SOC depth distribution under conventional and no-tillage in Georgia (millet/clover/cotton/rye rotation for 4 years), Texas (wheat/soybean rotation for 10 years), and Alberta/British Columbia (small-grain crop rotation for 7 years). Each location is shown with mean annual precipitation, temperature, and average SOC concentration; *, **, and *** indicate significance between the tillage systems within P£0.1, P£0.01, and P£0.001, respectively (modified from Franzluebbers 2002)