Sustainable Farming of Bioenergy Crops

Sustainable Farming of Bioenergy Crops

C H A P T E R 23 Sustainable Farming of Bioenergy Crops Adrian Muller Research Institute of Organic Agriculture FiBL, Zurich, Switzerland; Institute ...

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23 Sustainable Farming of Bioenergy Crops Adrian Muller Research Institute of Organic Agriculture FiBL, Zurich, Switzerland; Institute for Environmental Decisions, Swiss Federal Institutes of Technology (ETH), Zurich, Switzerland email: [email protected]

O U T L I N E Introduction


Criteria for Sustainable Farming and Sustainable Food Systems Conventional Agricultural Production Sustainable Agricultural Production Sustainable Food Systems What is Sustainable Bioenergy Production? Ways of Comparisons

409 409 409 410 410 410

INTRODUCTION Is bioenergy a sustainable energy source? A positive answer to this question is a key if bioenergy shall become a significant sustainable energy source for future societies. The answer to this question depends on various aspects of the production and use of bioenergy. The most prominent topic there is the greenhouse gas (GHG) balance, as this is the key motivation to investigate bioenergy at all. For most cases, the GHG balance is positive, albeit not at a tremendously high rate and negative values are due to large emissions from direct and indirect land use change (ILUC), e.g. if palm oil plantations are established on peatland rainforest (Faist Emmenegger et al., 2012; PBL, 2010; Fargione et al., 2008). Clearly, in the use phase, GHG emissions are counted as zero due to the overall assumption of renewable biomass provision for bioenergy. However, over the whole life

Bioenergy Research: Advances and Applications

Sustainability Criteria for Biofuel Production Biomass Use

410 412

How Much Bioenergy may be Produced Sustainably? Global Bioenergy Potential Bioenergy Potential on Farm Level

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cycle of bioenergy, GHG emissions arise at various steps, in particular, in the agricultural production phase (Faist Emmenegger et al., 2012). The GHG balance usually plays the role of a fundamental decision criterion in favor or against bioenergy types. With a zero or negative balance, bioenergy will not contribute to and may even adversely affect climate change mitigation. However, even in this case, one could promote one argument for bioenergy, namely that it replaces nonrenewable energy sources with renewable ones. This is particularly attractive for liquid fuels as there are currently no other alternatives available than liquid fuels from biological sources. In the following, we often focus on liquid biofuels as the discussion of sustainability of bioenergy is developed furthest for those and most data are available for those bioenergy types. The findings on the sustainability of agricultural production of crops for liquid biofuel, however, apply to agricultural production of any bioenergy type and we will

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also report findings for biogas production, for example, on which there is also a considerable amount of research. Besides the GHG balance, many other criteria are needed to assess the sustainability of bioenergy. They range from environmental impacts of the agricultural production process over emissions in the use phase (e.g. nitrous oxide emissions from biomass-fueled power plants) to socioeconomic aspects, such as production costs or effects on labor (Elbehri et al., 2013; Faist Emmenegger et al., 2012; Delucchi, 2010). The assessments of environmental impacts show that most bioenergy crops perform worse than the fossil fuel baseline regarding many criteria of sustainability in agricultural production. The best performance is realized for some residue or forestry-based bioenergy sources such as fuels from wood products (Faist Emmenegger et al., 2012). The negative performance of bioenergy crops is due to the fact that current conventional agricultural production has many adverse environmental effects (Matson et al., 1997; IAASTD, 2009; see e.g. Tomei and Upham, 2009 on soy-based biodiesel in Argentina for an exemplification). As important as these findings on environmental performance is, the key point in the current discussion besides the GHG balance is land competition between bioenergy crops and food production (Rathman et al., 2010; Delzeit et al., 2010; HLPE, 2013). Bioenergy crops compete for fertile land with food crops and bioenergy use of food crops such as maize directly competes with food use. This dynamics has been behind the volatile and high food prices in recent years as, e.g. for maize and grain in 2007e2008 (NYT, 2007; HLPE, 2013). There are several options available to address all these challenges. First, there are alternative ways of agricultural production that reduce the adverse impacts from farming, a key example being organic agriculture (Rossi, 2012). These alternatives usually focus on systemic aspects of the whole production systems, emphasize closed nutrient cycles, sustaining soil fertility and plant health and the role of ecosystem services, e.g. for pest and disease control. Second, how strong land competition may become depends on the concrete situation, i.e. on total bioenergy demand, relative prices for food and energy, the policy environment with incentives and regulations, etc. Very circumspect planning of any larger scale bioenergy strategy seems crucial to have a chance to avoid such adverse effects (e.g. Damen, 2010 for Thailand, wa Gathui and Ngugi, 2010 for vulnerable regions in Kenya, or Palola and Walker, 2010 on oil palms). Standards and certification are also suggested as a means to address the potential land use competition, e.g. by explicitly excluding bioenergy from fertile croplands (e.g. GEF et al., 2013). Third, new forms of bioenergy emerge (IEA, 2010). Instead of the so-called “first-generation” bioenergy that is based on use of oil, starch or sugar

