The nitrogen cycle is the constant movement of nitrogen throught the biosphere and atmosphere. Atmospheric nitrogen is mostly contained as nitrogen gas (N2). The process by which autotrophic plants obtain this nitrogen is through fixation and assimilation. Nitrogen fixation is the process by which nitrogen gas is converted to solid chemicals like ammonia and various nitrates. This is mostly done through soil bacteria. Some of these nitrogen fixing bacteria like Rhizobium live directly on the roots of some plants. Assimilation is the process where plants absorb these nitrate and ammonium ions from the soil through their root systems. The plants then use this nitrogen to make biomolecules like amino acids and nucleic acids.