Diversity of Diazotrophic Bacteria and Non-Symbiotic N2-Fixation in Australian Cropping Soils.

Non-symbiotic (NS) nitrogen fixation by diazotrophic bacteria is an important N input and is highly desirable for the economic and environmental sustainability of nutrient limited agricultural systems in Australia. Little is known about the genetic diversity of diazotrophs and their total abundance in Australian cropping soils. Populations of diazotrophic bacteria were quantified using qPCR and their genetic diversity assessed by targeted nifH gene sequencing using surface soils from cropping regions of southern, western and eastern Australia. A total of 225,000 sequences were clustered at 5% amino acid dissimilarity resulting in 40197 OTUs representing 208 unique closest similarity taxa. Alpha- and beta-proteobacteria were the dominant members (28-63% abundance). OTUs with the highest similarity to Verrucomicrobiae, Bacillales and Clostrida accounted for up to 5% each.

Estimates of NS N₂ fixation, measured using a laboratory based microcosm (¹⁵N isotope) method, ranged from <0.15 to 2.3 mg N fixed/kg soil/day. N fixation was influenced by soil type, time of sampling (in crop vs. non-crop period), crop and mineral nitrogen levels. Non-symbiotic N₂ fixation was not always related to diazotroph diversity but rather related to the relative abundance of specific members and soil characteristics. NifH diversity and N fixation were highest in the clay soils of eastern Australia compared to the sandy and loam soils in southern and western Australia. Overall, these results suggest that soil type and climate based management solutions are needed to gain maximum benefits from NS N₂ fixation.