pastureland vs grassland

pastureland and grassland both are nouns.

Word	Noun	Adjective	Verb	Adverb
pastureland	Yes	No	No	No
grassland	Yes	No	No	No

As nouns, grassland is a hypernym of pastureland; that is, grassland is a word with a broader meaning than pastureland:

pastureland: a field covered with grass or herbage and suitable for grazing by livestock
grassland: land where grass or grasslike vegetation grows and is the dominant form of plant life

pastureland (noun)	grassland (noun)
a field covered with grass or herbage and suitable for grazing by livestock	land where grass or grasslike vegetation grows and is the dominant form of plant life