In 712, Emperor Ruizong passed the throne to Li Longji, who took the throne as Emperor Xuanzong. In 713, after Emperor Xuanzong converted Yong and Luo Prefectures to special municipalities, Song was made the mayor of Jingzhao Municipality (京兆府, i.e., the old Yong Prefecture). As of 715, he was serving as chief imperial censor (御史大夫, "Yushi Daifu"), when, on one occasion, he was accused of not ordering for greater force to be applied when a criminal was being caned, and he was demoted to be the prefect of Mu Prefecture (睦州, in modern Hangzhou). He was later made the commandant at Guang Prefecture (廣州, roughly modern Guangzhou, Guangdong). It was said that at that time, the people of Guang Prefecture used bamboo to build houses, and therefore the houses were prone to fires. Song promoted the use of bricks, which greatly reduced the damage of fires, and the people of the region were grateful for this.