Digestate Composition Affecting N Fertiliser Value and C Mineralisation

A variety of organic feedstocks can be used for anaerobic digestion, resulting in digestates with different compositions, affecting the fertiliser value. Therefore, two experiments were conducted to assess (1) differences in the nitrogen (N) fertiliser value of seven digestates from different feedstocks in a 2-year field experiment with spring wheat, and (2) the degradability of organic matter (OM) in the digestates within an aerobic incubation experiment. In the field, mineral fertiliser equivalents were in a range of 18-60% (1st year) and 39-83% (2nd year). Fertiliser properties could describe 58.9-74.2% of the N offtake variance among digestates. In the incubation experiment, digestates produced 720-1900 mg CO2-C kg(-1). After 56 days, 61% of organic C added by food waste digestate has been mineralised, compared to 16-22% for the other digestates. Digestate composition (C/N, C-org/N-org, carbonate, cellulose, lignin, and crude fibre) could explain 90.4% of the CO2 evolution. In both experiments, digested food waste stood out among digestates with the highest N offtake and highest OM mineralisation. In conclusion, differences in fertiliser value and OM degradability could be related to compositional variations. However, apart from food waste, the composition had only minor influence on digestate performance after soil application.