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

Intensively managed agricultural pastures contribute to N(2)O and N(2) 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 N(2)O and N(2) fluxes under a simulated ruminant urine event. Repacked soil cores were treated with (15)N enriched urea and maintained at near saturation (−1 kPa) or field capacity (−10 kPa). Soil inorganic-N, pH, dissolved organic carbon, N(2)O and N(2) fluxes were measured over 63 days. Fluxes of N(2), attributable to codenitrification, were at a maximum when soil nitrite (NO(2)(−)) 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 NO(2)(−) concentrations are shown to contribute to codenitrification, particularly at −1 kPa.