Subsoil denitrification is a potential sink for leached nitrate (NOj") that may otherwise contaminate ground water. A field trial was undertaken to assess the importance of subsoil denitrification and to examine the role of leached NOj" on subsurface nitrous oxide (N2O) (a potent greenhouse gas) dynamics. We monitored NOj", Br", N2O, CO2, and O2 concentrations in alluvial vadose materials to a depth of 7 m over a 32-mo period following the application of N (400 kg ha~^ as NH4NO3) and Br~ to potatoes {Solanum tuberosum L.) followed by a second application (400 kg ha"^ as NH4NO3) to ryegrass (Lolium multiflorum Lam.) 24 mo later. Our sampling system consisted of an array of ceramic cups and permeable silicone tubing chambers to sample soil solution and gases. Following rainfall and irrigation, subsoil N2O concentrations increased rapidly. Within days of NO3" leaching below 1 m, high concentrations of NO3~, Br~, and N2O were observed at 7-m depth. Based on N to Br" ratios, and NO3~ leaching estimates from drainage amounts and leachate NO3~ concentrations, 5 to 10% of the fertilizer and soil N was denitrified. Based on N2O flux estimates and NO3~-N/Br~ ratios, almost all of the net N2O production occurred in the subsoil above the gravel material (1-m depth). In the gravel matrix the NOj'-N/Br" ratio did not change indicating a low capacity to attenuate NO3".Abbreviations: ECD, electron capture detector; NP, neutron probe; STP, standard temperature and pressure; TDR, time domain reflectrometry; WFPS, water-filled pore space.

Understanding nitrogen (N) processes within the vadose zone is important to es mate N losses to groundwater systems. A fi eld trial was undertaken to examine the dynamics of nitrate (NO 3 ) and nitrous oxide (N 2 O) in an alluvial gravel vadose zone underlain by shallow groundwater. Synthe c urine (980 kg N ha −1 ) with a bromide (400 kg Br ha −1 ) tracer was applied to the surface of a 10-by 20-m plot, and changes in subsurface NO 3 , Br, and N 2 O concentra ons were compared with those from an adjacent plot, with just the Br tracer applied. Soil solu on and air were monitored at mul ple depths (from 0.2 to 5 m) over an 18-mo period. Transport of solu on was rapid in the gravel material with some Br transported to 3-m depth immediately a er the urine applica on. N 2 O was produced within the soil a er urine applica on. NO 3 to Br ra os indicated denitrifi ca on in the soil above the gravels, but none within the alluvial gravel vadose zone. In the "-urine" plot N 2 O concentra ons increased with depth, with upward fl uxes above the water table to the soil, sugges ng N 2 O produc on near the water table. A diff erent pa ern was observed in the "+urine" plot where N 2 O was produced both at the soil zone above the gravels and near the water table leading to both upward and downward N 2 O fl uxes. Overall N 2 O fl uxes in the soil zone were greater than at the water table.Abbrevia ons: DNR, denitrifi ca on and nitrate ammonifi ca on; ECD, electron capture detector; TDR, me-domain refl ectometry.