grassland vs piece of land

grassland and piece of land both are nouns.

Word	Noun	Adjective	Verb	Adverb
grassland	Yes	No	No	No
piece of land	Yes	No	No	No

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

grassland: land where grass or grasslike vegetation grows and is the dominant form of plant life
piece of land: an extended area of land

Other hypernyms of grassland include biome, parcel, parcel of land, piece of ground, tract.

grassland (noun)	piece of land (noun)
land where grass or grasslike vegetation grows and is the dominant form of plant life	an extended area of land