Nitrous Oxide Dynamics in a Deep Soil-Alluvial Gravel Vadose Zone Following Nitrate Leaching

Subsoil denitrification is a potential sink for leached nitrate (NO3-) 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 NO3- on subsurface nitrous oxide (N2O) (a potent greenhouse gas) dynamics. We monitored NO3-, Br-, N2O, CO2, and O-2 concentrations in alluvial vadose materials to a depth of 7 m over a 32-mo period following the application of N (400 kg ha(-1) as NH4NO3) and Br- to potatoes (Solanum tuberosum L.) followed by a second application (400 kg ha(-1) 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 NO3--N/Br- ratio did not change indicating a low capacity to attenuate NO3-.