Nitrogen dynamics in soils fertilized with digestate and mineral fertilizers: A full field approach

Highly stabilized digestate from sewage sludge and digestate-derived ammonium sulphate (RFs), were used in a comparison with synthetic mineral fertilizers (SF) to crop maize in a three-year plot trial in open fields. RFs and SF were dosed to ensure the same amount of mineral N (ammonia-N). In doing so, plots fertilized with digestate received much more N (+185 kg ha^-1 of organic N) because digestate also contained organic N. The fate of nitrogen was studied by measuring mineral and organic N in soil at different depths, ammonia and N₂O emissions, and N uptake in crops. Soil analyses indicated that at one-meter depth there was no significant difference in nitrate content between RF, SF and Unfertilized plots during crop season indicating that more N dosed with digestate did not lead to extra nitrate leaching. Ammonia emissions and N content in plants and grains measured were also similar for both RF and SF. Measuring denitrification activity by using gene makers resulted in a higher denitrification activity for RF than SF. Nevertheless, N₂O measurements showed that SF emitted more N₂O than RF (although it was not statistically different) (7.59 ± 3.2 kgN ha^-1 for RF and 10.3 ± 6.8 kgN ha^-1 for SF), suggesting that probably the addition of organic matter with digestate to RF, increased the denitrification efficiency so that N₂ production was favoured. Soil analyses, although were not able detecting N differences between SF and Rf after three years of cropping, revealed a statistical increasing of total carbon, suggesting that dosing digestate lead to carbon (and maybe N) accumulation in soil. Data seem to suggest that N₂O/N₂ emission and organic N accumulation in soil can explain the fate of the extra N dosed (organic-N) in RF plots.