Kharaj

"A comparison between pre-Islamic documents and those of the Islamic period reveals that conquering Arabs increased the land taxation without exception. Thus, raising taxes of each acre of wheat field to 4 dirhams and each acre of barley field to 2 dirhams, whereas during reign of Khosro Anushiravan it used to be a single dirham for each acre of a wheat or barley field. During the later stage of Umayyad Caliphate, conquered and subjugated Persians were paying from one fourth to one third of their land produce to the Arab Empire as kharaj."