15N natural abundance studies in Australian commercial sugarcane

The measurement of natural N-15 abundance is a well-established technique for the identification and quantification of biological N-2 fixation in plants. Associative N-2 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 N-2 fixation, even though high populations of N-2 fixing bacteria have been isolated from Australian commercial sugarcane fields and plants. In this study, delta(15)N measurements were used as a primary measure to identify whether Australian commercial sugarcane was obtaining significant inputs of N via biological N-2 fixation. Quantification of N input, via biological N-2 fixation, was not possible since suitable non-N-2 fixing reference plants were not present in commercial cane fields. The survey of Australian commercially grown sugarcane crops showed the majority had positive leaf delta(15)N values (73% >3.00parts per thousand, 63% of which were <5.00parts per thousand), which was not indicative of biological N-2 fixation being the major source of N for these crops. However, a small number of sites had low or negative leaf delta(15)N values. These crops had received high N fertiliser applications in the weeks prior to sampling. Two possible pathways that could result in low delta(15)N values for sugarcane leaves (other than N-2 fixation) are proposed; high external N concentrations and foliar uptake of volatilised NH3. The leaf delta(15)N value of sugarcane grown in aerated solution culture was shown to decrease by approximately 5parts per thousand with increasing external N concentration (0.5-8.0 mM), with both NO3- and NH4+ nitrogen forms. Foliar uptake of atmospheric NH3 has been shown to result in depleted leaf delta(15)N values in many plant species. Acid traps collected atmospheric N with negative delta(15)N value (-24.45+/-0.90parts per thousand) from above a field recently surface fertilised with urea. The delta(15)N of leaves of sugarcane plants either growing directly in the soil or isolated from soil in pots dropped by 3.00parts per thousand 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 delta(15)N values measured in the sugarcane crops at these sites.