Biological nitrogen fixation is the process that changes inert N 2 into biologically useful NH 3. This process is mediated in nature only by N-fixing rhizobia bacteria (Rhizobiaceae, α- Proteobacteria) (Soerensen sørensen And, sessitsch). 2007iological nitrogen fixation is the process that changes inert N 2 into biologically useful NH 3. This process is mediated in nature only by N-fixing rhizobia bacteria (Rhizobiaceae, α- Proteobacteria) (Soerensen sørensen And, sessitsch). 2007