Ruminant urine patch nitrification and N₂O flux: effects of urine aucubin rate in a laboratory trial

Livestock urine deposition onto grazed New Zealand pastures drives soil nitrous oxide (N₂O) emissions. Urine patch N₂O emissions can be reduced by inhibiting nitrification, a key urine patch nitrogen (N) transformation pathway. Aucubin, a secondary metabolite found in the pasture herb species plantain (Plantago lanceolata), is a potential nitrification inhibitor and could be excreted in urine by animals grazing plantain. In this laboratory trial, three rates of aucubin application (10, 50, and 100% of the highest theoretical aucubin excretion rate from cows grazing 100% plantain pasture) were added to urine and assessed for their potential to inhibit nitrification and subsequent N₂O emissions. Significantly reduced N₂O flux and soil ${NO}_3^{-}$ concentrations, aligned with significantly increased soil ${NH}_4^{+}$ and soil surface pH, indicated that aucubin inhibited nitrification from 5 to 17 days after urine application. However, this period of inhibition was not sufficient to reduce overall N₂O emissions during the 35 day experiment.