Dynamics of Nitrogen Transformations in the Soil Fertilized with Digestate from Agricultural Biogas Plant

The biogas production from dedicated crops creates an additional organic fertilizer which may, at least partially, substitute synthetic_-N fertilizers. The digestates are characterized by an elevated NH_4-N content; therefore, they may supply more readily-available N to the crops, compared to manures. Thus, the aim of the study was the analysis of N dynamics in the soil fertilized with the digestate from agricultural biogas plant fed mainly with maize silage with addition of poultry manure and potato pulp. A laboratory incubation experiment was conducted for 56 days and the soil was sampled from the field fertilized with the same digestate under the conditions of regular farming practices. In both the incubation experiment and the field study, the digestate supplied the soil in NH_4-N. The inorganic_-N transformation showed a similar overall pattern with some differences. In the incubation experiment, after the application of the digestate in the amount corresponding to the fertilizer dose of 170 kg N ha^-1, the NH_4-N content decreased rapidly during the first 14 days from 61.54±5.65 mg N kg^-1 to 19.02±4.12 mg N kg^-1 and then at the day 42 – to values close to zero. In contrast, the NO_3-N content increased from 6.35±0.35 mg N kg^-1 to 50.65±4.73 mg N kg^-1 at day 14 and further to 79.06±13.95 mg N kg^-1 at day 42. In the field, the elevation of the NH_4-N content after digestate application was less pronounced as a consequence of lower application rate (114 kg N ha^-1); however, the rapid drop in the ammonium-N content from 20.41±9.18 mg N kg^-1 at day 0 to 14.80±9.75 mg N kg^-1 at day 7 followed by a slow decrease until the day 56, was observed. The average soil NO_3-N content was rather constant in the first 7 days after fertilization and the rapid nitrification occurred in next 49 days resulting in the nitrate_-N increase to 32.97±24.46 mg N kg^-1. The overall pattern of N dynamics in the soil fertilized with digestate was the same in the incubation experiment and under the field condition, even though the studied soils showed some dissimilarities. Rapid ammonium_-N transformation to nitrate_-N may create favourable conditions for nitrate leaching; therefore, the farm management techniques should be focused on nutrient recycling and N loss prevention.