Nitrous oxide from the agricultural water system contaminated with high nitrogen

The agricultural water system especially contaminated with high nitrogen was surveyed in Saitama Prefecture, Japan. The anthropogenic nitrogen, such as fertilizer, livestock wastes, etc., caused the occurrence of high nitrate-nitrogen (20–30 mgN/l) in the groundwater in this area. The concentration of dissolved N₂O in the groundwater ranged from 0 to 28.2 μgN/l. The main source of N₂O in the groundwater seemed to be nitrified N from the stockbreeding areas. As this groundwater flowed into the organic matter rich areas, such as paddy fields and a small river, denitrification actively occurred and much N₂O was produced there. Dissolved N₂O concentration in these places ranged from 10 to 400 μgN/l and N₂O gas flux from the small river was extremely high (about 500 μgN/m² · min). While nitrate-nitrogen could be removed in these areas, these areas often served as significant N₂O sources. The effective and feasible method to decrease the N₂O emission ratio while keeping the denitrification activity high should be explored.

To demonstrate the efficiency of putting sulfur into soil as a denitrification electron donor, the column and the batch experiments were carried out. When denitrification proceeded sufficiently, N₂O formation in the soil column packed with elemental sulfur was kept low. In the case of adding CaCO₃ for pH adjustment and adding elemental sulfur or iron sulfide, N₂O production was suppressed. This indicated the possibility to decrease N₂O emission by adding sulfur into the soil and proceeding sulfur denitrification.