Liu, Shuguang, Tan, Zhengxi, Li, Zhengpeng, Zhao, Shuqing, and Yuan, Wenping, 2010, Are soils of Iowa USA currently a carbon sink or source? Simulated changes in SOC stock from 1972 to 2007: Agriculture, Ecosystems & Environment, v. 140, no. 1–2, p. 106-112.