Fig. 6. Relationship between the fraction of net aboveground C (TAC) that is returned as manure (FR) and the net C productivity required to meet C inputs that maintain the original soil organic C (SOC) in the wheat–fallow (WF) and wheat–oat–barley–hay–hay (WOBHH) rotations (from Eq. [18]). The greater inputs needed to maintain the original SOC in the WF rotation compared to the WOBHH rotation in combination with the lower proportion of TAC returned as roots in WF rotations result in a greater TAC needed to maintenance SOC. The actual values of TAC for three WF rotations and the WOBHH–Nil rotation are plotted as horizontal lines. When the WF lines reach the TAC line for the WF rotation they proceed down to the corresponding value of FR. The TAC for the WOBHH rotations are all above the TAC necessary to maintain SOC in the WOBHH rotation so only the point is shown. Values of FR at which the TAC meets maintenance requirements can be calculated with Eq. [19]