From what I'm reading, since this is a lifetime easement you'll treat it as a sale and report it as such on SCH D. Apparently, it doesn't matter that it doesn't reduce the amount of taxable land you own. But it does reduce your cost basis in the property overall, "as if" you outright sold the property. For the simplest and best write-up I can find that explains this, see https://www.bergankdv.com/resources/blog/proceeds-from-an-easement-or-right-of-way-2/
So basically, you have to figure that portion of land the money was paid for. Then figure what percentage of your original purchase price of the property applies to that portion of land the perpetual easement payment was for. That will be your cost basis. If the money you were paid exceeds that cost basis (I'm sure it will) then you have a taxable gain. You'll only pay tax on the gain.
