N natural abundance studies in Australian commercial sugarcane

The measurement of natural ¹⁵N abundance is a well-established technique for the identification and quantification of biological N₂ fixation in plants. Associative N₂ fixing bacteria have been isolated from sugarcane and reported to contribute potentially significant amounts of N to plant growth and development. It has not been established whether Australian commercial sugarcane receives significant input from biological N₂ fixation, even though high populations of N₂ fixing bacteria have been isolated from Australian commercial sugarcane fields and plants. In this study, δ¹⁵N measurements were used as a primary measure to identify whether Australian commercial sugarcane was obtaining significant inputs of N via biological N₂ fixation. Quantification of N input, via biological N₂ fixation, was not possible since suitable non-N₂ fixing reference plants were not present in commercial cane fields. The survey of Australian commercially grown sugarcane crops showed the majority had positive leaf δ¹⁵N values (73% >3.00‰, 63% of which were >5.00‰), which was not indicative of biological N₂ fixation being the major source of N for these crops. However, a small number of sites had low or negative leaf δ¹⁵N values. These crops had received high N fertiliser applications in the weeks prior to sampling. Two possible pathways that could result in low δ¹⁵N values for sugarcane leaves (other than N₂ fixation) are proposed; high external N concentrations and foliar uptake of volatilised NH₃. The leaf δ¹⁵N value of sugarcane grown in aerated solution culture was shown to decrease by approximately 5‰ with increasing external N concentration (0.5-8.0 mM), with both $NO_3^ - $ and $NH_4^ + $ nitrogen forms. Foliar uptake of atmospheric NH₃ has been shown to result in depleted leaf δ¹⁵N values in many plant species. Acid traps collected atmospheric N with negative δ¹⁵N value (-24.45±0.90‰) from above a field recently surface fertilised with urea. The δ¹⁵N of leaves of sugarcane plants either growing directly in the soil or isolated from soil in pots dropped by 3.00‰ in the same field after the fertiliser application. Both the high concentration of external N in the root zone (following the application of N-fertilisers) and/or subsequent foliar uptake of volatilised NH₃ could have caused the depleted leaf δ¹⁵N values measured in the sugarcane crops at these sites.