Assessment of N<sub>2</sub> fixation by understorey acacias in recurrently burnt eucalypt forests of subtropical Australia using <sup>15</sup>N isotope dilution techniques

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The N2 fixation by understorey acacias in repeatedly burnt dry and wet sclerophyll forest sites in subtropical Australia was assessed using 15N isotope dilution techniques. In the first study, aboveground parts of established acacias and nonlegume reference plants at the two sites were collected from plots subjected to varying fire frequencies for measurements of 15N natural abundance ( 15N). The 15N values of acacias and nonlegumes were similar, and substantial variation in these 15N values precluded any evaluation of N2 fixation. In the second study, 15N enrichment method was used to examine N2 fixation of acacia seedlings grown for 5 months in glasshouse pots of top 10-cm soils collected from the fire plots at the two sites. In Acacia leiocalyx (Domin) Pedley at the dry site, whole plant per...