Understanding How Much Nitrogen Used By Pasture Comes from Soil or Applied Fertiliser to Improve the Long-Term Sustainability of Australian Dryland Dairy Pasture Systems.

The efficiency of fertiliser nitrogen (N) applied into pasture systems is often based on short term biomass response to fertiliser (e.g. every 28 days). However, pastures will utilise both soil N and fertiliser N, and applied N could be immobilised and re-mineralised in the soil for subsequent uptake. Understanding how much soil N is available will inform better N management. A ¹⁵N-urea microplot experiment was conducted as part of a field trial on a rainfed dairy pasture on a Tenosol in south-eastern South Australia from May to October 2014 (growing season) to determine i) how much soil and fertiliser N was utilised by the pasture, and 2) how long the applied N could provide a productivity benefit to the pasture. ¹⁵N-urea was applied once (May) at 3 rates (50, 67 and 84 kg N/ha) per with unlabelled urea applied in 4 subsequent fertilisation events. More than 50% of the N taken up by the pasture by the first harvest was derived from soil, with fertiliser derived nitrogen being 38, 41 and 46% for the 50, 67 and 84 kg N/ha application rates respectively. Over the growing season a total of 28-33% of the ¹⁵N was recovered in the pasture, with greater recovery at the lower N rate and first 2 harvests. At the end of the growing season 13-15% of the ¹⁵N remained in the soil (11-13%) and roots (2%) with the majority recovered in the top 5 cm of soil (8-10%). More than 52-59% of the applied N was lost, presumably from ammonia volatilisation and denitrification. The results show that N remains available in the soil over time, and it is possible that this could be better utilised by manipulating N inputs.