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

Abstract

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 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⁻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–.