Spatial distribution characteristics of dissolved N₂O in unconfined groundwater of an agricultural catchment affected by significant fertilizer application

Abstract

Spatial distribution characteristics of nitrous oxide (N₂O) in an unconfined aquifer were examined, based on changes in concentrations of N₂O, nitrate-nitrogen (NO₃ ^--N) and other chemical components with the groundwater flow of an agricultural catchment affected by significant fertilizer application. N₂O concentrations were about 4μgN L^-1 in the upstream area, and were positively correlated with NO₃ ^--N concentrations. These results suggest that the nitrification process influences N₂O concentrations in upstream areas. In the downstream area, N₂O concentrations in deeper groundwater (> 15 m) were significantly higher (13 μgN L^-1), but they were very low in shallower groundwater (< 15 m), below the detection limit. Spatial distributions of dissolved oxygen (DO) and dissolved organic carbon (DOC) suggest that shallower groundwater is characterized by a strong reducing condition and high organic carbon content compared to deeper groundwater. These results suggest that a complete denitrification process (NO₃ ^-→N₂) occurs in shallower groundwater, whereas an incomplete denitrification process causes the increase of N₂O concentrations found in deeper groundwater.