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

Biogas production from dedicated crops creates additional organic fertilizer which may, at least partially, substitute synthetic-N fertilizers. Digestates are characterized with elevated NH4-N content therefore may supply more readily available N to crops comparing to manures. Therefore the aim of the study was analysis of N dynamics in the soil fertilized with 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 soil was sampled from the field fertilized with the same digestate in conditions of regular farming practices. In both incubation experiment and field study digestate supplied soil in NH4-N. The inorganic-N transformation showed similar overall pattern with some differences. In the incubation experiment, after application of digestate in the amount corresponding to fertilizer dose of 170 kg N ha-1, the NH4-N content decreased rapidly during first 14 days from 61.54±5.65 mg N kg-1 to 19.02±4.12 mg N kg-1 and then at day 42 to values close to zero. Contrastingly, the NO3-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 NH4-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 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 slow decrease until the day 56 was observed. The average soil NO3-N content was rather constant in the first 7 days after fertilization and the rapid nitrification occurred in next 49 days resulting in nitrate-N increase to 32.97±24.46 mg N kg-1. Overall pattern of N dynamics in soil fertilized with digestate was the same in incubation experiment and in the field condition, even though studied soils showed some dissimilarities. Rapid ammonium-N transformation to nitrate-N may create favourable conditions for nitrate leaching therefore farm management techniques should be focused on nutrient recycling and N loss prevention