Utilization of cattle slurry, biogas digestates and separated digestates by injection to organically managed spring barley

Nutrient scarcity in organic farming is an important obstacle to the growth of the sector. This study aimed to design a novel anaerobic digestion system for enhancing nitrogen (N) availability and abundance by co-digestion of grass-clover biomass and cattle slurry and a series of post-treatments. The digestates were separated, and the solid fraction was dried and stripped of ammonia. By a novel approach, the liquid fraction was used in a desulfurizing filter for the biogas producing an acidic liquid that was enriched with ammonia resulting in a sulphur-nitrogen-rich fertilizer product (LiqNS). The fertilisers were assessed for their fertilizer value under organic field conditions by direct injection before sowing spring barley. Two field studies were conducted to compare the N fertilizer replacement value (NFRV) of cattle slurry, digestates from co-digestion of cattle slurry and grass-clover, the liquid fractions, the dried fibre fraction of digestates, LiqNS, and mineral fertilizers under organic crop management. A third field study also compared the effect of crop management practices (organic vs. conventional) on NFRV. The field trials showed that cattle slurry, co-digested digestates, liquid digestates, and LiqNS had high NFRV of 80–90 %, with liquid digestates showing consistent performance. Co-digestion of grass-clover and cattle slurry did not change NFRV significantly compared to untreated cattle slurry. Conversely, the dried fibre fraction (DF) exhibited negative NFRV in spring barley due to nitrogen immobilization but no negative yield effect in a N-fixing faba bean crop. Anaerobic mono-digestion of cattle slurry improved NFRV by 16 % compared to untreated cattle slurry. By organic crop management, higher NFRV was estimated than by conventional management, due to lower yield response to mineral fertilization by organic management (11.9 percentage points lower nitrogen use efficiency), but mostly similar responses to organic fertilization under organic and conventional management. A reason for this could be the use of surface application of mineral N fertilizer favouring weed growth in the organic system, while such effects were avoided in both systems by injection of the slurries. Anaerobic digestion and post-treatments of digestates are valuable methods for enhancing nutrient efficiency and availability in organic farming. However, assessing the N fertilizer value of organic manures requires careful consideration of experimental management practices. Additional research is necessary to understand the different responses to mineral N fertilizers by organic management.