Biological nitrogen fixation by two Acacia species and associated root-nodule bacteria in a suburban Australian forest subjected to prescribed burning

Purpose: Prescribed burning is a forest management practice which can lead to nitrogen (N)-limited conditions. This study aimed to explore whether biological N fixation (BNF) remained the main source of N acquisition for two understorey Acacia species in a Eucalyptus-dominated suburban forest of subtropical Australia, 3 to 6 years after prescribed burning. Root-nodule bacteria associated with these acacias were also characterised to unravel the differences in rhizobial communities between sites and species. Material and methods: Two sites, burned 3 and 6 years before sample collection, were selected within a dry subtropical forest of south-east Queensland, Australia. Leaves were collected from individuals of Acacia disparrima and A. leiocalyx at each site to determine leaf total carbon (C) and N content, C and N isotope composition (δ C and δ N) and the percentage of N derived from atmospheric N . Nodules were harvested from both acacia species at each site to isolate root nodule bacteria. Bacterial isolates were processed for 16S rDNA gene sequencing. Results and discussion: Generally, no differences were found in plant physiological variables between the two acacia species. Six years after the fire, both species still depended upon BNF for their N supply, with a higher dependence in winter than in summer. Fire, although of low intensity, was likely to have created a N-limited environment which induced the reliance of legumes on BNF. Root nodule bacteria were dominated by non-rhizobial endophytes, mainly from the Firmicutes phylum. No difference in nodule bacterial diversity was found between sites. The relative abundance of rhizobial genera varied amongst plant species and sites, with a shift in dominance from Bradyrhizobium to Rhizobium species between sites 1 and 2. Conclusions: Our results show that even 6 years after burning, ecosystem remained under N stress and BNF was still the main mechanism for N acquisition by the understorey legumes. 2 2 13 15