contents of crops to manufacture biodiesel, “second-generation” bioenergy relies on the lignocellulosic contents of the crops. This allows a much wider range of crops to be utilized for biofuel production, in particular nonfood woody crops such as switchgrass or Eucalyptus (IEA, 2010). It also allows utilization of basically the whole plant and of crop residues for biofuel production. This reduces land demand per unit energy for bioenergy. However, it is not expected that the second-generation biofuels will play a significant role in the near future as much research is still needed (IEA, 2010). An aspect that is largely missing in this discussion on environmental impacts of bioenergy production and land competition is the role of biomass quantities. Large quantities of biomass are needed for bioenergy strategies that significantly contribute to the global energy supply. On the other hand, biomass plays a key role as a fertilizer in sustainable agricultural production systems. There is thus a direct competition for biomass between exporting it from the agricultural production system for bioenergy use and recycling it as a fertilizer (Muller, 2009). Data and studies on how much biomass may be exported for bioenergy use from sustainable agricultural farming systems are scarce, but indications that it will not be much dominate, as we will discuss further down. The key topic in this discussion is thus less whether agricultural production of bioenergy crops can be done in a sustainable way, as sustainable agricultural production is well established and can be implemented for any crop production and as options to reduce land competition are around in principle, albeit challenging to implement. The question is thus rather how much bioenergy can be sustainably produced in such a context where biomass is not a waste output from agricultural production but an essential fertilizer input in sustainable agricultural production systems and where fertile land is needed for food production. In this chapter, we focus on the sustainable production of bioenergy crops with a focus on the farm level. This thus covers the farm operations, but it does not cover processing, transport, storage and use of bioenergy. Possible disposal of waste after bioenergy use is shortly addressed in the context of fertilizer use (cf. Section Bioenergy Potential on Farm Level). This chapter also covers more aggregate aspects of agricultural production such as land and water resource use and more systemic aspects related to the whole food system, such as the competition for land, water and biomass between food and bioenergy production. In this context, we also address general aspects of food security and in the conclusions also relate to the role of meat and milk production as another sector that competes for scarce land resources with crop-based food production. We restrict this analysis to agricultural bioenergy. We thus address forestry only occasionally and do not


address aquaculture such as for algae-based biomass production for energy use or biomass production in industrial contexts, such as in bioreactors (Chen et al., 2011). As biological processes are involved in all these cases, the key topics we discuss in the following are relevant also there: resource use and resource competition, environmental impacts of the production (e.g. water and air pollution, intoxication due to pesticide use, etc.), and effects on the sustainability of the food production system as a whole. On the other hand, harvesting the energy capture potential of photosynthesis in purely industrial production of biomass in bioreactors could be an option, as the production system can work well separated from the natural environment. For such production, no ecosystem inputs such as fertile soils and water bodies are used as inputs or sinks (e.g. for nutrient runoff) and thus potentially depleted. Environmental impacts can thus be kept to a minimum. Such operations do however not refer to agricultural but rather to industrial production of biomass for bioenergy use and we do not further pursue this here. Furthermore, they are still in the research and development phase and commercial use is not expected in the near future (Pragya et al., 2013). This chapter is organized as follows. In Section Criteria for Sustainable Farming and Sustainable Food Systems, we discuss the criteria of sustainable agricultural production and sustainable food systems. In Section What is Sustainable Bioenergy Production? we assess how bioenergy production may be implemented if it needs to meet these criteria. We also address sustainability criteria for bioenergy production as formulated by a range of institutions and relate them to the criteria for sustainable agricultural production from Section Criteria for Sustainable Farming and Sustainable Food Systems. Section How Much Bioenergy may be Produced Sustainably? provides some discussion on how much agricultural bioenergy may be produced in a sustainable way and Section Conclusions concludes.

