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

Abstract

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.

Nitrate and Nitrous Oxide Dynamics Under Urine Application in an Alluvial Gravel Vadose Zone

“…Overall, N 2 O fluxes were greatest in the soil and subsoil soon after urine application. This is consistent with Thomas et al (2012) who reported that, based on N 2 O flux estimates and NO 3 –N/Br ratios, almost all of the net N 2 O production occurred in the subsoil above gravel material (depth of 1 m). This indicates that in these environments, once N from urine is leached below the subsoil in the gravel matrix, net N 2 O production in the vadose zone is small and that there will be little attenuation of leached NO 3 before it is transported to groundwater.…”

“…The declining NO 3 –N/Br ratios in the soil zone (0.6 and 1.0 m) during the winter spring period (July 2007–October 2007) indicated that some denitrification may have occurred (Fig. 3 and Table 1) as reported in similar conditions by Thomas et al (2012), though the changing ratios may have been a result of differential N uptake in the root zone followed by a leached pulse with different ratios. Within the underlying alluvial gravel vadose zone, there was no evidence that NO 3 –N to Br ratios were changing due to denitrification.…”

“…After irrigation in October 2007 there was a significant drop in NO 3 –N to Br ratios in the soil zone (0.2‐ and 1‐m samplers) suggesting denitrification was occurring at these depths. We are not aware of any evidence for preferential uptake of either NO 3 or Br by ryegrass (Thomas et al, 2012). This period coincided with warmer early summer air temperatures and increased soil water contents from the onset of irrigation.…”

Mechanisms leading to enhanced soil nitrous oxide fluxes induced by freeze–thaw cycles

“…In other cases, much of the N 2 O trapped at depth may be removed as melting water infiltrates the subsurface and recharges the groundwater. This N 2 O loss mechanism is poorly studied and rarely quantified (Outram and Hiscock 2012;Thomas et al 2012).…”