It depends.
If the amount you received for the easement is greater than the basis of the portion of the property affected by the easement, you need to reduce your basis in that part of the property to zero, and then treat the excess amount as a recognized gain.
Otherwise, it just reduces your basis.
According to Easements in the IRS' Publication 225, Farmer's Tax Guide:
The amount you receive for granting an easement is usually considered to be proceeds from the sale of an interest in the real property. It reduces the basis of the affected part of the property. If the amount received is more than the basis of the part of the property affected by the easement, reduce your basis in that part to zero and treat the excess as a recognized gain.
