from sandy soil, 2-22 and 15-53 kg N ha −1 yr −1 , respectively (Aronsson et al., 2011).Leaching at Log ården the same year as emission was measured showed values of 18-22 kg N ha −1 yr −1 , which confirms a low leaching from clay soil (Stenberg et al., 2011;Wess én et al., 2011).The low leaching from the clay soil compared to the IPCC default may indicate a rather low indirect emission from clay soils, making the overall picture somewhat better than estimated in Table 3, but the N-leaching from sandy soil may be similar to the default values.We have shown the LCA to be very sensitive to the size of the N 2 O emissions, both direct and indirect, where a small change can make it possible or impossible to reach the goal.

N 2 O as a consequence of agriculture in general
The expected future population growth needs agricultural production to increase by 70 % to fulfil demands for food over the next 50 yr (FAO, 2009) raising the question: how much more N will be required to meet both increased food needs and biofuel Introduction

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Full production.And how much N 2 O emission will result?Can agriculture management be designed for an overall low N 2 O emission?One attempt to answer this was by van Groenigen et al. (2010), who showed that fertiliser addition rates below 200 kg N ha −1 give minimum N 2 O emissions per unit of yield (8 g N 2 O-N kg −1 crop N).Attempts to increase the yields with higher N additions will decrease the nitrogen use efficiency and inevitably increase N 2 O emissions.However, there should be no departure from attempting to achieve the optimum level of yield and minimum level of N 2 O per unit of yield, consequently the option to increase the yields of food and feed crops, thus releasing land for biofuel cropping, is limited.But even the best management options leave reactive nitrogen behind, which can possibly be converted into N 2 O over time, providing some explanation for the Crutzen et al. (2008) emission factor of 3-5 % of the new N being converted into N 2 O-N.Due to the transfer of reactive N from agricultural land to surrounding ecosystems indirect emissions will also take place in the year of application and in following years (IPCC, 2006;Well and Butterbach-Bahl, 2010), as mentioned above.These emissions are included in the estimate produced by the method of Crutzen et al. (2008), but on a regional or country scale the overall emissions can be both lower and higher than the global average emission factor of 3-5 % of the newly fixed nitrogen.
Arguments have been raised that the atmospheric N 2 O increase depends also on other factors besides the nitrogen fixation, e.g. the roles of an increased livestock production and mineralisation of soil nitrogen as substantial additional sources of liberated N (Davidson, 2009).We agree that taking virgin land into cultivation has been, and will continue to be a source, through the release of nitrogen stored in the soil organic matter, but N 2 O from livestock production is already implicitly included in the Crutzen et al. (2008), concept, because the reactive N has entered the ruminant N cycle either as fertiliser on grass, or by BNF by fodder legumes and soya beans used in feed supplements.
The large overall emission calculated by the Crutzen et al. (2008) method may not always be welcomed by biofuel producers and authorities, who have argued that a lower Introduction

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Full direct emission factor is more appropriate, since only emissions directly associated with the cropping for bio-ethanol should be included and emissions that anyhow should result from the land, so-called background emission, can be subtracted (Ahlgren et al., 2009).A similar way to lower the emission is to use a reference case: an alternative land use for which hypothetical emissions can be subtracted from the actual production emissions [EC directive 98/70/EG].However, we argue that a major part of such emissions is anthropogenic, resulting from past agricultural management (IPCC, 2006), and like present land use will influence future emissions.Thus the emissions are an integral part of agriculture and cannot be subtracted.Therefore we have not made any background subtraction in our simple LCA, which shows three factors having a major influence on the possibility of reaching the goal of 35 % GHG savings: harvest size, refinery emissions and soil N 2 O emissions.
We now turn to the question of whether we can put numbers on the N 2 O emission at a regional scale.The most important conclusion is that emission measurements in northern systems on mineral soils cropped with cereals and oilseed rape span a yearly nitrous oxide emission range from nearly zero to >10 kg N 2 O-N ha −1 yr −1 , but for 75 % of the cases the emission is below 4 kg ha −1 yr −1 with an average 2.8 kg N 2 O-N ha −1 yr −1 for fertilised agricultural mineral soils (Fig. 2).The variability between different investigations can be due to shortcomings in the intermittently performed measurements.But since the data were collected in different systems there are also different influencing factors and biological variation in soil may be huge.An emission below 1.5 kg N 2 O-N ha −1 yr −1 , needed to fulfil a GHG saving of 35 %, was found for less than 50 % of the data collected.And also the Log ården clay soil had this necessary low emission.But because of the low yields it was still barely possible to fulfil the 35 % rule, and not possible to achieve a 50 % GHG reduction as will be required in 2017.
The Mellby farm LCA estimations resulted in an increased GHG emission compared with fossil fuel, due to both low yields and "high" emission size.We here included only mineral soils since this are the typical soils used for arable cropping across Sweden, but unless adequate account is taken of the contribution of

