In a large bowl, with an electric mixer on medium speed, beat butter and brown sugar until well blended. Beat in eggs until smooth, scraping down sides of bowl as needed. In a small bowl, stir baking soda in 1 tablespoon hot water until dissolved; beat into butter mixture.
In another bowl, mix flour, cinnamon, nutmeg, cloves, ginger, and salt. Stir or beat into butter mixture until well blended. Stir in pecans and citron.
Divide dough in half and shape each portion into a log about 11 inches long and 1 1/2 inches in diameter. Wrap logs separately in waxed paper and freeze until firm enough to slice, at least 1 hour.
Unwrap dough. Cut logs crosswise into 1/4-inch-thick rounds and place about 1 inch apart on buttered or cooking parchment-lined 12- by 15-inch baking sheets.
Bake cookies in a 350° oven until lightly browned, 8 to 12 minutes; if baking more than one pan at a time, switch pan positions halfway through baking. Let cookies cool on sheets for 5 minutes, then use a wide spatula to transfer to racks to cool completely.