The Australian sugar cane industry is under increasing pressure from global competitors and environmental groups to achieve a more economical and sustainable production system. Sugar cane production in Australia has a large N fertiliser input and has potentially serious downstream environmental consequences.
The use of N fertiliser in Australian sugar cane increased significantly over the last 50 years with more than 100 000 tonnes of N fertiliser now applied annually (Taylor and Brodie 2002). Fertiliser use efficiency of sugar cane has remained low with only about a third of plant N being derived from the current annual N fertiliser application (Vallis et al. 1996). The remaining fertiliser N is either added to the soil mineralisable N pool or lost from the production system through the processes of leaching, denitrification and to a lesser extent volatilisation. Large N fertiliser applications and low N fertiliser use efficiency are a major constraint to achieving a more economical and environmentally sustainable sugar cane production system in Australia. Current research into N fertiliser use in Australian production systems has concentrated on tailoring N fertiliser recommendations to soil type and crop N requirements, placement and timing of N fertiliser applications and identifying alternative plant N sources.
This thesis addresses the potential of biological nitrogen fixation (BNF) by endophytic N2-fixing bacteria to provide sugar cane within the Australian production system with an alternative N source. The endophytic N2-fixing bacteria G. diazotrophicus, Burkholderia spp. and Herbaspirillum seropedicae and H. rubrisubalbicans have been isolated from sugar cane in Brazil where they have been attributed as the organisms responsible for contributing fixed N2 to sugar cane. Isotopic 15N dilution and N balance studies have shown positive N contributions to sugar cane from BNF. BNF in Brazilian sugar cane was variety dependent and reliant on adequate P, K and Mo nutrient additions and year round water conditions conducive to good plant growth. N2-fixation by endophytes has advantages over rhizosphere fixation systems in that the N2-fixing organisms are located within the plant where they are provided with a microsite that has potentially less competition for substrate than in the rhizosphere, and allows a more direct transfer of fixed N2 product to the host plant.
This thesis examined the key issues for the diazotroph sugar cane association in Australia:
- are natural populations of N2-fixing endophytes present within Australian sugar cane varieties;
- what is their distribution within the plant, and the effect of variety, season and cane growing region;
- are they influenced by N fertiliser and what BNF contribution can they make to Australian sugar cane.
Natural populations of the endophytic N2-fixing bacteria G. diazotrophicus, Burkholderia spp. and Herbaspirillum spp. were isolated from Australian sugar cane. These bacteria were identified by a range of biochemical, physiological and morphological traits when compared to type strains. Populations were found to be largest within the root and upper sugar cane stem regions. Root populations ranged from 102 to 106 cells g/d.w. and stem populations ranged from 102 to 105 cells g/d.w. G. diazotrophicus and Herbaspirillum spp. populations did not vary across growing season, Burkholderia spp. were larger prior to harvest than during the main vegetative crop growth phase. Populations of these diazotrophs varied between sugar cane varieties and across cane growing regions of Australia. There was limited variation in N2-fixing bacteria populations between two sugar cane varieties Q135 and Q138 within regions and considerable population variations across regions. Nitrogen fertiliser had a significant effect on N2-fixing bacteria populations. Annual applications of 150 kg N/ha or larger were found to significantly decrease populations however these rates had the largest sugar cane yields (cane and sugar t/ha). A 15N enriched pot trial conducted over two crop cycles showed positive BNF contributions to five Australian and two Brazilian sugar cane varieties. Although the pot culture system restricted sugar cane growth, BNF contributions of 5 to 16% were estimated. Brazilian sugar cane varieties SP70-1143 and SP71-1406 had 10 to 14% nitrogen derived from the air (Ndfa) while Australian sugar cane varieties had an estimated 5 to 16% Ndfa.
15N natural abundance measures within field grown sugar cane plants and associated reference plants indicated some N fixation was occurring, but the delta 15N values for both sugar cane and reference plants was so variable that the method was considered unreliable as a survey method. A planted field trial with carefully designed juxtaposition of selected reference and sugarcane plants was recommended as the only way to satisfactorily use this method.
This study measured large natural populations of endophytic N2-fixing bacteria within Australia sugar cane varieties across the cane growing regions of Queensland. These populations were strongly influenced by large N fertiliser additions. The demonstration of small, positive BNF contributions to Australian sugar cane highlights the potential of BNF and the importance of variety selection.
A better understanding of the conditions and requirements for endophytic N2-fixation will allow the system to be optimised. Quantification of the contribution of BNF to sugar cane in the field is required and can be achieved through careful design and planning of field or large lysimeter 15N enriched N balance and natural 15N abundance experiments. BNF by endophytic diazotrophs has potential to contribute to the N requirement of Australian sugar cane. Such a supply of an N via BNF to sugar cane has important economic and environmental benefits.