How is the Zakat calculated for an inherited agricultural land that has not reached the Nisab, to which a new plot of land has been added with the intention of selling it? Are the values of both lands combined to calculate the Nisab, or is the inherited land not included in the calculation?

Inherited land is not subject to zakat because it is not considered trade goods. Similarly, purchased land is not subject to zakat unless you are certain it is for trade. As long as you intend to sell it only when needed, this is considered hesitation in intention, and therefore, no zakat is due on it merely based on this intention. Rather, zakat becomes obligatory on its price after it is sold, provided it reaches the nisab (minimum threshold) and a full hawl (lunar year) passes over it. However, if you purchased it with the firm intention of selling it when its price increases, then it is considered trade goods, and zakat is obligatory on it.