An Innovative System for Optimising Nitrogen Fertiliser Applications to Sugarcane Crops.

Nitrogen fertiliser management is increasingly important for sustainable sugarcane production. We tested a new concept for nitrogen management in sugarcane, the N Replacement system. This system aligns nitrogen applications with long-term, actual cane production (as opposed to potential production), relying on soil nitrogen cycling to ‘buffer’ differences in crop N needs and N fertiliser supply to individual crops. We conducted 11 experiments, each over two to five years, in a wide range of environments in Australia to determine yields (cane and sugar) and nitrogen surpluses in the N Replacement system relative to farmers’ conventional management. Further, at one site, nitrate lost in runoff was measured for three years. Average yields in the N Replacement treatment were similar to those under farmers’ conventional nitrogen management, with a trend through time for yields to increase relative to conventional management. After four crops, this trend resulted in cumulative sugar yields being significantly higher in the N Replacement treatment. Average nitrogen applications and nitrogen surpluses were reduced by approximately 35% and 50%, respectively, in the N Replacement treatment. Our results showed that, in individual crops, the difference between crop N needs and N fertiliser supply could be up to 30% without loss of yield, supporting the concept of aligning nitrogen applications with long-term actual production, not potential production. At the site where runoff was measured, both nitrate-N lost in runoff and the nitrogen surplus was ~60% lower with N Replacement over three years. Our results show that the N Replacement system has promise for optimising nitrogen fertiliser applications to sugarcane crops, reducing environmental losses and maintaining production. Further evaluation of the system is needed, and there is scope for determining more site-specific values of parameters in the system.