Assessment of N2 fixation by understorey acacias in recurrently burnt eucalypt forests of subtropical Australia using 15N isotope dilution techniques

Abstract

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 (δ15N). The δ15N values of acacias and nonlegumes were similar, and substantial variation in these δ15N 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 percent N derived from the atmosphere (%Ndfa) was 25% in unburnt soil, 66% in periodically burnt soil, and 42% in annually burnt soil (reference Alphitonia excelsa (Cunn. ex Fenzl) Reisseck ex Benth.), compared with 33, 69, and 66%, respectively (reference Corymbia variegata (Hook.) K.D. Hill and L.A S. Johnson). In Acacia oshanesii F. Muell and Maiden at the wet site, whole plant %Ndfa was 9% in unburnt soil, 41% in quadrennially burnt soil, and 50% in biennially burnt soil (reference Dodonaea triquetra Andr), compared with -4, 26, and 55%, respectively (reference Eucalyptus pilularis Sm).