( ( ( converting ammonia ) ( to ( nitrogen gas ) ) ) ( , ( which ( ( returns ( to ( the atmosphere ) ) ) . ) ) ) )
(ROOT (S (S (VP (VBG converting) (NP (NN ammonia)) (PP (TO to) (NP (NN nitrogen) (NN gas))))) (, ,) (NP (WDT which)) (VP (VBZ returns) (PP (TO to) (NP (DT the) (NN atmosphere)))) (. .)))
converting ammonia to nitrogen gas, which returns to the atmosphere.
(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))))) (. .)))