Abstract
BackgroundEnergy crop production for biogas still relies mainly on maize, but the co-digestion of alternative energy crops (legumes, amaranth, ryegrass, flower mixtures) with maize can have several advantages. First, a greater biodiversity in the fields; second, an enrichment of essential trace elements in biogas substrates (cobalt, nickel, manganese, and molybdenum); and third, less use of artificial trace element additives.MethodsIn two randomized field trials, 12 different variants of field crops in sole, double and intercropping were tested over a 2-year period. Dry matter yield, trace element content of the crops, and soil parameters like soil texture, pH, and soil element concentration were determined. The trace element concentrations in biogas plants resulting from input mixtures of energy crops (legumes, amaranth, faba bean, and ryegrass) and maize are calculated.ResultsHigh dry matter yields were obtained for ryegrass, maize, winter faba bean maize, intercropping winter faba bean/triticale-maize, and intercropping rye/vetch-maize. The double croppings with maize reached highest total yields (ca. 30 t DM ha−1). Total element deliveries from the harvest reveal large differences between the variants and the trace elements. Cobalt is provided most by summer faba bean maize and intercropping of winter faba bean/triticale-maize. Ryegrass can deliver the greatest amounts of Manganese and Molybdenum to biogas plants.When these energy crops are added to conventional maize input for biogas production, the trace element concentration in the fermenter can be raised significantly, e.g., 0.03 g Co t−1 FM can be attained compared to 0.003 g t−1 with maize silage input only. Sufficient Co can be provided by addition of manure to the input mixture.ConclusionsAlternative energy crops in combination with maize ensure a good dry matter yield per year and provide significantly more trace elements. However, these substrate mixtures alone do not provide enough trace elements, particularly Co. However, enough Co can be supplied by a small addition of manure.
Highlights
Energy crop production for biogas still relies mainly on maize, but the co-digestion of alternative energy crops with maize can have several advantages
Aims of the study This study shows results of an interdisciplinary project which combines agronomic,chemical and microbiological methods for an optimized and more sustainable biogas production with energy crops
A promising outcome of this study was that faba bean, amaranth, and ryegrass have a much greater concentration of essential trace elements for biogas production than maize, triticale, or winter rye intercropped with vetch
Summary
Energy crop production for biogas still relies mainly on maize, but the co-digestion of alternative energy crops (legumes, amaranth, ryegrass, flower mixtures) with maize can have several advantages. Energy crop production for farmers offers many advantages, e.g., more stable delivery contracts for biogas substrates leading to stabilization of the revenues, the production of organic fertilizer in the form of biogas residue and a larger biodiversity on the fields [2]. In Germany, maize is the most commonly used energy crop with 72%, followed by grass (12%) and 7% of cereal whole crop silage [3]. There are some negative traits about long-term maize cultivation, because it leads to soil degradation and decrease in soil organic matter. A 27-year field experiment showed that soil organic carbon decreased by 30% for continuous maize growth compared to only 3% for a permanent grass cover [4]. When maize is grown in monoculture, it may promote pests like the European corn borer (Ostrinia nubilalis) [6]
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