Nitrogen cycling in summer active perennial grass systems in South Australia: non-symbiotic nitrogen fixation

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Non-symbiotic nitrogen (N2) fixation by diazotrophic bacteria is a potential source for biological N inputs in non-leguminous crops and pastures. Perennial grasses generally add larger quantities of above- and belowground plant residues to soil, and so can support higher levels of soil biological activity than annual crops. In this study, the hypothesis is tested that summer-active perennial grasses can provide suitable microsites with the required carbon supply for N2 fixation by diazotrophs, in particular during summer, through their rhizosphere contribution. In a field experiment on a Calcarosol at Karoonda, South Australia, during summer 2011, we measured populations of N2-fixing bacteria by nifH-PCR quantification and the amount of 15N2 fixed in the rhizosphere and roots of summer-active perennial grasses. Diazotrophic N2 fixation estimates for the grass roots ranged between 0.92 and 2.35 mg 15N kg–1 root day–1. Potential rates of N2 fixation for the rhizosphere soils were 0.84–1.4 mg 15N kg–1 soil day–1 whereas the amount of N2 fixation in the bulk soil was 0.1–0.58 mg 15N kg–1 soil day–1. Populations of diazotrophic bacteria in the grass rhizosphere soils (2.45 × 106 nifH gene copies g–1 soil) were similar to populations in the roots (2.20 × 106 nifH gene copies g–1 roots) but the diversity of diazotrophic bacteria was significantly higher in the rhizosphere than the roots. Different grass species promoted the abundance of specific members of the nifH community, suggesting a plant-based selection from the rhizosphere microbial community. The results show that rhizosphere and root environments of summer-active perennial grasses support significant amounts of non-symbiotic N2 fixation during summer compared with cropping soils, thus contributing to biological N inputs into the soil N cycle. Some pasture species also maintained N2 fixation in October (spring), when the grasses were dormant, similar to that found in soils under a cereal crop. Surface soils in the rainfed cropping regions of southern Australia are generally low in soil organic matter and thus have lower N-supply capacity. The greater volume of rhizosphere soil under perennial grasses and carbon inputs belowground can potentially change the balance between N immobilisation and mineralisation processes in the surface soils in favour of immobilisation, which in turn contributes to reduced N losses from leaching.