Replacing silage maize for biogas production by sugar beet – A system analysis with ecological and economical approaches

In a holistic methodological approach, we linked field trial data with different modeling approaches to answer the question if sugar beet roots offer an ecological and economical efficient alternative to silage maize as a substrate for biogas production. Field trials were conducted at highly productive sites in Germany, representative for Central Europe, and tested both biomass crops in continuous cultivation and in crop rotations with winter wheat. In these trials, estimated methane yield of silage maize was generally higher (6837 to 8782Nm3ha−1 a−1) than of sugar beet roots (3206 to 7861Nm3ha−1 a−1) and both biomass crops reached highest yield in crop rotations. Under the nonobservance of technical effects, substrate production costs (€ per Nm3 methane) were higher for sugar beet roots and a nationwide modeling showed that, in most of the German districts, it would need to be reduced by 10 to 25% in order to reach economical competitiveness with silage maize. However, at a farm level, sugar beet for biogas production was economically advantageous when introduced with a share of 10 to 16% into the individual farm's cultivation program mainly due to high yield stability reducing the economical risk. However, a decrease in gross margin (€ ha−1) was likely to occur. In the field trials, different ecological impacts of crop cultivation were assessed but did not highlight one of the two biomass crops in comparison. However, it was evident that cultivating them in three years long crop rotations with two years of winter wheat provoked lower risks of loss of soil organic matter (−122 to −20kg humus-C ha−1 a−1) or N-leaching (40 to 62kgNha−1 in three years) than in continuous cultivation. In contrast, the continuous cultivation of silage maize and sugar beet showed lower greenhouse gas emission (7652 to 11,074kg C-dioxide-equivalents ha−1 in three years) than the crop rotations with winter wheat. Overall, we conclude that sugar beet roots can offer an efficient alternative to silage maize as a substrate for biogas production. However, to raise sugar beet's competitiveness, dry matter yields should be increased without increasing production costs and ecological impacts.