BGD Introduction
Full organic soils, the average emissions will be underestimated.Of the arable land in Sweden, 9 % is classified as having organic soils, where stored carbon and nitrogen is released by mineralisation, causing high emissions: 10 kg N 2 O-N ha −1 yr −1 when cultivated for cereal crops (Kasimir Klemedtsson et al., 2009), and 28 % of the organic soils in Sweden are used in this way (Berglund and Berglund, 2008).If there is no regulation controlling which soil type can be used for biofuel production, these emissions should be added and included in the typical soil emission values for the region, resulting in an average increase of almost 1 kg N 2 O-N ha −1 yr −1 , which we can conclude would make biofuel production from arable crops impossible under the rules of 35 % (and 50 %) GHG savings.

Prediction of N 2 O emission
To predict the size of the N 2 O emission from an area of land, it is necessary to know which environmental factors have the most impact.The IPCC revised Tier 1 method, basing emissions solely on N inputs, is a convenient way to estimate the emissions, as it is easy to obtain the data, but as Fig. 2 shows, the influence of the fertiliser on the emission can be quite small.However, this concept has been used in many life cycle analyses (Smeets et al., 2009), even though it does not work well in some circumstances.Problematic cases are where soil organic matter contributes to the emission, from the release of nitrogen accumulated into the ecosystem long ago.Thus in principle we need attempts like those of Freibauer and Kaltschmitt (2003) and Stehfest and Bouwman (2006), where other influencing factors besides nitrogen addition were also included.Unfortunately, these methods resulted in estimates that were much higher than the Swedish measurements.Important factors in the Freibauer and Kaltschmitt method were the C and N content of the soil; this could be an indication of turnover of organic matter and release of N, but the contents themselves do not predict the rates of N turnover processes such as mineralisation, nitrification, denitrification and assimilation, that have a major influence on the actual N 2 O emissions.There is a need for continued method development for nitrous oxide estimations at a local level, including Introduction

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Fig. 1 .Fig. 2 .Fig. 3 .
Figure1 source category is thus shown separately because of the large total impact, and we use a low emission value for N fertiliser production, since products produced in lowemission factories with catalytic destruction of N 2 O dominate the Swedish market.Also we assume a conventional rate of fertiliser addition, 120 kg N ha and E EM is the sum of emissions caused by energy use and manufacturing (management, transportation, ethanol refinery and emission caused in N manufacturing), y is the crop yield converted into ethanol (MJ ha −1 ), direct emissions are 3/4 and indirect emissions are 1/4 of the total N 2 O emissions, division by 298 (GWP) converts CO 2 into N 2 O and multiplication by 28/44 converts N 2 O into N 2 O-N.
069 if pH 5.5-7.3,F 4 = 0 if sandy soil and =0.43 if clay soil, F 5 = 0.02 if coastal temperate climate, F 6 = 0 if cereal crop and = −0.35if grass and = 0.44 if other crop as rapeseed, F 7 = 1.99 if data is obtained during year-long measurements.

Table 1 .
Results of Swedish field N 2 O measurements.

Table 2 .
Comparison between measured and calculated direct nitrous oxide emission.Measured data are from Mellby and Log ården in Sweden, having mineral soils cropped with cereals, with the addition of mineral fertilisers.