There are several free living bacteria in soil that fix nitrogen including species of azotobacter, clostridium and klebsiella. Rhizobium only fixes nitrogen when in symbiotic association, in root nodules, with leguminous plants. The oxygen sensitive nitrogenase is protected by plant produced leghaemoglobin and the plant obtains fixed nitrogen from the bacteria.
See: frankia. The assimilation of atmospheric nitrogen by soil bacteria and its release for plant use on the death of the bacteria.A capability of some bacteria to harness nitrogen from the open air environment and use it to synthesises [[amino acids.