CRITERIA FOR SUSTAINABLE FARMING AND SUSTAINABLE FOOD SYSTEMS Conventional Agricultural Production Over the decades from 1960 to 2010, agricultural production has increased more than threefold and per capita provision of calories has increased by one-third (FAOSTAT, 2013). This has greatly contributed to feeding an ever-increasing population (from 3109 to 6.7109 billion over this period). This development is usually subsumed under the label “Green revolution” (IFAD, 2001; Evenson and Gollin, 2003). Starting in the 1960s, this development was based on a strict focus on monocropping with high yielding species and varieties,


irrigation and mechanization where available and increased use of mineral fertilizers, pesticides and herbicides. The successes of the green revolution are evident, but so are the downturns related to it (Matson et al., 1997; DFID, 2004). The focus on monocropping, chemical fertilizers, pesticides, irrigation and mechanization has left an increasingly negative legacy regarding adverse effects on soil fertility, i.e. increased soil degradation, salinization and depletion of water bodies, on intoxication of the environment, biodiversity loss, loss of ecosystem services, on eutrophication of water bodies and animal health (Matson et al., 1997). Current agriculture is well able to feed the world and will be able in 2050 to feed more than 9 billion people, given projected yield increases realize (Alexandratos and Bruinsma, 2012). The challenge is not the average supply of calories per capita but their distribution globally and the fact that a third is lost or wasted globally (Godfray et al., 2010). However, in the light of the adverse effects of the green revolution and climate change, such yield increases may be compromised and sustained agricultural production calls for alternative cropping practices and a fundamental shift in the agricultural production system (IAASTD, 2009; Mu¨ller et al., 2010).

Sustainable Agricultural Production A new revolution in agricultural production is thus needed. On the production level, a sustainable future agricultural system needs to focus on mitigating and avoiding the adverse effects of current agricultural practices. It needs to focus on crop diversity, ecosystem services, soil protection and fertility, nutrient and water use cycling, biocontrol of pests, diseases and weeds and reduced pesticide use. A range of alternative production approaches are available (Eyhorn et al., 2003; Pretty et al., 2006; Rossi, 2012), such as agroecologybased approaches, focusing on utilization of ecological concepts (Altieri, 1995), or integrated pest management, focusing on reducing pesticide use via managing pest populations in such a way that damages remain low (Bajwa and Kogan, 2002). The role model for these alternative approaches is organic agriculture with its ban on most pesticides, focus on soil fertility, plant health and closed nutrient cycles, utilization of optimized crop rotations and crop diversity, organic fertilizers and ecosystem functions for pest and weed control (FAO, 2002; Eyhorn et al., 2003; IFOAM, 2006). Organic agriculture is the role model as it addresses all adverse effects of conventional production, adopts a systemic approach and is well established and tested for decades and embedded in a context of governance, information provision, training and extension institutions that make it the best-developed alternative production system. Organic agriculture performs better than conventional



agriculture with respect to most environmental indicators on a per-hectare basis (Schader et al., 2012). The biggest drawback is its generally lower yields (Seufert et al., 2012; De Ponti et al., 2012; Badgley et al., 2007). Lower yields predominantly manifest in comparison to high-yielding intensive conventional agriculture. In developing countries, in a context of currently nooptimal conventional production systems, organic yields are on par or even higher for well-managed organic farms. The lower yields can result in a less favorable per kilogram produce assessment of environmental impacts for some products in organic agriculture (Schader et al., 2012). We emphasize that we do not address socioeconomic aspects of organic production here, such as the need for information and extension services to train farmers and potential challenges of the conversion from organic to conventional agriculture. Interestingly, the key principles and practices of organic agriculture become increasingly important in conventional agriculture, mainly due to the increasing need to contribute to climate change mitigation and adaptation but also due to the increasingly important discussion on global biodiversity losses. Optimized crop rotations with deep-rooting forage legumes and use of organic fertilizers, for example, are promoted in the context of climate change mitigation and adaptation to improve soil fertility and increase soil organic carbon levels (Smith et al., 2008) and reducing nitrogen loads are key to protect biodiversity.

consumption of animal products that are mainly based on grassland feed (and some by-products of food production) thus comprise an optimal option for a sustainable food system (e.g. preliminary results from the FAO-SOL-model, Schader et al., 2012).

WHAT IS SUSTAINABLE BIOENERGY PRODUCTION? Ways of Comparisons When assessing the sustainability of energy crop production, the first criterion is usually its GHG performance with regard to a fossil baseline. For this comparison, the baseline fuel mix plays a crucial role, as the increasing importance of unconventional fossil fuel sources such as oil sands will increase GHG emissions from the baseline and its general environmental impacts, thus relatively improving the performance of bioenergy production (Faist Emmenegger et al., 2012). This bears the danger that biofuel options with increasing environmental impacts and less favorable GHG balances become relatively more sustainable. Here, we adopt a different focus as we are primarily interested in the sustainability of bioenergy production with reference to sustainability in agricultural production systems and in food systems in general. This takes all sustainability criteria into account and does not focus on the GHG balance.

