An improved way to determine nitrogen fertiliser requirements of sugarcane crops to meet global environmental challenges

Nitrogen (N) fertiliser management is increasingly important in sugarcane production as imperatives to reduce environmental impacts of N escalate. In this paper we report testing of a new concept for N management in sugarcane, the N Replacement system. This system relies on soil N cycling to 'buffer' differences in crop N needs and N fertiliser supply to individual crops, and aligns N applications with actual cane production over the longer-term rather than potential production. In 11 experiments, conducted in a wide range of environments over two to five crops, cane and sugar yields in the N Replacement treatment were similar to those achieved with the farmers' conventional N management, with a trend over successive crops for yields to increase relative to conventional management. At sites where experiments ran for at least 4 years, this trend resulted in cumulative sugar yields being higher in the N Replacement treatment. Average N applications were 35% lower in the N Replacement treatment, and N lost to the environment was estimated to be similar to 50% lower. Soil N 'buffering' was adequate to maintain sufficient N supply to crops even when yields were up to 30% greater than expected. Thus, it is not necessary to align fertiliser applications to potential sugarcane yields, which are rarely achieved in practice. Our results show that the ecologically-based N Replacement system has promise to deliver superior environmental outcomes without significantly reducing production of sugarcane, and potentially other semi-perennial crops, in the tropics and subtropics. Further evaluation of the system will be beneficial, and there is scope for determining more site-specific values of parameters in the system. However, care must be taken to evaluate the system over sufficient time frames (e.g. > 2 crops) so that productivity improvement trends in the N Replacement system can be expressed.