; and (C) such other factors as the Secretary determines to be appropriate. (2) Preference In selecting sample sites under paragraph (1), the Secretary shall give preference to sites that have been used for soil testing previously. (3) Number of sample sites The Secretary shall select sufficient sample sites under paragraph (1) to analyze changes in soil carbon across regional resource areas, as determined by the Secretary, over time, taking into account the geographical size and heterogeneity of each regional resource area. (d) Inventory (1) In general Every 5 years, the Secretary shall prepare an inventory of soil carbon stocks on eligible land, under which the Secretary shall collect measurements at each sample site selected under subsection (c). (2) Methodology The Secretary shall ensure that the methodology developed under section 3(a) of the Advancing Research on Agricultural Climate Impacts Act of 2023 is used for purposes of conducting measurements at each sample site under the program. (3) Minimum number of samples The Secretary shall measure a sufficient number of samples to analyze changes in soil carbon at each sample site selected under subsection (c) for each inventory under paragraph (1). (4) Characteristics In preparing the inventory under paragraph (1), the Secretary shall document the following characteristics for each sample site selected under subsection (c): (A) Soil type and texture. (B) Land use history, including conservation activities. (C) Environmental characteristics, such as temperature and precipitation. (D) Other characteristics, as determined by the Secretary. (e) Individual protections (1) Protection of private property rights Before selecting a sample site under subsection (c) or conducting any measurement under subsection (d)(1), the Secretary shall obtain authorization from the owner of the eligible land. (2) Voluntariness requirement The Secretary may not require authorization under paragraph (1) or any other participation in any activity under this section as a condition on