Sustainable Food Systems Agricultural production is only one aspect of food systems. Sustainability in agriculture also needs to address more encompassing concepts such as food security and land availability on a regional level. If agriculture would switch to organic, 10e20% more land would be needed due to lower yields, given diets do not change and the same amount of wastage is produced as today. However, dietary change is a key topic for sustainable food systems, as a large part of agriculture’s environmental charge stems from animal husbandry. Reducing meat, egg and dairy product consumption levels would greatly help to reduce environmental pressure from agricultural production. Focusing on feeding animals on grasslands and not on food crops such as soy and maize would reduce the need for land, as calorie production from crops is much more efficient than from animals. Furthermore, about 30% of agricultural production are lost or wasted globally (Godfray et al., 2010). Reducing this would also contribute to reduced agricultural land use. Such reduced land use would on the other hand reduce pressure to further increase yields. Organic production in combination with reduced waste and lower

Sustainability Criteria for Biofuel Production There exist several approaches proposing sustainability criteria for bioenergy (e.g. Cramer et al., 2007; EU, 2009; IEA, 2010; RSB, 2011; GEF et al., 2013, where the last two are very detailed). They usually cover some goal for GHG emission reductions, e.g. a 35% reduction of aggregate emissions over some time period with respect to the baseline as suggested in EU regulations (50% from 2017 to 60% from 2018 onwards, EU, 2009). While this seems a clear criterion, its assessment is complex. The choice of different default values, soil carbon stock data and land use change definitions, for example, is behind the huge differences in GHG balances between two GHG calculation tools as assessed in Hennecke et al. (2013), one of them being the tool used by the Roundtable of Sustainable Biomaterials (RSB) whose sustainability criteria are discussed below. Other aspects are decisive as well. The choice of the time horizon over which aggregate reductions have to be achieved and the choice of the social discount rate, which influences the relative importance of current and future emissions, also greatly influence the outcome (De Gorter and Tsur, 2009).


The sustainability criteria proposed for biofuel production that relate to agricultural production and the food system, address land competition, biodiversity, environmental impacts on soil, water and air, and social aspects. Land competition, resp. absence thereof and the related food security are by far the most prominent criteria in the discussion (HLPE, 2013). Potential drivers for land competition are many. First, there is the fact that bioenergy crops need fertile land to achieve economically interesting yields. Biofuels on marginal lands are some option in smallholder and communityself-sufficiency contexts, but for commercial supply of biofuels in significant shares of total global energy demand, production on fertile land that potentially is used for food production is necessary (cf. Section Bioenergy Potential on Farm Level). Similarly, bioenergy crop production depends on water availability and nutrient inputs as any other agricultural production system. Thus, a second potential competition is not only on fertile lands, but also on land with sufficient water availability (Lysen and van Egmond, 2008), in particular in the context of climate change, where water scarcity will become a prevalent problem in many regions (Meehl et al., 2007). Third, relative price differences between bioenergy and food production will be and have been a key driver behind land competition as without further regulation, land will be allocated to the most profitable production. Stated differently, an increasing demand for biofuels leads to higher prices, which triggers an increasing supply for it, with corresponding land use (HLPE, 2013). It has to be emphasized that land competition between food and other uses is not new and relative profitability has always been a key driver behind this. As Nhantumbo and Saloma˜o (2010) state, “Competition for higher-value resources existed well before the biofuels campaign was initiated. In this sense, biofuels production per se cannot be blamed for land use conflicts, as the same types of conflicts have occurred in other economic activities. But, in conjunction with other activities like mining, forestry and tourism, biofuels projects further exacerbate competition for land, water and other resources” (p. 4). The key point is thus that biofuel expansion increases the pressure on the already scarce resource of fertile land. In principle, policy measures can be used to mitigate these adverse effects. However, their implementation is often riddled with difficulties and land-use rights protection, enforcement of laws and regulations, etc. have to be carefully considered when establishing potentially promising institutions for sustainable land use. Nhantumbo and Saloma˜o (2010) illustrate these challenges for the case of Mozambique and draw a rather pessimistic picture. The land use debate is further complicated by ILUCs (Wicke et al., 2012). Those arise, for example, if biofuel


