Trace element delivery for biogas production enhanced by alternative energy crops: Results from two-year field trials

Abstract

Background: 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. 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. Methods: In 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. Results: High 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. Conclusions: Alternative 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.