Managing Global Resources for a Secure Future

Ruminant livestock urinary nitrogen (N) deposition on to pasture soil leads to significant soil nitrous oxide (N2O) emissions. N2O is a potent greenhouse gas and ozone depleting substance. The degradation pathway for urea, the dominant N form in urine, is well known. However, the fate of non-urea urine N compounds (NUNCs) in the urine patch has not been directly studied. The objective of our study was to track the degradation of 15N labelled NUNCs in pasture soils. We hypothesized that NUNCs would degrade within days of application and result in elevated soil inorganic N or N2O emissions. Previous research indicates NUNCs degrade in two linear cascades: creatine degrades to creatinine; while hypoxanthine degrades to allantoin with both xanthine and uric acid sequential intermediaries. Using 15N labelled precursor NUNCs, creatine and hypoxanthine, allows for sequential compounds to be expressed and the full potential contribution of creatine and hypoxanthine to N2O or inorganic-N pools. Vials containing soil from a perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) pasture were seeded with perennial ryegrass. The 15N-labelled NUNCs, creatine and hypoxanthine, were added at a rate of approximately 100 ug N g-1 dry soil (5 atm %) and sampled every 12-24 hours for 102 hours. Samples included N2O flux, soil inorganic N (NH4+ and NO3-), and plant N content. The experiment has recently been completed and analysis is in progress. The results will be presented at the conference.