Influence of soil moisture on codenitrification fluxes from a urea-affected pasture soil

Intensively managed agricultural pastures contribute to N₂O and N₂ fluxes resulting in detrimental environmental outcomes and poor N use efficiency, respectively. Besides nitrification, nitrifier-denitrification and heterotrophic denitrification, alternative pathways such as codenitrification also contribute to emissions under ruminant urine-affected soil. However, information on codenitrification is sparse. The objectives of this experiment were to assess the effects of soil moisture and soil inorganic-N dynamics on the relative contributions of codenitrification and denitrification (heterotrophic denitrification) to the N₂O and N₂ fluxes under a simulated ruminant urine event. Repacked soil cores were treated with ¹⁵N enriched urea and maintained at near saturation (-1 kPa) or field capacity (-10 kPa). Soil inorganic-N, pH, dissolved organic carbon, N₂O and N₂ fluxes were measured over 63 days. Fluxes of N₂, attributable to codenitrification, were at a maximum when soil nitrite (NO₂^-) concentrations were elevated. Cumulative codenitrification was higher (P = 0.043) at -1 kPa. However, the ratio of codenitrification to denitrification did not differ significantly with soil moisture, 25.5 ± 15.8 and 12.9 ± 4.8% (stdev) at -1 and -10 kPa, respectively. Elevated soil NO₂^- concentrations are shown to contribute to codenitrification, particularly at -1 kPa.