Anaerobic co‐digestion of perennials: Methane potential and digestate nitrogen fertilizer value

Co-digestion of crop biomass improves the traditional manure-based biogas yield due to an increased content of easily degradable carbon compounds. In this study, the methane potential of three perennials (grass, legumes, and grass+legume) was determined using various concentrations together with animal manure. The N mineralization dynamics in soil and the N-fertilizer value of the derived digestates was subsequently tested in both a soil incubation study and a pot experiment with spring barley as model crop.
Digestion of all tested crops together with a manure-based inoculum increased the cumulative methane yield 4 to 5 times compared to digestion of the inoculum alone, with the highest increases observed with pure grass as a substrate. However, the biomethane potential decreased along with increasing grass biomass concentration. In the plant pot experiment, all tested digestates increased barley shoot biomass by 40-170%, to an extent statistically comparable to mineral N fertilizer. However, the application of the digestate originating from fermentation with pure grass resulted in lower plant growth and a more fluctuating soil mineral N content throughout the incubation study compared to the other digestates.
The integration of grass-legume mixtures or sole legumes into anaerobic digestion systems as co-substrate for manure seems to be promising, especially when considering their high dry matter and methane yield ha-1 and the possibility to substitute mineral N fertilizer inputs utilizing leguminous biological N2 fixation capacity. This could contribute to the diversification of cropping systems for bioenergy production.