limitation imposed by section 26(a) for any taxable year, reduced by (B) the sum of the credits allowable under subpart A (other than this section) for the taxable year, such excess shall be carried to the succeeding taxable year and added to the credit allowable under subsection (a) for such succeeding taxable year. (2) Limitation No credit may be carried forward under this subsection to any taxable year following the tenth taxable year after the taxable year in which the credit arose. For purposes of the preceding sentence, credits shall be treated as used on a first-in first-out basis. . (b) Continued use not inconsistent with conservation purposes A contribution of an interest in real property shall not fail to be treated as a National Scenic Trail conservation contribution (as defined in section 30E(b) of the Internal Revenue Code of 1986) solely by reason of continued use of the real property, such as for recreational or agricultural use (including motor vehicle use related thereto), if, under the circumstances, such use does not impair significant conservation interests and is not inconsistent with the purposes of the National Trails System Act ( 16 U.S.C. 1241 et seq.). (c) Study regarding efficacy of National Scenic Trail conservation credit (1) In general The Secretary of the Interior shall, in consultation with the Secretary of the Treasury, study— (A) the efficacy of the National Scenic Trail conservation credit under section 30E of the Internal Revenue Code of 1986 in completing, extending, and increasing the number of National Scenic Trails (as defined in section 30E(b) of such Code), and (B) the feasibility and estimated costs and benefits of— (i) making such credit refundable (in whole or in part), and (ii) allowing transfer of such credit. (2) Report Not later than 4 years after the date of the enactment of this Act, the Secretary of the Interior shall submit a report to Congress on the results of the study conducted under this subsection. (d) Conforming amendment The table of sections for