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 grass — clover leys in the crop rotations, for example) and the biogas slurry can be used as a fertilizer. Meyer and Prie — fer (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, 7TWh/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 6—7% 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 4—10% 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 first- generation 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 10—20% 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 60—120% due to improved N fertilization of cash crops), or cash crop yields are reduced by 10—30%. 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 time—as 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.