expansion in one region (e.g. sugar cane in southern Brazil) leads to land use change in another region (in this case, deforestation for livestock production in northern Brazil). The rationale behind this example is the fact that expanding sugarcane in the South is at the expense of already existing pastures in this region, that then themselves relocate at the expense of other uses such as natural forests (Andrade De Sa et al., 2013). Such effects are very difficult to clearly identify and assess (Wicke et al., 2012). This is also the case in the detailed analysis in Andrade De Sa et al. (2013) who find only very weak significant statistical effects. Nevertheless, there is evidence from many descriptive studies that the potential presence of such mechanisms must not be neglected (PBL, 2010). ILUC is not only relevant for the competition between different land uses but also for the GHG balances of biofuels, as it can have considerable negative effects on those (PBL, 2010; Faist Emmenegger et al., 2012). Biodiversity criteria mainly refer to the ban of using forests or protected areas for bioenergy production (e.g. Cramer et al., 2007; EU, 2009) or to being attentive not to use invasive species as bioenergy crops (UNEP, 2010). The use of protected areas can also be seen as a particular aspect of land competition from biofuel production. As mentioned above, biofuel production competes not only with food for land but also with other uses, such as biodiversity protection and also with fiber and biomaterial production that all depend on land availability. Invasive species are seen as a potential danger, due to already existing cases but also due to general characteristics of biofuel crops that also correlate with invasiveness (e.g. fast growth or tolerance to wide range of soil and climate conditions, UNEP, 2010). Much less prominent in this discussion are the adverse effects of current agricultural production on biodiversity (mainly due to overfertilization with nitrogen and pesticide use), albeit those are a key driver behind biodiversity losses (Galloway et al. 2008). This is mentioned in Bindraban et al. (2009) and adoption of agricultural practices with low negative effects on biodiversity is a criterion in GEF (2013), but not in EU (2009) or RSB (2011). Other environmental impacts largely remain rather unspecific in the criteria suggested, although the range of adverse environmental effects of current agricultural production as described above will also realize in bioenergy cropping systems. EU (2009) for example only posits that the production has to meet the Community environmental requirements and in GEF et al. (2013), water contamination is assumed to be no issue if legal requirements are met. The size of the adverse environmental effects depends on the types of crops. Grassland or wood products usually perform better than annual crops, for example, WBGU (2009). Somewhat more



detailed criteria are usually given in reference to soilrelated aspects such as soil fertility and soil organic carbon contents (see e.g. Cramer et al., 2007; EU, 2009; RSB, 2011; GEF et al., 2013). Regarding soil organic carbon, some sustainability criteria explicitly exclude bioenergy cropping on peatlands and other carbon-rich soils (EU, 2009). On the other hand, some bioenergy crops are judged to be advantageous for soil carbon levels, mainly grassland and forest-based bioenergy. The effect on soil carbon is not that clear for some perennial crops and rather negative for annual crops (WBGU, 2009). Social sustainability criteria, finally, sometimes tend to be formulated on a very general level. EU (2009), for example, only requires that source countries for bioenergy have “ratified and implemented” (p. 97) a range of conventions referring to labor rights, gender aspects, etc. RSB (2011) and GEF et al. (2013), on the other hand, are quite detailed on the social aspects that cover a range of important criteria for social sustainability in agricultural production. RSB (2011) and GEF et al. (2013) also make long-term economic viability of bioenergy projects a criterion for their sustainability assessment. It is not mentioned as a criterion, but bioenergy crops can have some risk-spreading characteristics as they can increase production diversity of a farm and as their demand and price dynamics likely follows different patterns than food or fiber crop demand and prices. Some types of bioenergy crops can also be used for direct on-farm energy provision without much investment needs, such as Jatropha, for example. These energy crops thus have the potential to increase energy access and reduce workload of women considerable in case they have to collect fuel-wood from far away, which is a common situation in many poor regions in developing countries. Energy crops can also provide specific income sources for women, as many case-studies show (Karlsson and Banda, 2009). However, there is similar evidence of problematic situations from case-studies, and whether a bioenergy project is advantageous for single farmers, the community and women in particular strongly depends on the concrete design and institutional context. Further example of positive cases are given in Practical Action Consulting (2009), and some negative cases for example in Ribeiro and Matavel (2009), focusing on Jatropha in Mozambique. The choice of sustainability criteria for bioenergy thus reflects the classical topics of sustainability criteria, with a focus on environmental aspects and climate change in particular. An additional aspect is land competition, which is covered extensively in the discussion. The focus on environmental criteria is understandable as bioenergy is a climate change mitigation strategy and prime impacts of agricultural production are in the environment. However, besides GHG emissions and, partly, biodiversity, the assessment of environmental criteria

remains rather weak. For a comprehensive sustainability assessment, the topical breadth and depth in analysis must be improved. Generally, bioenergy production has the same impacts as any agricultural production and sustainability in bioenergy production largely links to sustainable agricultural production.

