Comparative study on effects of digestate and cattle slurry application on N dynamics in fertilized soil

Abstract

Anaerobic digestion of energy crops and wastes of different origin are widely recognized as a climate-neutral source of energy, while digestate became valuable fertilizer substituting animal manure. To compare the effect of soil fertilization with digestates and cattle slurry on nitrogen dynamics we ran an incubation experiment. The loamy sand soil was amended with: (1) anaerobically digested maize silage with chicken droppings and potato pulp (MSD); (2) anaerobically digested common reed silage (CRD) and (3) raw cattle slurry (CS). All fertilizers supplied NH4-N to the soil. However, the addition of CS resulted in the highest NH4-N content in the soil. The NH4-N dynamics showed a similar overall pattern with some differences in the first week of incubation. Soils fertilized with CR and MSD showed a rapid transformation of NH4-N resulting in its decline in the first two weeks after digestate application with the further slower decrease, while in soil fertilized with CS this process was more gradual. However, in all cases after 42 days, the NH4-N content decreased to values close to zero, which remained until the end of incubation on day 56. Contrasting to NH4-N, the content of NO3-N was initially low and similar in all treated soils and control. The NO3-N concentration in the samples treated with CR and CS rose rapidly from day 2 to day 28. In the soil fertilized with MSD, the NO3-N content increased gradually and reached its maximum on the 42nd day of the experiment. Addition of different organic fertilizers to the soil triggered a range of processes including nitrification, immobilization, and emission, however, the most pronounced process was nitrification of the applied NH4-N. The pattern of inorganic-N transformation suggests that digestate can be used successfully as organic fertilizer but fast nitrification may lead to enhanced loss of NO3-N by leaching. Therefore, application of digestate should follow the same rules as traditional liquid organic fertilizers.