Is zakat obligatory on a large plot of land owned by an individual as a guarantee for the future, selling a portion of it when needed, while keeping the rest for his children, knowing that its value is one million dollars and his net annual income is three thousand dollars, and if it is obligatory, how is it paid annually?

There is no zakat (definition) on land if it is for saving or building, but only if it is for trade. Net annual income is subject to zakat if it reaches the nisab (definition) and a hawl (lunar year) has passed on it. To facilitate this, zakat can be paid on all existing money when the first hawl is due, even if a hawl has not passed on some of it, and this would be considered an advance zakat payment.