This study benchmarked N2O and CO2 emissions in heavily (>1 Nitrogen t annually) manured and fertilised sandy soils in temperate Australia and demonstrated the mitigation of N2O emissions using the nitrification inhibitor (NI) 3,4-dimethylpyrazole phosphate (DMPP). Three field trials conducted on celery crops from 2014 to 2016 showed that the traditional practice of adding composted chicken manure (4.5–8.0 t ha–1) to the surface of sandy soils produced large spikes in daily emissions of N2O (up to 1860 g ha–1 day–1) and that further spikes occurred with each subsequent fertiliser application. Pre-crop incorporation of manure (8–9 t ha–1) rather than surface application reduced the daily N2O-N emission flux from 1800 to <200 g ha–1 day–1. The use of DMPP, applied to both manures and fertilisers, reduced cumulative N2O emissions during the cropping period by 64–76%, with greater reductions occurring for surface-applied manures and fertilisers. DMPP-treated plots had higher levels of ammonium-N in soil. CO2 emissions were not affected by DMPP, indicating no negative effect on microbial activity. There was no difference in yields or product quality between the standard grower practice plots and plots treated with DMPP at the full or half rate, indicating overuse of N inputs and no negative effects of DMPP. Because N2O emissions were markedly reduced and yields unaffected by lower rates of NI-treated inputs, the use of NIs, particularly with manures, is an excellent cropping practice for these sandy soils to reduce the negative effect of excess N on the environment and to avoid the overuse of fertilisers and manures by growers. This study showed that the cumulative emissions of N2O for celery production over the cropping period using the standard grower practice translated to annual emissions of 27–68 kg N ha–1 year–1, among the highest recorded for Australian crops.