N2O and N2 gas fluxes, soil gas pressures, and ebullition events following irrigation of 15NO3–-labelled subsoils

Abstract

We examined the fate of N₂O following the addition of labelled nitrate and subsequent irrigation. Repacked silt loam soil columns, 1 m deep, were wetted up and instrumented with pressure transducers, soil profile gas samplers, and time domain reflectometry rods. Combined substrates (glucose- and ¹⁵N-enriched nitrate) were injected at 0.45 m depth. N₂O, N₂, and NO were monitored in the soil profile and headspaces. When soil profile N₂O gas concentrations became elevated, an irrigation event was applied. Immediately prior to the irrigation event, confined drainage (no drainage outlet) and unconfined drainage (lateral drainage outlet at 0.9 m depth) treatments were implemented. Soil profile gas pressures increased following irrigation with pressure changes at 0.375 m chronologically linked to increased pressure pulses in the headspace. Irrigation contributed to decreases in N₂O gas concentrations in the soil profile. N₂O and N₂ displaced in drainage from the unconfined treatment represented 0.01 and 2.3% of the gas in the soil profile immediately prior to irrigation, respectively. Following irrigation, soil gas pressures increased to a maximum of 11.8 kPa at 0.825 m soil depth in the confined drainage treatment but only reached 4.3 kPa at the same depth in the unconfined drainage treatment. It is suggested that ebullition events could possibly contribute to the increased and variable fluxes of N₂O, commonly observed, immediately following rainfall or irrigation.