Effects of different manuring systems with and without biogas digestion on soil mineral nitrogen content and on gaseous nitrogen losses (ammonia, nitrous oxides)

Nitrogen (N) is the most susceptible nutrient to transformations affecting plant availability. These transformations include mineralization, immobilization, nitrification and denitrification, as well as leaching and ammonia volatilization. Use of stable wastes and other residues for biogas digestion may reduce N-losses. It is the purpose of this paper (i) to assess the effects of biogas digestion on soil mineral N (SMN) content in spring and autumn, (ii) to compare NH₃ volatilization following superficial application of different manures to a cereal crop, (iii) to compare greenhouse gas emissions of differently treated liquid slurry after its application via injection into closed slots, and (iv) to compare greenhouse gas emissions of differing manuring treatments within a whole organic stockless cropping system. The SMN content in autumn was not influenced by digestion of slurry. However, if cover crops and crop residues were harvested for digestion instead of leaving it on the field, a strong decrease of the SMN content was measured. Ammonia volatilization after application from digested slurry was higher than the volatilization from undigested slurry, likely due to the effect of the higher ammonia content and higher pH. Organic manuring by application of liquid effluents of the biogas digester, by incorporation of green manures with a narrow C/N ratio or by mulching aboveground biomass of a clover/grass-ley, resulted in a strong increase in N₂O emissions. The balance showed a 38% decrease in N₂O emissions for a whole arable organic stockless cropping system when crop residues and the clover/grass-ley were harvested, digested, and the effluents were reallocated within the same cropping system, in comparison to mulching and incorporation of the biomass as green manure. Injection of liquid cattle slurry resulted in a strong increase of N₂O emissions. The results provide some evidence that denitrification during nitrification was the driving force for the measured emission peaks. It was concluded, that biogas digestion of field residues resulted in a win-win situation, with additional energy yields, a lower nitrate leaching risk and lower nitrous oxide emissions. However, the propensity to ammonia volatilization was higher in digested manures.