Does biological N2-fixation contribute to nitrogen requirements in Australian sugarcane?

There have been reports of Nz-fixation supplying considerable amounts of nitrogen (N) to sugarcane in Brazil. To date, however, there has been no measure of the input of N into Australian sugarcane via biological nitrogen fixation (BNF). This study aims to identify Nz-fixing sugarcane cultivars and investigate the input of N, via BNF, into commercial sugarcane cultivars grown in Australia using nitrogen isotopic techniques. The leaf 615N of sugarcane was sampled for a range of cultivars at eight commercial sugarcane sites in Queensland and New South Wales, and the potential contribution from BNF calculated. At seven of the sites sampled the leaf 615N values of the sugarcane was greater than 4%0, a 615N value range not indicative of substantial BNF inputs. The remaining site (Broadwater) however, had 615N values less than 4%0. At four of the sites, reference plants were sampled allowing for the calculation of relative input of N from BNF (%NBNF) for the sugarcane at these sites. For two sites, the calculated %NBNF showed no N input from BNF, while the remaining two sites (Harwood and Broadwater) showed moderate N inputs (13.1 to 44.6%). At one of these sites (Harwood), this result is seen to be an artifact of the technique. The encouraging Broadwater result appears to be site specific. The same sugarcane cultivars sampled from other sites had higher 615N values, and calculations of %NBNF did not support BNF as an N source at these sites. The Broadwater site had unique soil conditions, which may have either resulted in lower leaf 615N values or have been favourable for Nz-fixation in the sugarcane at this site. There is no strong evidence from this study indicating BNF to be a major source of N for the sugarcane sampled. However, there appears to be variation in both the 615N value and the calculated %NBNF between sites and cultivars. Further investigation is required to fully utilise the 615N technique and to see if under certain conditions sugarcane may derive substantial N from BNF sources.