Groundwater-induced emissions of nitrous oxide through the soil surface and from subsurface drainage in an Andosol upland field: A monolith lysimeter study

Nitrous oxide (N₂O) produced in shallow groundwater has two emission pathways to the atmosphere: dissolution in subsurface drainage and groundwater and later degassing from water surfaces open to the atmosphere, and upward gas diffusion. N₂O undergoing upward diffusion through the soil surface cannot usually be distinguished from N₂O produced in the topsoil. To evaluate the emission pathway and rate of groundwater-induced N₂O, we conducted a one-year experiment using monolith lysimeters containing 1 m-long undisturbed Andosol. We measured emission of N₂O via the soil surface and dissolved N₂O emitted via subsurface drainage from the non-planted lysimeters under two conditions without fertilizer-nitrogen (N) addition: (1) with the groundwater table at 0.9 m depth (GW), and (2) without any groundwater table (nonGW). Total soil surface N₂O emissions in the GW and nonGW treatments were 21.0 ± 6.3 and 17.0 ± 1.1 mg N m^-2yr^-1, respectively (mean ± standard error, n = 3), and the difference between the two treatments was not significant. Total dissolved N₂O emissions via drainage in the GW and nonGW treatments were 11.40 ± 5.68 and 0.42 ± 0.03 mg N m^-2yr^-1, respectively. The presence of groundwater significantly increased dissolved N₂O emission under zero fertilizer-N addition. This is due to the one to three orders of magnitude higher concentration of dissolved N₂O in the GW treatment. Our results indicate that the presence of groundwater increases total N₂O emissions from an Andosol upland field via these two pathways.