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Carbon Balance of the Breton Classical Plots over Half a Century

^a Pacific Northwest National Laboratories, 901 D Street S.W., Ste. 900, Washington, DC 20024-2115
^b Dep. of Renewable Resources, University of Alberta, 4-42 Earth Sciences Bldg., Edmonton, AB T6G 2E3, Canada

View larger version (29K): [in a new window]   Fig. 1. (A) Plot layout, (B) contour lines spaced at 1-m vertical interval, and (C) slope classes of the Breton Classical Plots

View larger version (28K): [in a new window]   Fig. 2. Aboveground plant C productivity per rotation cycle at the Breton Classical Plots

View larger version (22K): [in a new window]   Fig. 3. (A) Relationship between simulated and observed changes in soil organic C (SOC) (kg C ha-1 yr-1). Change in SOC is the SOC at each sampling date minus SOC at 1939. Simulated SOC at each date was calculated with Eq. [10] using parameter values reported in Table 7 for the Cum model. (B) Mass of SOC was simulated using adjusted values of k to distinguish wheat–fallow (WF) from wheat–oat–barley–hay–hay (WOBHH) rotations and compared with observed values for the Breton plots over 51 years. Adjustment entailed multiplying each k by 1.3 for the WF rotations and dividing them by 1.3 for the WOBHH rotations

View larger version (21K): [in a new window]   Fig. 4. Relationship between annual addition rate of C and measured annual change in soil organic C (SOC) over 51 yr

View larger version (18K): [in a new window]   Fig. 5. (A) Carbon content of each of compartments C1, C2, and C3 in the wheat–fallow (WF) rotation using adjusted k values and model-derived additions that maintain original SOC content at 51 yr. See Table 7 and Fig. 3 and 4 for model information. (B) Trend of total soil organic C (SOC) in each rotation as calculated using the three-compartment model with model-derived additions that maintain the original SOC at 51 yr. Additions needed to maintain the original SOC are greater for the WF than the wheat–oat–barley–hay–hay (WOBHH) rotations

View larger version (21K): [in a new window]   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]