Do we need to avoid nitrogen fertiliser applications to urine patches

This project aimed to quantify the additional nitrogen (N) lost by leaching when fertiliser N was applied on top of a urine patch. A lysimeter study previously suggested an additive effect of fertiliser and urine on N leaching at the urine patch scale at high rates of N fertiliser. Here, we report the use of a process-based model (APSIM) to extend the lysimeter results to a wider range of scenarios to estimate the benefit (reduction) to paddock-scale N leaching by avoiding N fertiliser on urine patches. Based on a Horotiu silt loam in the Waikato region of New Zealand, modelling suggested that avoiding fertiliser (nine x 25 kg N/ha/year) on top of urine patches would decrease N leaching from 58 to 49 kg N/ha/year or 16% of the annual total. However, when rerun without N fertiliser applications in summer (a more realistic scenario for the summer-dry Waikato region), modelled N leaching was 54 and 49 kg N/ha, i.e. a saving of 5 kg N/ha or 9% of the total from not fertilising urine patches. The critical period for fertiliser application was late summer-autumn when repeated applications had a relatively larger effect on N loss. These results need to be set in the context of a Waikato climate where, on average, moisture becomes limiting to pasture growth in summer. Further work is required to extend this analysis to different soil types and climates.