Preheat oven to 350°.
Pierce potato with a fork; place on a paper towel in microwave oven. Microwave at high 7 minutes, turning after 4 minutes. Wrap potato in paper towel; let stand 5 minutes. Peel potato; mash to measure 1 cup.
Place butter in a small microwave-safe bowl. Microwave at medium 20 seconds or until soft. Stir in pecans and 2 tablespoons sugar.
Lightly spoon flours into dry measuring cups; level with a knife. Combine all-purpose flour and next 6 ingredients (all-purpose flour through nutmeg) in a large bowl, stirring with a whisk. Add mashed sweet potato, juice, oil, and egg, stirring until well blended.
Spoon batter into an 8 x 4-inch loaf pan coated with cooking spray. Drop pecan mixture by spoonfuls over top of loaf; gently press into batter.
Bake at 350° for 1 hour or until a wooden pick inserted in center comes out clean. Cool in pan 5 minutes on a wire rack. Remove from pan; cool completely on wire rack.