No sugar yield gains but larger fertiliser 15N loss with increasing N rates in an intensive sugarcane system

High nitrogen (N) fertiliser inputs together with a low N use efficiency in intensive sugarcane systems indicate substantial losses of fertiliser N to the environment and therefore the need to reconsider N fertiliser rate management. However, the effect of N fertiliser rates on the fate of fertiliser N in plant and soil, and on N fertiliser loss is largely unknown. This study established the response of 15N fertiliser recoveries to N rates on a tropical commercial sugarcane farm in Australia on a Luvisol. The experiment over 10 months included three N fertiliser rates, 150, 200 (the recommended rate) and 250 kg N ha−1 applied as 15N-labelled urea (5 atom%) and a non-fertilised control. Fertiliser 15N uptake increased from 39 to 65 kg N ha−1 with increasing N rates, accounting for 26% of the N applied across N rates. Fertiliser 15N contributed 22–33% to total N uptake and increased soil-N uptake compared to the control. Fertiliser 15N in the soil down to 1.0 m at harvest was consistent across N rates (36–39 kg N ha−1), indicating a fixed capacity of the soil to immobilise fertiliser N. Increasing N rates did not contribute to increased sugar yield or retention of soil N but increased N fertiliser loss, ranging from 48 to 60% of the applied N. This study demonstrates that 50% of the applied N was lost from an intensive sugarcane system under current best management practices, highlighting the need to target these large environmental N losses in future recommendations.