( ( Though ( nitrogen ( comprises ( ( ( about 78 ) % ) ( of ( ( the ( Earth 's ) ) atmosphere ) ) ) ) ) ) ( , ( ( ( plants and ) animals ) ( ( ( ( do n't ) necessarily ) ( have ( ( an ( easy time ) ) ( getting ( ( all ( the nitrogen ) ) ( they need ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN Though) (S (NP (NN nitrogen)) (VP (VBZ comprises) (NP (NP (QP (RB about) (CD 78)) (NN %)) (PP (IN of) (NP (NP (DT the) (NN Earth) (POS 's)) (NN atmosphere))))))) (, ,) (NP (NNS plants) (CC and) (NNS animals)) (VP (VBP do) (RB n't) (ADVP (RB necessarily)) (VP (VB have) (NP (NP (DT an) (JJ easy) (NN time)) (VP (VBG getting) (NP (NP (PDT all) (DT the) (NN nitrogen)) (SBAR (S (NP (PRP they)) (VP (VBP need))))))))) (. .)))
Though nitrogen comprises about 78% of the Earth's atmosphere, plants and animals don't necessarily have an easy time getting all the nitrogen they need.
(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))))) (. .)))