Can 15N- isotopic methods be used to estimate plant associated nitrogen fixation in hybrid perennial sorghum?

Matthew T. Newell1, Timothy E. Crews2 and Richard C. Hayes3

1 NSW Department of Primary Industries, Cowra Research and Advisory Station, Binni Creek Rd, Cowra, NSW 2795,
2 The Land Institute, 2440 E. Water Well Rd., Salina, KS 67401, United States of America,
3 NSW Department of Primary Industries, Wagga Wagga Agricultural Institute, PMB, Wagga Wagga, NSW 2650


A redesign of current cropping systems that incorporates grain production from perennial species has been proposed as a method of sustainable staple grain production. Perennial sorghum is one such species under development as a grain crop, which has been suspected of hosting endophytic nitrogen fixing bacteria in its stems or crowns. A pilot study using isotope dilution methodology was used to measure the percent nitrogen derived from the atmosphere (%Ndfa) in perennial sorghum. Plants were grown in three metre columns over 90 days set in the field. Three treatments providing high (HN), low (LN) and zero (0N) N applied as urea were compared, with each treatment consisting of a perennial sorghum plant grown in the same column with Lolium rigidum and L.perenne. Additional nitrogen from urea significantly (P< 0.05) increased total shoot and root dry matter, compared to the 0N treatment. Interestingly, perennial sorghum plants in the 0N treatment had significantly (P< 0.05) longer roots and a higher root:shoot ratio (P< 0.05) compared to both additional N treatments. This suggests that N scarcity stimulated increased root allocation. Perennial sorghum showed no evidence of N2 fixation under the HN and LN treatments at all sampling dates. However, over time in the 0N treatment there was a trend for increasing levels of biologically sourced N with an average of 30 %Ndfa at the final sampling date, although calculations varied with the reference species used. The current study provides evidence of N2 fixation in perennial sorghum but highlights concerns about the adequacy of the reference plant δ15N values.


The Australian Society of Agronomy is the professional body for agronomists in Australia. It has approximately 500 active members drawn from government, universities, research organisations and the private sector.

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David Marland Photography Graham Centre for Agricultural Innovation, Charles Sturt University

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