Biomass Use The assessment of proposed sustainability criteria for bioenergy production shows that the competition for biomass between bioenergy use and for fertilizing sustainable agricultural production systems is no topic. It is covered marginally in some other publications on sustainable bioenergy, e.g. in Bindraban et al. (2009) or Blonz et al. (2008), although it is of key relevance for sustainable agricultural production. Some publications address this topic as a caveat of agricultural or forestry residues use, as exporting too much of them causes soil carbon losses and soil degradation (e.g. WBGU, 2009). Only Muller (2009) takes up this topic in depth. The export of biomass from the fields for bioenergy use also exports nutrients that have to be replaced by other fertilizers, i.e. mineral fertilizers. The overuse of mineral fertilizers is however a key driver behind many environmental problems of current agricultural production. Sustainable agricultural production systems are based on closed nutrient cycles and organic fertilizers (compost from crop residues, roots and residues that remain on and in the fields, and manure from livestock operations). Those are keys for soil fertility and increased soil organic carbon levels (Lal, 2008; Gattinger et al., 2012). The nutrient export becomes particularly relevant for second-generation biofuels, where basically the whole plant can be used and no unused residues remain, resp. where cellulosic residues from any crops can be utilized (IEA, 2010). This is even suggested as a strategy to mitigate land use competition, as residues come without additional land requirements and feedstock for second-generation biofuel is claimed to often grow on marginal lands (IEA, 2010, 2011). On marginal lands in particular, high organic matter inputs are key to improve soil fertility, though. Also for bioenergy crops, yields tend to be lower and erratic on marginal lands and economic viability of bioenergy projects is often given on fertile land only (Bindraban et al., 2009). Thus, regarding the biomass competition, the most promising options to avoid land use competition seem particularly problematic.

HOW MUCH BIOENERGY MAY BE PRODUCED SUSTAINABLY? As illustrated above, sustainable production of biomass for energy use is in principle possible. Thus,


the key question is how much biomass may be produced sustainably on a global level. We basically discern two types of approaches to this question. First, there are various assessments of the global biomass potential for energy use. They are based on assessments of the suitability of global land areas for biomass production and corresponding yields, usually imposing the condition that food, feed and fiber demand need to be met. The second approach focuses on single farms or farming systems and estimates how much bioenergy may be produced in such, given the specific agronomic characteristics. These latter studies can also involve experimental data from case studies.

Global Bioenergy Potential A range of literature assesses the global bioenergy potential employing various models and assumptions. WBGU (2009), Chum et al. (2011) and HLPE (2013) contain some recent reviews of this literature. Some very gross global comparisons are illustrative. If all harvested biomass today (including crops, forage, wood, and residues) would be used for energy use, this would cover about one-third of today’s energy supply (HLPE, 2013). This total harvested biomass corresponds to about 230 Exajoule (EJ) of primary energy per year. Chum et al. (2011) give a gross estimate for the technical bioenergy potential of 100e300 EJ/a in 2050, showing a wide range of uncertainty, though. This amount of bioenergy would very roughly cover between 10% and 60% of total primary energy supply in 2050, which ranges between 500 and 1000 EJ/a, based on 164 scenarios (Chum et al., 2011, Figure 10.3). Nevertheless, this number is illustrative as it roughly corresponds to a situation, where a biomass quantity equaling the total current biomass production was used for bioenergy in 2050. Current biomass energy use is about 50 EJ/a. Chum et al. (2011) base these estimates on a literature review that assess the physical biomass production potential based on land suitability and crop yields. These assessments also rely on considerably gains from yield increases and agricultural technology progress. While Chum et al. (2011) evaluate these numbers rather positively, HLPE (2013) considers them very problematic. WBGU (2009) in detail presents their model for the assessment of bioenergy potentials, relating to clearly stated general sustainability boundaries (biodiversity conservation, food security, climate change and acidification mitigation, and soil protection) and arrive at an estimate of 80e170 EJ/a. A key drawback in these estimates is the lack of economic and agronomic considerations in a systematic way. Both Chum et al. (2011) and HLPE (2013) emphasize the role of economic aspects, i.e. costs of such bioenergy development. WBGU (2009) also emphasize


