( ( Nitrogen fixation ) ( ( is ( ( the conversion ) ( of ( ( atmospheric ( nitrogen gas ) ) ( into ( organic ammonium ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Nitrogen) (NNP fixation)) (VP (VBZ is) (NP (NP (DT the) (NN conversion)) (PP (IN of) (NP (NP (JJ atmospheric) (NN nitrogen) (NN gas)) (PP (IN into) (NP (JJ organic) (NN ammonium))))))) (. .)))
Nitrogen fixation is the conversion of atmospheric nitrogen gas into organic ammonium.
(ROOT (S (NP (NN Nitrogen) (NN gas)) (VP (VBZ comprises) (NP (NP (QP (RB about) (NNS three-fourths))) (PP (IN of) (NP (NP (NN earth) (POS 's)) (NN atmosphere))))) (. .)))