N natural abundance studies in Australian commercial sugarcane

Abstract

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 NO3- and NH4+ 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.