Spatial distribution characteristics of nitrous oxide (N
2O) in an unconfined aquifer were examined, based on changes in concentrations of N
2O, nitrate-nitrogen (NO
3 --N) and other chemical components with the groundwater flow of an agricultural catchment affected by significant fertilizer application. N
2O concentrations were about 4μgN L
-1 in the upstream area, and were positively correlated with NO
3 --N concentrations. These results suggest that the nitrification process influences N
2O concentrations in upstream areas. In the downstream area, N
2O 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 (NO
3 -→N
2) occurs in shallower groundwater, whereas an incomplete denitrification process causes the increase of N
2O concentrations found in deeper groundwater.
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