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Nitrous oxide is a potent greenhouse gas (ca.200-to 300-fold-stronger than that of carbon dioxide) and will lead to long term globe warming and the depletion of stratospheric ozone layer.Agriculture soils are recognized a major source of nitrous oxide due to microbial activities on large N input.N2O production is generally primarily attributed to aerobic autotrophic nitrification and anaerobic heterotrophic denitrification.Soil for the laboratory microcosms was collected from a paddock that was used for grazing by sheep at the AgResearch Grasslands campus at Palmerston North, New Zealand.Soil microcosms with urea and water amendment plus 12CO2 were constructed to monitor the nitrogen transformations and measure the changes of ammonia-oxidizing organisms and denitrifiers.And next, DNA-based stable isotope probing (SIP) microcosms (microcosms above with 13CO2 instead of 12CO2) Were constructed to trace the active ammonia-oxidizing organisms which grow with Carbon source 13CO2 and Energy source from nitrification and might produce N2O as a byproduct of nitrification.