that their estimate is purely technical given some sustainability boundaries and that the economic potential likely is considerably lower. But only few studies address market interactions between food and energy crops in economic equilibrium models and none of those was used to assess the bioenergy potential presented. Supply costs curves for various bioenergy crops resp. food crops for energy use are provided for illustration in Chum et al. (2011), for example, but no consequences on crop prices in interaction with demand are derived from this. Infrastructure and access to the suitable land are neither addressed explicitly. However, to realistically assess any competition for land between food and energy crop production, biomass and land markets need to be included in the model, as the relative profitability of food or bioenergy production on a certain area of land will drive production decisions and food and bioenergy supplydunless strong governmental regulations are imposed on the bioenergy and land markets. Thus, even if the biomass potential for energy uses is very large, its effect on food market prices needs to be assessed in detail to derive robust statements on land competition. The second aspect that is missing is agronomic characteristics of biomass production. The estimates reviewed in Chum et al. (2011) consider some sustainability criteria when trying to assess how much biomass may be produced without additional conversion of forests and grassland to cropland, which may be the effects on biodiversity, or via investigating whether bioenergy crops may even serve to improve degraded soils. However, agronomic aspects of the crop production system are largely neglected. Water requirements and potential problems related to that are mentioned explicitly, but are not captured explicitly in the models referred. Furthermore, any crop production needs to be fertilized if yield decreases after some years and soil degradation should be avoided. Yield assessments for biomass production do however not differentiate for nitrogen inputs. In addition, fertilizing with mineral fertilizers only is not enough, as organic fertilizers are crucial to halt soil degradation (e.g. Lal, 2008; Blanco-Canqui and Lal, 2009). This problem is mentioned in Chum et al. (2011), but it does not become effective for determining the biomass potential although it directly conflicts with the basic mechanism of bioenergy cropping, which is exporting a high amount of biomass, resp. using biomass residues formerly left on the field. Utilizing the biomass potential referred to above may thus result in considerably increased total global fertilizer use with respect to today (and in addition, crop production has to increase by 70% as well, Alexandratos and Bruinsma, 2012). This would put additional pressure on the nitrogen cycle and lead to corresponding adverse environmental effects (Erisman et al., 2010). WBGU (2009)



contains a review of several bioenergy crops that also contains some agronomic aspects. They conclude that only grasslands and some forestry have a sustainable potential for bioenergy provision. This is also reflected in their model, which assesses the bioenergy potential on additional grassland and forestry use, as only those meet their sustainability boundaries, besides some use of residues.

Bioenergy Potential on Farm Level As we have seen in the previous section, assessments of the global bioenergy potential are based on land use and land availability consideration subject to several sustainability criteria. These assessments thus tend to disregard agronomic boundary conditions. WBGU (2009) is one exception and also explicitly includes such aspects on a very aggregate level in their model, by assuming that only 60% of residues can be used for energy production technically (and only 30% economically), given that part of the residue biomass needs to be left on the fields in order to avoid soil degradation. In contrast to such global or regional assessments, farm or farming system-based assessments are in principle able to account for such agronomic boundaries. Rossi (2012) reviews a range of sustainable farming systems as options for sustainable biomass production. He points out the role of biomass as a fertilizer and for soil fertility, but does not provide quantitative assessments of how much biomass may be exported from these systems for bioenergy use. Even more, the case studies presented in Rossi (2012) often do not address bioenergy production at all but only illustrate the advantageous performance of the respective farming system along a range of sustainability criteria. There is however other research that provides detailed quantitative analysis. Meyer and Priefer (2012) for example discuss the potential of biogas production in organic agriculture, based on case-study farms in Germany. Biogas fits neatly into organic production systems, as in organic farms, much biomass that can be used as feedstock for biogas plants is around (from grassclover leys in the crop rotations, for example) and the biogas slurry can be used as a fertilizer. Meyer and Priefer (2012) provides also some forecast on the potential for such bioenergy production in Germany, assuming that the biogas is used for electricity production and also utilizing the heat generated in the power plants. Assuming 20% of agricultural production being organic (political goals for 2020 are 20% in Germany) and equipped with biogas facilities, 7 TWh/a electricity could be provided plus 50% of this energy in heat. Assuming a total electricity demand of 535 TWh/a in Germany in 2030 BMU 2011), similar biogas production on all farms would provide 6e7% of this (35 TWh/a). Also, Anspach

