Effects of anaerobic digestion of organic manures on N turnover and N utilization

Animal manures and plant-based manures are used for biogas production by anaerobic digestion (AD). After AD the concentration of ammonium-N in manure is increased and the concentration of decomposable C is decreased. Thus, the potential first year fertilizer value of the manure can be increased by the treatment. However, pH is also increased by AD thereby increasing the risk of ammonia losses. The objective of this paper was to compare N turnover in soil after application of digested and corresponding undigested manures, and to compare N fertilizer values of digested manures after direct injection or surface-banding in cereals. After AD of pig and cattle slurry the increase in potential plant availability was equivalent to 10-25% of total manure N. AD of cattle faeces and a mixed cattle diet increased the net mineral N release in soil even more to about 60 and 80% of total N, respectively. The present results indicate that the plant availability of N of digested plant materials is similar to that of digested cattle slurry. After surface-banding of digested manures rich in fibers, such as cattle and plant-based manures, significant ammonia loss can be expected resulting in relatively poor utilization of manure N