Is zakat due on an apartment purchased for investment purposes (price appreciation and rental income), or only on its rental income?

If you intend to sell the land when its price rises, then it is considered a trade asset, and its value is assessed every year (hawl), and a quarter of a tenth of its value is to be paid. The income from it is subject to zakat (definition) if it reaches the nisab (definition) by itself or when combined with other money, and a year passes over it, and a quarter of a tenth is to be paid from it. If you have old money that has reached the nisab, then you pay zakat on the income along with it when the hawl of the old nisab is completed, as an advance payment of zakat. Alternatively, you can set an independent hawl for the income and pay zakat on it when its hawl is completed, even if it does not reach the nisab by itself, provided it reached the nisab when combined with the initial money.