Nitrogen mineralization from digestate in comparison to sewage sludge, compost and urea in a laboratory incubated soil experiment

This paper evaluated, in a laboratory incubated soil, the properties of digestate as a nitrogen fertilizer in comparison with sewage sludge, compost and urea, this last as a typical mineral fertilizer. The incubation period lasted for 90 d and during this time, pH, CO2 and NO3 - evolution were measured. The maximum concentration of nitrate was reached in the incubated microcosm fertilized by urea (133 mg kg-1 after 62 d), and that of digestate was very similar (113 mg kg-1). Soil treated with compost showed a slower nitrate evolution. A significantly negative correlation was detected between cumulative nitrogen nitrified at the end of the trial, and the values of the C:N ratio of the biomasses used (compost, sludge and digestate) (mg kg-1 N - NO3 - vs. C:N, r=-0.94, n = 3, p < 0.05), and between the alkyl-C content at the end of the experiment (mg kg-1 N - NO3 - vs. alkyl-C, r = -0.95, n = 3, p < 0.05). As expected, pH decreases and soil respiration (CO2 evolution) were also well correlated with the content of nitrate. Considering that about 90% of the nitrogen content in the digestate is short acting, the results obtained indicate that the nitrogen rate of mineralization in digestate is very similar to that of urea, confirming that digestate could replace traditional mineral fertilizers.