(2009) finds that biogas production fits well into organic production systems. Using biogas slurry as fertilizer has also some additional advantages regarding yields, environmental impacts and weed control (as seeds of weeds e.g. in manure are killed in the biogas digester). The potential of biogas production is also recognized by authors of more aggregate studies, e.g. Bindraban et al. (2009). This biogas production is assumed to work largely without bioenergy cropping and only uses residues and manure. Thus, it does not lead to competition with food production. Currently, the reality in Germany is different, though, as co-substrates are imported to a significant part in biogas digesters and part of those are specifically grown for biogas production (e.g. maize). Another body of literature focuses on energy selfsufficiency of organic farms, motivated by the unsustainable use of fossil fuels also in organic production systems (Carter et al., 2012; Christen and Dalgaard, 2013; Halberg et al., 2008; Oleskowicz-Popiel et al., 2012; Pugesgaard et al., 2013). Those studies are from Denmark and serve as further illustration for the bioenergy production in sustainable agricultural production systems. They generally find that energy self-sufficiency of organic farms is possible and that sometimes even some small energy surplus can be generated. Carter et al. (2012) are somewhat different, as they focus on a GHG life-cycle analysis and do not address nutrient recycling aspects at all. Pugesgaard et al. (2013) find that energy self-sufficiency is also possible with nitrogen self-sufficiency. The energy selfsufficiency described in these studies comes at the expense of increased land demand or lower yields, though a fact that is not emphasized in these studies but that is crucial for our more encompassing assessment of sustainable bioenergy production. Fredriksson et al. (2006) find 4e10% increased land demand for energy self-sufficiency of the farm. We emphasize that self-sufficiency means that such a farm does not produce any energy for the wider society. In Fredriksson et al. (2006), this is achieved with utilization of firstgeneration bioenergy, thus the agronomy is similar to ordinary food production and biomass exports are also similar. Halberg et al. (2008) achieve energy selfsufficiency and improved nutrient availability by using land that has been set-aside in the baseline (8.5% of total farmland) for energy production. It is not discussed which environmental effects this has. Pugesgaard et al. (2013) use 10e20% of the farm area for biogas feedstock production and report lower food yields. Either are milk yields reduced by more than 50% due to lower cattle numbers (while cash crop yields are increased by 60e120% due to improved N fertilization of cash crops), or cash crop yields are reduced by 10e30%. The scenario with 120% increased cash crop utilizes additional 20%


farmland of meadows and is thus not fully comparable to the baseline. Also in this case, energy production thus comes at the expense of lower yields or higher land use. A clear assessment of what this means regarding food security is however not possible, as the differences should be translated in total calorie and protein provision for human nutrition. Interesting though is the fact that part of this energy provision is possible in scenarios that go along with some dietary change only, as animal products are reduced.

CONCLUSIONS Our analysis shows that bioenergy without land competition is difficult. While general land use models exhibit quite some potential for bioenergy production also under several sustainability constraints, they lack a due assessment of nutrient use, supply and demand in the agricultural production phase. On-farm studies reveal that increased land use or reduced yields cannot be avoided even for moderate bioenergy generation (e.g. to make a farm energy self-sufficient) unless only biogas is produced. We draw several conclusions from this assessment of sustainable farming of bioenergy crops. First, for a thorough assessment of the sustainability of bioenergy, systemic views have to be adopted. It is not enough to assess the GHG balance on a life-cycle basis. Bioenergy as a climate change mitigation strategy needs to be analyzed in the context of the whole food system including agricultural production. Much work has been done in this direction. Land use modeling and also sustainability criteria for bioenergy account for a wide range of aspects, such as the competition for land. However, as a second point, we want to emphasize that fertilization and nutrient cycles play a minor role in the assessment of bioenergy and its sustainable production only. This is a significant lack in analysis, as biomass plays a key role as fertilizer in sustainable agricultural production systems and as feedstock for bioenergy production. Agronomic aspects of crop fertilization and nitrogen use need to play a significant role in sustainability assessments of bioenergy. Third, we may point out biogas production as one viable option, where biomass can in principle be used for both ends at the same timedas feedstock for biogas plants and as fertilizer in the form of biogas slurry, after having passed through the biogas digester. Biogas production can be designed in such a way that it fits into agricultural production systems without additional land demand. However, as promising as it is for local energy generation, the aggregate potential remains small. In addition, it is no option for producing liquid biofuels.


Fourth, land competition is a key challenge, in particular for liquid biofuel production. Many models to assess the bioenergy potential globally or regionally exist, but they should be improved by adding much more detailed interaction with the energy markets. Such models need to be able to capture land use allocation based on the relative profitability of energy or food production. Most models focus on assessing physical potentials which is a key basis for this, and they mention economic constraints for developing the technical bioenergy potential, but how strong a land competition will emerge hinges on such relative profitability, resp. prices and on demand and supply elasticities, i.e. how much demand and supply changes with prices. In addition, these land use models need to incorporate agronomic aspects. Nitrogen demand of energy crops, corresponding fertilizer demand, its environmental effects and linkages between yields and nutrient inputs need to be captured in much more detail to arrive at reliable conclusions. If it comes to assessing bioenergy potentials in the context of sustainable agricultural production systems, the need to capture fertilizer and nutrient dynamics in more detail is directly linked to biomass flows that must be captured adequately between energy and fertilizer use. Fifth, some improved standard for sustainable bioenergy could help in this. We thus suggest to combine the RSB (2011) and GEF et al. (2013) standards and to enhance them with agronomic aspects related to nutrient and biomass use and recycling.

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