Origin of forage crops and manure used as substrate in co-digestion affects methane yield and microbial community structure

In farm-scale biogas systems, different kinds of manure are the most important substrate for anaerobic digestion, but result in low biogas yields. Biogas production can be increased by complementing the manure with forage crops, in co-digestion. In this study four continuous stirred anaerobic laboratory-scale reactors were operated with grass-clover (GCS) and whole-crop barley silages (WCB) in co-digestion with manure from organic and conventional dairy production systems in order to evaluate the importance of substrate chemical and microbiological composition for biogas production, degree of degradation and gas quality. In a second experiment, the substrates were complemented with protein and starch (soybean meal and wheat grain) to balance the chemical composition and increase the organic load. The co-digestion resulted in additive effects on biogas production, but no synergistic effects. The highest biogas yield were obtained in reactors receiving WCB independent of manure types, for both experiments (7416 mL day-1 respectively 10978 mL day-1). The degradation efficiency, measured as the reduction in volatile solids, was on average six percentage units higher in the reactors receiving manure from conventional compared with organic dairy cows, probably because of a higher level of undigested fibre and proteins in conventional cow manure. Microbiological analysis by illumina sequencing illustrated low impact of the manure on the reactor community and only small differences between the reactors receiving GCS and WCB. However, addition of soybean meal and wheat grain changed the community in all reactors.