Codenitrification: An Important Processes Controlling Nitrogen Cycling in Grazed Grasslands.

Pasture-based livestock systems are associated with considerable losses of reactive forms of nitrogen (N) to the environment such as nitrate leaching, ammonia and nitrous oxide (N₂O) emissions. Research has focused on losses to air and water due to the health, economic and environmental impacts of reactive N.  Di-nitrogen (N₂) emissions are still poorly characterized, both in terms of the processes involved and their magnitude, due to methodological constraints.  Relatively few studies have focused on quantifying N₂ losses in vivo and fewer still have examined the relative contribution of the different N₂ emission pathways, particularly in grazed pastures.  Cow urine was amended with 98 atom% ¹⁵N-labelled urea resulting in a urine N concentration of 10 g N L^-1 and a ¹⁵N enrichment of 45 atom% excess. Two litres of urine was applied to replicated monolith lysimeters at a rate of 100 g N m^-2 and N₂ and N₂O emissions were measured over 123 days using the static chamber technique_. Headspace N₂ and N₂O samples were analyzed for ¹⁵N by isotope ratio mass spectrometry in the UC Davis Stable Isotope Facility. Contributions of true denitrification and co-denitrification to N₂ emissions were calculated using the ¹⁵N flux method.  The study found that N₂ emissions accounted for 95% of gaseous N loss, with 55.8 g N m^-2 emitted as N₂ by the process of co-denitrification, compared to only 1.1 g N m^-2 from true denitrification.  This study highlights the importance of co-denitrification in contributing to N dynamics in grazed grasslands which were dominated by N₂ emissions.